Before you read too much further, I would like to clarify some potential misconceptions that may pop up regarding what some may define as a log-builder. First, I am not contracted with nor am I a representative of any log home company, nor am I simply a kit-builder, who in reality is one whose only role in the process is basically that of an assembler; a contractor who assembles the highly processed, pre-fabricated log home “kits” that have been produced by others. Those individuals and the companies they represent differ greatly from what I do and how I approach the craft of building with logs, and as you read on I hope that you will agree.
Although the basic construction methods are different, my approach to the crafts of timber framing and log building is intertwined with the long-held belief in traditional, handcrafted, wood-to-wood timber joinery. I have incorporated many facets of traditional timber framing in my log building philosophy. The log building system and dovetail corner notching that I use were chosen not only for their aesthetic characteristics, but more so for the fact that this design and joinery method has been historically proven to be a structural success.There are many ways to “modernize” the crafts of timber framing and log construction, but my belief is to stay close to the building patterns of America’s pioneering past, and use these historical structures as my inspiration.
I will be working with a very traditional log building system which incorporates your choice of wall log thickness, either 6”x12” or 8”x12” rectangular timbers that historically were hewn to shape with the broadax, and today are sawn to final dimension on a bandmill. This log profile produces an even, symetrical horizontal chink-gap between log courses. I will also be offering, for those interested in a more authentic, traditional look, a log profile where the log is flattened on only two sides, and the chink joint follows the natural contour of each log. Most log structures are defined by the distinct and prominent style of corner notching they display, and the “Appalachian” dovetail is the log corner profile that is used in conjunction with these massive wall-logs. The materials, specifications, and corner notching details featured in this log wall system, combined with the preventive measures I’ve taken in addressing potential air and water infiltration are explained in much more detail further along.
I prefer to use only full-length eastern white pine logs (with no mid-wall splices) in the construction of the cabins that I build. Partnered along with material handling issues and my local mills saw-length capacity, these factors will dictate the size of the log structures. Eastern white pine is available in my local in lengths up to 24’, while 30’ long logs can be obtained a further distance away. Ideally, a log structure using full-length logs with exterior dimensions of 30’ x 30’ is as large as I prefer to build. Larger structures can be built, but this would require the employment of “scarf” (splice) joints. This scarf joinery, to create a seamless, aesthetically pleasing longer timber from two shorter ones, is quite time consuming to layout and fabricate, while still achieving the tight joinery tolerances that I expect of myself. There would be many of these scarf joints in the wall system of a larger structure. But we can certainly discuss this option if a larger structure is in your plans.
Each dovetailed corner notch, and all wooden joinery is laid out and handcrafted with the traditional hand-tools of the past (squares, chalk-line, wooden mallet, framing chisels, hand saws, etc). The handcrafting of your log cabin demands great care, time, and patience, and coupled with the fact that I work alone, it does take me longer to complete a project than others who use a more mechanized approach. As a famous log builder once said, “Mistakes in wood are damn permanent, so take your time and do it right”, and I work under the guidance of that quote.
In addition to the features and specifications that you will read about, I have my own mobile derrick, a mini-crane on wheels that I use inside the structure (on a temporary sub-floor) in the raising and stacking of wall-logs that have been pre-staged on the exterior perimeter of the structure. This eliminates the added expense of having to keep a boom-truck or larger crane onsite for an extended period of time, and eliminates any potential damage or disturbance to the surrounding landscape.
What follows is a brief historical background of the Appalachian dovetail log structure and the pioneers from our past that introduced this still-enduring log building method to our American culture, and also a brief anecdote on how log structures were once historically defined based solely on their log-wall profile.
Building with logs that were hewn with the broadax to have flat interior and exterior surfaces was a technique developed by and brought to this country by immigrants from Sweden and Finland. The log house was an unknown in America until 1638, when the first and only purely Scandinavian settlement, called New Sweden, was founded along the Delaware River, at what is now Wilmington. While the Swedish brought their log-building techniques to Delaware and Maryland, the early 1700’s saw Germans and Scotch-Irish begin to settle in Pennsylvania. While these ethnic groups had a long-standing cultural background in log construction in Europe, and were comfortable in its continuation here in pioneer America, it seems that only the English had no experience building with logs and instead clung to their own familiar building practice of timber framed houses sided with sawn clapboards. The ensuing years produced a melting pot of log construction methods, as ethnic groups migrated from previously settled areas and contributed building techniques that were unique to their European heritage. Generally, historians seem to agree that the hewn, dovetailed log structure as we know it today evolved from the cultural building patterns of the Germans, Scandinavians, and Scotch-Irish. This particular style of hewn log walls with dovetail corner notches became the dominant form of log construction in the Appalachian Mountains, resulting in it commonly being referred to as the “Appalachian Dovetail.”
I’ve found it interesting to note the historical difference in terminology as it relates to log building, past and present. I’m referring to the distinction between a house and a cabin. In the pioneer landscape of early America, as opposed to modern-day vocabulary, it seems the size of the structure was of little relevance as to how it was termed. A log cabin was built of round logs, and often disparagingly referred to as a pole-shack, usually as temporary shelter and the first living quarters built as soon as a clearing in the forest permitted. It was hastily erected, sometimes with only the earth as a floor, with little time to be spent on craftsmanship. Oftentimes, slave’s quarters were constructed of round logs and referred to as cabins. On occasion, in the absence of a chimney, a large opening was left in the roof to allow smoke from cooking fires to escape. Historically, it seems that there has been a negative connotation attached to the word cabin as it pertained to a log dwelling, with that negative implication being all but erased with the passage of time. By contrast, a log house was built of hewn logs, flattened on two, or occasionally all four sides, with carefully fitted dovetail corner notches, and was of a much higher degree of craftsmanship. It was somewhat of a status symbol for the settler, indicating that he and his family had prospered enough to rise above the crude, round-log cabin of his past. However, many dovetailed log structures were soon sided over, in an effort to emulate the refined, town-dwelling Englishman and his house of timber frame construction covered with sawn clapboards.
Log Shell Materials & Specs
Wall Logs- A superior-quality log structure begins with superior-quality material, which, when combined with appropriate and knowledgeable sawing techniques, are the initial requirements that will have the most impact on the integrity of your log building. The sawmill where I obtain the timbers that will be incorporated into your log structure is given a set of standards regarding the quality/grade of logs and acceptable sawing methods needed to produce structurally sound timbers for a log structure. Eastern white pine is the wood specie that is used exclusively in the 6”x12” and 8”x12” Dovetail Log system. While other log builders may mix woods within a log wall system, I believe it best to use only one specific wood species. Different woods exhibit different rates of shrinkage, and incorporating a mix of woods can result in an uneven shrinkage and settling timetable for a given log structure. Only boxed-heart timbers are used in the construction of your log cabin. This sawing method produces a timber where the pith/heart, or the center of the tree remains in the center of the timber along its entire length, resulting in a much more dimensionally stable wall log. Another benefit in the conversion process from round log to sawn rectangular profile is that most of the weak, rot-prone sapwood is removed from the logs, leaving the dense heartwood which is more resistant to decay and insects. By specifying only boxed-heart timbers when ordering from the sawmill, and not accepting halved or quartered logs (where the heart is far off-center or not present), the undesirable drying characteristics common to these sawing methods is eliminated.
Another material-quality checkpoint is that all wall logs are thoroughly inspected with regards to spiral grain patterns, to ensure their proper placement within the wall. Virtually all logs/timbers, regardless of how straight they may appear, have some varying degrees of left or right-hand spiral grain. This can readily be seen as drying checks (cracks) develop on the log surface. These checks will follow the fiber alignment of the log. Logs with a slight-to-moderate left-hand spiral tend to be more prone to distortions during the drying process and are placed only in the lower 1/3 of the wall, where the weight of the building and/or alignment pins will hold them in place. Logs that slightly spiral to the right can be used anywhere in the wall system, as they have more balanced stresses within and will remain stable. I will not accept logs with anything more than slight-to-moderate grain run-out, or any other naturally occurring potential structural defect to be incorporated into your log cabin.
Although I go to great lengths to ensure quality logs go into your log structure, you may feel more comfortable using logs that have been structurally graded by an accredited log grading agency. Or perhaps your local building inspector will require grade-stamped logs. In either of these instances, I can supply building logs that have been structurally graded according to Log Home Council Grading Standards.
Once your logs arrive at my yard, great care is taken to insure that they maintain that sawmill fresh look. Upon arrival, fresh end-cuts are made and an end-grain sealer is applied to the freshly sawn timber ends to control end-grain checking (splits). This end-grain sealing process forces moisture to leave the timber through the long-grain surfaces (sides) and at a much slower rate than if allowed to exit the timber from its preferred and faster path of travel, which is via end-grain. All timbers are stacked and stored in my shop, with sufficient air flow between layers in the stack, to help inhibit any possible fungal growth. White pine is especially susceptible to blue stain, a fungus that attacks only the sapwood, and is present in airborne spores when temperatures range between 40 and 105 degrees. Structurally, it is of no consequence, but it is visually unappealing and difficult to eradicate once it takes hold. Keeping timbers dry and with good air-flow are a few preventive measures, and chemically inhibiting the fungi with a borate-based wood preservative is another important step that I take to keep your timbers looking their best.
Borate-based wood preservatives are derived from naturally occurring, water-soluble mineral salts that are mined in California. They are highly effective as a wood preservative in preventing fungal decay and are deadly to most wood-boring insects (termites, old house borers, carpenter ants, etc.), but are non-toxic to people, pets, and the environment. Some of you may be familiar with the old-time laundry detergent still found on grocery store shelves, 20 Mule Team Borax, in which borates are the primary ingredient. Borate salts are odorless, have no effect on the color of the wood, are compatible with most exterior stains (check with manufacturer), will not interfere with chink-joint adhesion, and are non-corrosive to metal fasteners. When applied to freshly sawn wood, the water-soluble borate solution uses the woods’ own high moisture content as a transport medium to naturally penetrate and diffuse deep within the log or timber. But, the water-soluble characteristic of borates can also work in reverse order unless some precautions are taken. It has been shown that the borate solution in timbers will tend to leach out of the wood once it has been exposed to rain, unless a high-quality stain/sealer is applied to logs soon after installation, usually within a few days. While your house logs are in my care, either in storage or in the various stages of fabrication, they are kept under cover at all times, with no possibility of rain exposure.
The standard surface texture on wall logs and interior timbers is that of a rough-sawn appearance. All logs and timbers are sawn on a band-mill, as this sawing method leaves the timber faces with a relatively smooth finish. For those that may prefer wall logs and timbers with a different aesthetic look, I offer as an option the very rustic and authentic-looking hand hewn finish.
Upon completion, all interior structural wood joinery is treated with an end grain sealer. This paraffin-based wax coating all but stops the natural moisture migration from end-grain surfaces, thus greatly reducing checking (cracks) while insuring the integrity of every wood joint. Exposed exterior end-grain on dovetail corner notches will tend to wick rain water or melting snow into the pores of the timber, leading to possible decay. To counter this, all end-grain corner notches are hand-planed smooth to close the pores, and then an exterior-grade sealer is applied to complete the sealing process.
Building System Details
The dovetail corner notch was by far the most common corner notch used in the log dwellings found in the mountain regions of Appalachia. It is a self-draining (slope of dovetail drains rain-water to exterior), self-locking timber joint that draws the corners of the building tighter as the weight of the upper logs is applied. Notch extensions are additional amounts of wood left on the log ends and aid in the protection of the dovetail notches from end grain deterioration. I allow for a 2” overhang, as that was most common in pioneer cabins.
Inherent in dovetailed log construction is the fact that the corner notches are designed to bear 70-80% of the weight of the building, resulting in constant compression on the notches while ensuring that the joinery remains tight. In order to maintain this tight corner notch seal over the life of the structure, an optional compression-spring & thru-bolt system can be installed near the dovetail corners and is explained in further detail as you read on. This eliminates the well-intentioned but often misguided use of spikes in log wall assemblies, as these spikes often function in reverse order of their intended use and end up holding wall logs apart as they shrink and dry.
Despite the most diligent of efforts aimed at the prevention of air and water infiltration, there still remains the ever-slight possibility that wind-driven rain may penetrate the corner joinery, as the constant expansion/contraction cycles common to wooden structures may create small openings in even the most well-crafted and tight dovetail corner notches. A vital element in addressing this issue is the employment of an open-cell, tri-folding foam gasket by Emseal Log Home Tape. This high performance sealant has been impregnated with acrylic-modified asphalt, and is placed at the vertical interfaces of all dovetail corner joinery. I’ve also taken one last preventive measure in addressing the threat of water penetration at corner joinery, in the form of a 1/8” deep channel that is sawn into the up-face of each dovetail notch that follows the natural down and outward slope of the notch and will direct any rain water that may find its way into a corner notch to the exterior of the building. This gutter channel is then covered by the down-face of each succeeding notch.
To ensure lateral wall stability at window and door openings and to keep logs from twisting, hardwood alignment dowels (1¼” x 20”) are placed on either side of openings and in long wall-length runs. These drift pins, as they are sometimes referred to, are wax-coated to assist them being driven in the matching diameter holes. The tight, compression-fit of the drift pin is necessary to keep the walls in a proper vertical plane and increase the rigidity of the log wall, while the wax coating reduces friction and allows the logs to shrink and settle with no vertical impediment.
Another measure taken that addresses both wall-alignment and settling issues at window and door openings is the use of angle iron splined jambs. A vertical tenon is sawn into the log ends to accept a slotted 3” x 6” window/door jamb, which is then re-enforced with 1½” x 1½” x ¼” angle iron, providing an extremely rigid framework and allowing the window/door assemblies the freedom of movement to settle along with the log walls. In addition, load-bearing wall support blocks are placed at window and door openings.
Dovetailed log construction is not only characterized by the distinctively joined dovetail corners, but also the horizontal gap between each log course known as the chink space, one of the prominent characteristics in American log building. Historically, this chink space was filled all sorts of materials ranging from moss to wood chips, to cow manure, and plastered over with various mixtures of clay, lime, straw or horse hair. Materials and solutions evolved over the years to include wire lath nailed to the logs and coated with a cement/clay mix. Today, there are many flexible, rubber-like chinking compounds on the market designed specifically for log homes to accommodate log movement. Some well known companies with excellent product lines and informative catalogs that have been around a while include www.sashco.com, www.permachink.com, and www.sansin.com. Another excellent source for many other log home products is a catalog put out by Schroeder Log Home Supply www.loghelp.com. Not only do they carry many products relevant to log construction, but there are also useful how-to write-ups concerning the application of chinking, interior/exterior stains, etc.
The chink joint in the dovetail log system averages about 2” to more closely resemble log cabins of the past. The underside of each log is grooved (¾”x ¾”) along both edges to accept a 3/8” strip of rigid spline board. The top of the log has a beveled edge onto which the bottom of the spline is nailed. This spline board is angled out and down, allowing the chink joint to effectively shed rain. Synthetic chinking compound is then applied over the spline board to complete the exterior chinking process. This forms a weather-tight seal, while still being flexible enough to stretch to accommodate the expansion/contraction cycles common to heavy timber structures.
Before the interior chinking and spline board is applied, the chink joint insulation is installed. There are a variety of insulation materials for this particular application, some more suitable than others. I’ve chosen to use natural sheep wool for the chink joint insulation from www.goodshepherdwool.com. According to their product literature, their wool batts are made from 100% pure virgin wool, with no acrylic fillers, and no artificial materials such as recycled carpet fibers that contain plastics and/or petrochemicals. Sheep wool has natural insulating properties with millions of tiny air pockets, has a rated R-value of approx. 3.5 per inch of material thickness, and is also flame retardant. In addition to these naturally occurring characteristics, the wool used in your log structure has been infused with natural borax for increased rot and insect resistance, with no harsh chemicals like boric acid or other boron compounds mixed in. Due to its crimped structural nature, sheep wool will retain its original shape within the chink joint and not be prone to settling or breaking down like other insulating materials can. In the unlikely event that moisture does penetrate the chink joint, sheep wool has the unique ability to absorb up to 30%-40% of its own weight in moisture without becoming wet to the touch. This inherent characteristic does not compromise wools’ natural insulating qualities, as is common to both fiberglass and cellulose. And unlike fiberglass, natural sheep wool handles nicely on-site as there are no respiratory or skin related irritations associated with its use.
After the exterior chinking process is completed, strips of wool 3”x 6”x 8’ are placed in the 2” chink joint from inside your cabin, allowing for a snug top/bottom and side-to-side fit with very little compression. The remainder of the interior chinking is done in the same manner as the exterior.
All chinking material is supplied by Square Peg Timber Joinery and is to be installed as part of the finish work by the owner or general contractor. Visit www.permachink.com for an excellent write-up on the proper methods of chink material application.
Through-bolt Fastening System (optional)
By this time you are surely well attuned to my affinity for building in a traditional manner, and I suppose that if you have continued reading to this point that you too also share an interest in traditional log structures. And traditional log structures from our American pioneer past did not employ any mechanical fasteners in their log wall systems. The settlers who built these early log cabins relied on the vertical compression of simple gravity in the hope that log corners would remain tight and came to expect the occasional intrusion of wind and a bit of rainwater. Living standards have increased significantly since that long ago time and the log home industry began searching for ways to help prevent air and water infiltration at corner notches. This brings us to the use of modern log-fastening systems which have evolved from simply spiking log courses together (which, unfortunately, many log builders still do), to the latest in self-tightening, compression-spring, through-bolt technology designed to keep your log structure sealed tight against the elements. Although I do have a strong aversion to visible metal fasteners, I do support their use if they are hidden from view and if they make the structure better for it.
The sealant system used in the chink space between log courses that was detailed previously is just one feature addressing potential air and water infiltration. Unlike frame construction where the layers of exterior skin protect against the elements, a log wall acts as both the structural frame and the first/last line of defense against air and water intrusion. Therefore, key elements in this regard begin with a quality building system, followed by a sound and effective plan to prevent air and water infiltration.
There are a few different mechanical fastening methods common in the log home industry, with each one having its own merits and applicable to the many different log building systems out there. I’ve chosen to incorporate (as an option) self-tightening, spring-loaded through-bolts in my log wall system for a variety of reasons. First let me say that without a well-constructed log wall assembly to begin with, no mechanical fastener will save it from its own shortcomings. Through-bolts are designed as a supplement to quality log building methods, not transform a poorly constructed log structure into a good one.
That being said, the through-bolt system used in the log structures that I build consists of 3’ lengths of ¾” diameter threaded rod connected with couplers that runs through the full height of the log wall from the top plate log all the way down to the foundation, where it is anchored to j-bolts imbedded in the concrete. It is augmented at the top plate log by a corrosion-resistant zinc nut and washers, beneath which sits a heavy-duty 6½” compression spring rated at 1000# of pressure that provides constant vertical compression within the log wall stack. This is followed by a ThruLoc self-tightening ratchet-type nut. The compression of the log wall stack is as follows: The top nut is manually tightened down which compresses the spring. The spring comes into contact with the ThruLoc self-tightening nut, forcing it to ratchet down the threaded rod and apply constant force where necessary. This system allows the logs to function as a solid unit and settle uniformly with no vertical hindrance as is common in log-on-log nailing systems.
Many “spring-less” through-bolt systems require frequent adjustments on the homeowner’s part, so that when the logs shrink and settle, manual tightening of the nuts will be necessary to maintain the constant compression and to take up the “slack” on threaded rod. In order to ensure maximum effectiveness while at the same time making this system as maintenance-free as possible, I’ve also decided to incorporate the ThruLoc self-tightening lock nuts within the assembly. With the constant downward force generated by the compression spring, solid wood-to-wood contact is maintained at the horizontal interfaces of dovetail corner notches, locking the logs in place, and when combined with the compression-fit alignment pins, will resist the logs tendency to twist and/or bow as they dry and shrink.
Now that you’ve read all the positives concerning through-bolts, I’d like to point out some difference of opinion regarding how necessary they may or may not be from those “in-the-know” within log building circles. As I have not used this through-bolt arrangement as of yet, I cannot vouch for the integrity of the system. In theory, the through-bolt assembly and all its parts make complete functional sense. In the real world practice of log construction however, I have heard varying scenarios regarding the effectiveness of the assembly and whether through-bolts are even needed in log structures other than in earthquake prone areas. Some log builders I have heard from believe in a through-bolt system so much that they will not build without their inclusion, while others feel that they are rather unnecessary and their addition tends to place a structure in the overbuilt category. As of now, I will be offering a through-bolt system as an optional feature, my intent being to not obligate you to pay for a structural element that may or may not make your log structure better for it. But personally, I see only structural positives by including through-bolts in a log wall, and would highly recommend the use of a through-bolt system. However, I feel that the final decision should be left up to each individual. You, in conjunction with your designer, building code official and engineer, can decide for yourself if a through-bolt system is right for your situation.
Timber Frame Loft
The 18’ x 24’ model cabin shown on this page features an upstairs sleeping loft that covers half the width of the structure, and is designed to provide 6’ of headroom while standing 3’ from an exterior wall. When a larger structure is desired and has an interior width that is wider than 16’, the loft is framed using massive posts and beams with pegged, wood-to-wood joinery and with an 8”x12” tie beam spanning the necessary distance and joined to the eave wall logs with a housed dovetail joint. This tie beam is supported at mid span by a braced and pegged 8”x 8” post. An 8”x12” “summer beam” (girder) runs perpendicular to the tie beam and carries 4”x 8” loft joists to the eave walls. It also features housed dovetail joinery on either end. Cabins with interior widths of 16’ or less can employ a simpler framing method of 6”x 9” loft joists covering this span between eave walls.
Timber Frame Roof System
The roof system most commonly used in Appalachian dovetailed log structures and elsewhere was the common rafter roof. In small, one-level structures with no knee-wall, the 6”x 8” rafter pairs are secured at the peak by a pegged tongue & fork joint and joined at roughly mid-span by 6”x 8” collar ties. These collar ties feature half-lapped, dovetailed joinery and are fastened to their rafter mates with white oak pegs. A stepped-lapped rafter seat, traditionally a favorite among joiners, locates and seats the rafters on the plate logs. These plate logs and their attachment are a critical structural component, as they must be up to the task of countering the roof thrust that is inherent in rafter framing, which is generated by roof loads that push down and out on the eave plates and (in story-and-a-half structures), the knee-wall logs below. One part of the solution is a 12/12 roof pitch, which provides maximum loft headroom while not looking too “peaky”. Other strategies employed include compound dovetail notches on the plates, the load bearing and structurally superior stepped-lapped rafter seat, and the vertical pinning of the knee-wall logs at 4 feet intervals with 1¼” hardwood dowels. These rods penetrate the top several courses of logs, and provide the required lateral wall stability to the loft knee-wall. Yet without a doubt, the most viable solution to negating the roof thrust imparted onto the knee-wall involves a posted structural ridge beam, in which the rafters are “hung” from the ridge beam, thus eliminating plate spread entirely. In the end though, engineering specs will have the final say in determining the appropriate roof framing system for your particular situation.
GRK’s RSS screws are used in the attachment of the rafters to the top plate. Where in the past it was quite common in log and timber frame structures to find a square peg securing the rafters to the top plate (and you may still use this traditional method of attachment if building code allows), I find it less of a compliance issue to use a fastener whose hold-down characteristics can be quantified to building officials.
An alternative to common rafter roof framing is the purlin framed roof, in which the gable ends, instead of being stick-framed, are built with logs that are cut progressively shorter to form a triangular continuation of the end walls. Massive timbers (purlins) span the full length of the building. When a log roof is framed in this manner, the weight of the roof is directed to the gable ends and interior support posts, with roof loads being vertical in nature and little outward pressure on the walls. This style of roof framing is much more common in round-log construction.
Timber Frame Porch System
Another standard feature that is included in the log shell package, and one that I feel compliments the rustic nature of a structure such as a log cabin, is a full timber frame front porch. Porch posts @ 6”x 6” feature mortise & tenon wood joinery, with pre-drilled “weep-holes” in the brace mortise bottoms to help facilitate rain water drainage that can collect in the brace pockets in the posts. Pegged braces rise to meet the 6”x10” porch plate, which is mortised with step-lapped rafter seats to accept the 4”x 8” rafters. All porch timbers that are exposed to the weather are cut from white oak heartwood, one of nature’s most rot-resistant species. Allowances for settling are provided in the form of adjustable Nortek screw-jacks under all porch posts.
Allowances for Log Settling
Settling, as applied to a log structure, refers to the vertical reduction in height of horizontally laid logs in a wall system due to the combination of cross-grain shrinkage in a log and the compaction of the logs from the weight of the wall and roof system. Green logs will shrink, and will do so more in cross-section than along their length, although eastern white pine is one of the most dimensionally stable of the softwoods and has some of the lowest shrinkage rates. When properly addressed and accounted for, settling can be a positive attribute in a log structure, as log walls and corner notches actually tighten up over time. ILBA Log Building Standards state that green logs must be allowed to settle ¾” per foot of wall height. That doesn’t mean the log walls will settle that much, but those accommodations must be accounted for. A variety of factors affect the total amount of settlement in a log wall, including moisture content of the logs, wood species, density of the wood, and roof loads. This height reduction usually occurs slowly and over a 5 year period in a heated building. Effective methods to counter any potential problems with logs settling include cut outs in header logs above door and window openings, Nortek settling jacks (which can be hidden by finish trim) under all vertical members, and not spiking window and door jambs at log end openings. To properly allow window and door units to settle along with the log walls, slotted and angle iron-splined window and door jambs are employed, as opposed to simply nailing the jambs in place. The consequence of nailing the jambs directly to the log ends is that as the logs gradually settle downward, the fixed-in-place jambs (and the attached windows) do not move with the logs, creating many problems. Also addressed with regards to settling are interior partition walls, gable stud framing, staircases, and generally any area where the natural settling of horizontal logs can be impeded by fixed vertical members.
Thermal Performance of Log Walls
When comparing the insulation qualities of a log wall vs. standard frame construction, the common R-value used to measure energy-efficiency cannot and should not be applied here. Eastern white pine, the species most commonly used in log construction, has one of the highest rated R-values at R-1.3 per inch of thickness. This yields an R-value of about R-8 for a 6” thick log wall and a little over R-10 for the 8” thick wall. Not too bad in itself, but compare this to the standard 6” stick framed wall packed with fiberglass which carries an R-19 rating, and the log wall lags a bit behind when using that standard of measure. But log walls march to a different drummer and draw their energy-efficiency not so much from an insulation standpoint but from the thermal mass of the logs themselves. Thermal mass refers to the ability of a material to store heat and then slowly re-radiate that heat (a time lag of around 8 to 12 hours) when the surrounding air temperature falls below that of the material. A concrete driveway on a sunny, hot summer day is an example of thermal mass because it absorbs, holds, and re-radiates heat well. Walking barefoot at noon on that driveway may be intolerable, but take that same barefoot stroll after dark and you will still feel the warmth under your feet. Log walls perform in this same manner. They act as a thermal storage system and delay the impact of outside air on inside air. In the winter, the log walls retain heat that is internally generated and re-radiate that heat, allowing the interior surface of your log walls to remain warm to the touch. Conversely, in the summer, they also absorb and deflect the heat of the outdoors, resulting in an interior that retains cool air and a wall that will feel cool to the touch.
Exterior Design Recommendations
Some exterior design features that may be common and appropriate in stud-frame construction should not be employed when building a log structure. In my opinion, the two biggest design flaws in log construction are foundations that permit the base course logs to be too close to the ground, and uncovered decks that are attached to the house. Both of these errors in design allow rain splash-back to hit the lower course logs, and/or melting snow to pile up against those bottom logs, creating a situation where eventual rot is a certainty. Water is the enemy of log homes. Dry logs do not deteriorate. Initially, splash-back will present itself as a maintenance issue, causing black mold stains on the lower 2-3 courses. This continued exposure eventually results in the base course log or logs rotting and needing to be replaced. Needless to say, this is a much different scenario than with a simple clapboard-sided building, where replacing a few pieces of siding would remedy the situation.
Another very important aspect in a properly designed log structure in terms of its longevity is sufficient roof overhang. A wrap-around porch would be the ideal design feature, as it offers full protection against sun and rain for wall logs and corner notches. A down-side to the wrap-around is that it does tend to limit the amount natural light entering the home, resulting in a somewhat darker interior. A well-designed log structure that features effective preventive-maintenance strategies incorporated within its exterior design are: foundations that place the base course log at least 16” above grade (20” is better), wide (24”minimum, 36” is better) roof overhangs at the gable and eave ends, and/or covered or wrap-around porches. If an uncovered deck is a must-have on your wish list, construct it as a stand-alone deck, that is, only the entryway to/from the house would be attached to the house. The deck itself should be located at least 12” from a log wall, thereby eliminating splash-back possibilities.
Keep all shrubbery at least 3’ from log walls, as all ground plantings tend to hold moisture-laden air, especially on the north side of buildings, and tend to cause mold and contribute to the decay process.
Avoid the tendency to stockpile firewood against log walls. The combination of wind-blown rain and leaves that will get lodged in between the pile and log wall is a recipe for mold and also offers a convenient pathway for insects to enter your home.
Not to be ignored when addressing the topic of water and log structures is the potential damage that water from inside the house can do. High-moisture areas like kitchens and baths generate water vapor that can seep into loft-joist pockets in the wall logs. What happens over time is the loft joist ends will shrink a bit in diameter, while the wall log (and the mortise the joist sits in) will show virtually no shrinkage (longitudinal) along its length. This shrinkage differential will result in small gaps appearing at the joist end and wall log interface. In the winter, the loft-joist ends, being notched in and located closer to the colder exterior surface of the wall log, will attract warm, moist air through these gaps that will seek to condense on the colder joist ends. Hidden frost can form on these joist end notches, which will eventually melt. Repeated wet/dry cycles over the years create conditions for decay, not only on the joists, but also in the wall logs. Any shrinkage gaps around joist-ends where moist interior air can get to should be filled with backer rod and caulked.
Wood Finishes for Logs & Interior Timbers
Assuming that you have taken the proper steps involving the exterior design of your log cabin, you will still have log walls that will be exposed to a variety of destructive natural elements. Log walls will need periodic maintenance for the life of the structure, an undeniable fact with not just log structures, but with all exposed exterior wood. But by choosing a high-quality exterior stain and applying it according to manufacturer’s specs, you can lengthen the time span between applications. Log structures need a superior product that has been specifically developed and tested for its unique requirements, and one of the most important decisions you will make once construction is completed is the quality of the finish/stain you will apply to the exterior of the logs. Don’t be tempted to purchase a stain/preservative from the big-box stores. There are many companies that manufacture excellent wood finishes that cater exclusively to the log home industry, with www.permachink.com, www.sashco.com, and www.sikkens.com among those that have been around awhile and have informative web sites and produce high-quality products. Permachink was one of the first companies on the scene over 20 years ago that offered maintenance products aimed at the log home industry. Their web site offers valuable tips on the do’s & don’ts concerning exterior maintenance of log walls, as well as a vast array of products that will keep your log structure looking great for many years. Take your time and research the different companies out there and the quality of their product.
One other point to consider, as your logs and interior timbers are still “green” (high moisture content), do not apply a non-breathable surface film-finish (polyurethane, paint) to them. White pine timbers dry at the rate of one inch per year, hardwoods will take twice that long to dry, at half an inch per year. Over the next several years, moisture will be slowly evaporating from the timber surfaces. If that moisture release is impeded by a solid surface coating, it will become trapped between the timber surface and the finish, resulting in probable mold spots and a cracked, peeling finish.
The 2x6 T&G boards that you may be using for gable wall/roof sheathing and finish flooring should be kiln-dried before installation. If not, cross-grain shrinkage will result in unsightly gaps at the interfaces between the tongue and the groove. Moisture migration will not be an issue if the boards are kiln-dried, so a non-penetrating film can be applied to these if desired. Some web sites to look into concerning floor finishes are www.vtpf.com/finishes, www.bonakemi.com, www.landarkwoodfinish.com, and www.woodfloorsonline.com.
All wall logs are vertically pre-drilled with 1¼” holes at various points to run the wires. The open 2” chink space in the dovetail system is an ideal location for wire and box placement. Other options include a built-up baseboard that provides an electrical chase behind the baseboard, and conventionally framed interior partition walls and floor system. Wiring up to loft areas can be laid in a pre-routed channel in the top surface of a rafter or beam and before the decking is nailed on. Be sure to check with local building codes for acceptable installation details concerning the wiring of your log structure.