LOG SETTLING can be UN-SETTLING

The International Code Council “Standard on the Design and Construction of Log Structures (ICC 400-2007)” defines settling as the combination of three separate phenomena: radial shrinkage, slumping and compaction. Generally, the most important, radial shrinkage is a decrease of the log’s diameter due to evaporation of the moisture in the wood cells.  This drying causes the majority of settling and must be considered when constructing a log home.

Compaction and slumping are typically a concern only with cope style logs.  Swedish-cope logs have a cross section similar to a pie with a missing piece.  The logs are stacked such that the cope on the bottom side is stacked on top radius of the log below it.  This commonly allows a small air gap between the logs in the middle of the cope. As the log dries, the cope opens up, and the top log sits lower on the bottom log.  This reduction in height is called slumping.  If the contact points between log courses are not uniform the wood fibers will be crushed by the bearing weight of the structure above.  This crushing of wood fibers is called compaction.

Longitudinal shrinkage (log length), where logs are oriented vertically, as in load bearing columns and posts, for all practical purposes is insignificant with regards to log home design.  In other words, due to the fact that wood cells are tube like, the length dimension of logs change very little.  Therefore it is important to note that screw jacks are used on load bearing columns because of the collective radial shrinkage of the logs in the outside wall, and not because of any longitudinal shrinkage of the column or interior stud walls.

Logs with large bearing areas or flat-on-flat surfaces, (like a D-profile or rectangular log) are generally not subjected to slumping and compaction but are commonly affected by radial shrinkage (log diameter).  Since we know that logs used in log home construction are stacked horizontally and vary widely regarding their moisture content, (See Log Moisture & Drying) both within the individual log and within log walls, it is extremely important to consider the potential radial shrinkage when designing a log home.

More specifically, most wood-cell shrinkage does not begin until the moisture content (MC) of the wood drops to below an average of 27 percent (actual range could be anywhere between 18 and 35 percent).  According to the Log Standard, four climate zones exist in the United States, based on temperature and humidity (dry, moist, warm-humid and marine).  Logs in these climate zones will have average final moisture contents of 10, 13, 14 and 15 percent respectively.  Other factors, such as how the house is heated, may also affect the final moisture content of the logs.  The greater the difference between the beginning and final moisture contents of the logs, the more settling will take place. With a 9-foot wall this settling may range from 1/2 inch to as much as 4 inches.

In theory, the solution to settling is easy according to the log home industry.  Since the log walls decrease in overall height, all other parts of the house must be constructed to settle at the same rate.  To do so the experienced log home builder may: 1) use adjustable screw jacks under logs or steel posts, 2) use slip joints around window, door and chimney openings, 3) attach cabinets to only one log, 4) use special flexible joints on rigid plumbing pipes, 5) construct steps that are slightly shorter in anticipation of final settled floor heights, 6) place standard log-home screws on an angle even though testing proves it does little, and 7) a few other creative techniques.  More complex issues involve interior load-bearing stud walls that need to be designed as beams supported by adjustable posts with a non-load bearing wall built around the post-and-beam support system.  I question whether log gables should even be used with this type of construction since stresses added to the ridge, caused by settling, can cause major problems.  Wow, this is quite a list of major things to consider when building a log home.  This theory becomes even more challenging because log shrinkage is unpredictable and never uniform.  In reality the majority of log home construction becomes more of an art, than a science.

In contrast, another option to deal with settling is to construct a home that makes settling insignificant.  The Log Standard states that if settling is less than 0.05 percent of the total log wall height (1/2 inch for an 8-foot wall) the log wall can be constructed and designed as a non-settling wall.  Non-settling log walls can be accomplished several ways.  The first is to construct the house out of logs with the same average moisture content as the climate zone the home is being built.  However as we have learned, this all sounds logical but is rarely practical as even the most rigorous kiln-drying schedules seldom bring the core moisture content of logs below 19 percent.

Second, some “Log Home” companies build homes with standard stud frame construction and cover them with half log siding.  This does eliminate the settling issue but technically they are not “logs.”  The advantages recognized by full logs such as thermal mass, sound dampening, and log stability are greatly diminished.
             
Thirdly, laminated logs, also referred to as an engineered log, is similar to a glued laminated beam and milled into a log profile.  This log, whose technology has been proven for decades, is composed of layers of wood pieces, usually 1-1/2 inches thick, glued together and milled into the shape of a common log profile.  Since the thickness of the wood pieces are industry standard, uniform drying is understood, repeatable, and easily accomplished in a dry kiln.  Like plywood, the laminated layers counteract each other to produce a dimensionally stable and stronger product.  As a result laminated logs, produced in accordance with laminated wood industry standards (ANSI A 190.1 and AITC 117) do not require any additional design, method nor any mechanism to combat settling typically required in log home construction.  Some are opposed to the glue lines seen at the homes’ intersecting corners however these are commonly hidden by using vertical log or post corners, additional sanding/sealing, or capping corners.

Economically, an engineered log will typically cost more upfront (0-15% more in terms of total home price) than an industry standard milled log but ultimately will be less expensive over the life of the home.  This is due to a number of specific factors but largely due to savings in initial home construction  (contrary to modifications necessary for a settling home), home maintenance, and overall satisfaction. 
           
The result of Settling is the most significant and yet most unpredictable factor to consider when choosing both the log supplier and log home builder.  Unfortunately the affects caused by settling are not completely manifested until you have lived in the home for months/years and your suppliers/builders are long gone.   If the potential dimensional changes of logs are not properly addressed in the construction of the log house, bad things can happen.  Air leaks between logs, windows crack, doors don’t close, cabinets bind, pipes break, roofs leak, floors swell, beams break, and caulking will become your second job.  Many log-home manufactures and contractors are well versed in the design nuances of log settling and have successful and even patented methods of dealing with it.  CAUTION: Even though the system is patented or unique, it may merely be a complicated solution to a simple problem—in other words, a marketing gimmick.  All log home things considered, laminated logs may very well be the best answer to the inherent natural log home issues created by settling.

 

Reference:
Alex Charvat, Alexander Structures, LLC, Conifer, CO.