The Macon Telegraph
Shrinkage and Settling in Log Walls
One of the great mysteries about building a log home is understanding settling in the log wall system. This phenomenon is present throughout all types of construction, from the foundation in the ground, all the way to the top of the home.
Settling in log walls is primarily a result of the slow drying of the logs causing reduced diameter as the lower moisture level drops and the home acclimates to its surroundings. A small diameter log of mostly sapwood will shrink more than a large diameter log that has had the outer sapwood layer removed, leaving a timber of heartwood. Location also has a lot to do with shrinkage because of the relative humidity. A log home in the low humidity of Colorado will shrink and settle more than one in the high humidity of Georgia.
To illustrate this, Ill use an Appalachian style log 6 inches thick with a height of 12 inches and a standard chinking space between the logs of 4inches. Settling is always cumulative as you go up a log wall. Allowing for 3/16" of shrinkage per log, you will need 6 courses of logs and 6 courses of chinking for an 8-4" wall and will need to allow approximately 1" of settling space above before installing interior frame walls or exterior doors. Normal size windows will not require as much settling space above them as a door. Remember, settling is cumulative and a window may only require the space of 3 logs and that translates into 9/16" of settling allowance.
To allow for settling and make building the home easy, better log home manufacturers have included systems to accelerate settling and allow the natural settling to occur without affecting windows, doors, interior walls, or porches attached to the home. There are three important parts of a settling accommodation system. A sliding jamb installed at any opening in the log wall keeps the logs perfectly straight while permitting them to settle, windows and doors are actually attached to the jamb itself, not the log. The result is that the movement of the logs does not affect the window or door and the window or door does not interfere with settling.
Through-bolts that run vertically inside holes drilled precisely in line keep the log walls perfectly straight and accelerate settling by the use of extremely strong compression springs. The steel through-bolts start at the sill log and go all the way to the top log at intervals of 6 feet or so around the home, where the compression springs are tightened down. In earthquake zones and high wind areas, these can be attached to the foundation itself.
Vertical posts cannot settle naturally, they require a method to allow for adjustment as the home settles. This is accomplished by using a screw jack built into the bottom of each post. A hole is drilled into the bottom of the post and a steel plate with a hole in it is centered over the hole on the post. A screw jack, consisting of a steel plate with a 1" diameter by 6" tall threaded rod welded to it, is bolted to the floor where a large nut and washer are threaded onto the rod. The post is then slid down over the threaded rod until it contacts the nut and washer. By turning the nut, the post can be let down as the settling of the home requires. Allowing for settling in a log home is necessary, the newer systems in place now make it easy to accomplish.
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