Settling
All homes experience some initial settling during and after construction. Log homes typically experience substantially more settling than conventionally-framed homes.
Settling is the loss of height in wall logs over time and may amount to up to ?-inch per foot of wall height in some handcrafted log homes.
The principal causes of settling are:
- Shrinkage of log diameter as the logs dry to a stable condition. Reaching a stable condition depends on logs reaching Equilibrium Moisture Content (EMC) with the homesite environment. EMC is reached when the log average moisture content is equal to the relative humidity of the homesite. The amount of time it takes for a home to reach EMC depends upon the amount of moisture in the logs when construction first begins, and the humidity levels at the homesite. Shrinkage of log diameter typically causes the greatest amount of settling.
- Wood fiber compression: over time, the weight of the structure will compress wood fibers, causing the wall logs to settle. Compression causes less settling than shrinkage.
- Spreading of long grooves in handcrafted homes and the kerfs cut into long grooves to control checking may cause additional settling.
Log Shrinkage
The drying process is the same for both standing dead trees and logs. As soon as a tree dies or is cut it starts the long process of losing the water trapped within the trunk, which is stored in two ways:
- Free water is water stored in the cavities inside wood cells.
- Bound water is water stored within- and chemically bound to- the walls of wood cells.
Free Water
As a log begins to dry, liquid water inside wood cells (free water) slowly turns to vapor and evaporates. This moisture evaporates both outwardly to the log surface, and longitudinally from the end grain. Logs lose (and absorb) moisture about 15 times faster through the end grain than through the sides of the log. Loss of free water does not cause shrinkage.
Bound Water
When all free water is gone but the cell walls still contain all the moisture they can hold, the log is said to have reach its Fiber Saturation Point (FSP). FSP varies among wood species from 25% to 30% moisture content. No shrinking takes place in logs until wood cells have reached FSP. All free water must be gone from cells before shrinking starts.
The cell walls of wood cells have multiple layers and are capable of holding a significant amount of water (bound water). The cell walls begin to lose bound water after all free water is gone from the cell cavities. As cells lose bound water they partially collapse. It’s this collapse of wood cells which causes wood to shrink
Dry Logs
Logs will stop losing water and shrinking when they reach equilibrium moisture content
(EMC) with the homesite. Logs with a moisture content equal to or less than 19% are termed “dry logs”. Although a log with 19% moisture is a ?dry? log and one with 20 % moisture is a ?green? one, these two logs will obviously differ very little in the amount of water they contain, so the terms “dry” and “green” and the establishing of 19% as the point of difference are industry standards that provide as a point of reference.
Many modern, preassembled handcrafted homes are built from standing dead trees.
Kiln-dried logs are available but are expensive.
The inspection concerns with log shrinkage and settling are:
- Doors and windows may bind or be damaged. Extra height should be allowed at these openings to accommodate settling.
- Parts of a home like porch roofs, lofts, and interior walkways may settle unevenly, so that they are no longer level or flat.
- Gabled ends may shrink at a rate different from standard wall logs. This condition can create gaps where the roof structure meets the top wall logs. For this reason, in log homes, gabled ends are sometimes conventionally framed. Conventional gabled end framing is sometimes covered with log veneer to match the appearance of the walls.
- Conventionally framed interior partition walls must have a space above or below each wall and where they attach to exterior log walls, methods must be used that will allow settling of the log wall without distorting or damaging the partition wall.
- Plumbing pipes or electrical conduit must be flexible or must be designed with a slip joint to avoid damage.
- Staircases and landings that are attached to exterior walls will be affected by shrinkage.
.Methods, systems and components designed to accommodate settling and resist seismic activity are often not readily visible at the time of an inspection and the Inspector should disclaim responsibility for identifying these systems or components and any problems or defects resulting from their installation or neglect.
Measuring Log Moisture Content
log moisture content cannot be measured with a pin moisture meter at the log surface. It would need to be measured with probes at a depth approximately 1/3 of the log diameter. This would give a rough idea of the log moisture content.
Many modern, preassembled handcrafted homes are built from standing dead trees.
Kiln-dried logs are available but are expensive.
Shrinkage/Settling Concerns
The inspection concerns with log shrinkage and settling are:
Doors and windows may bind or be damaged. Extra height should be allowed at these openings to accommodate settling.
Parts of a home like porch roofs, lofts, and interior walkways may settle unevenly, so that they are no longer level or flat.
- Gabled ends may shrink at a rate different from standard wall logs. This condition can create gaps where the roof structure meets the top wall logs. For this reason, in log homes, gabled ends are sometimes conventionally framed. Conventional gabled end framing is sometimes covered with log veneer to match the appearance of the walls.
- Conventionally framed interior partition walls must have a space above or below each wall and where they attach to exterior log walls, methods must be used that will allow settling of the log wall without distorting or damaging the partition wall.
- Plumbing pipes or electrical conduit must be flexible or must be designed with a slip joint to avoid damage.
- Staircases and landings that are attached to exterior walls will be affected by shrinkage.
Methods, systems and components designed to accommodate settling and resist seismic activity are often not always readily visible at the time of an inspection and the Inspector should disclaim responsibility for identifying these systems or components and any problems or defects resulting from their installation or neglect.