Checking

Depending on wood species and drying conditions, logs shrink 1.5 to 2 times as much across their width as they do in length. Since most shrinking occurs across the diameter of the tree, most cracking occurs longitudinally, along the grain.

Wood dries from the outside to the inside, meaning that wood cells nearest the surface of a log will lose bound water long before bound water is gone from the log core.

Because wood near the log surface is shrinking and wood in the core is not, stresses are created in surface wood which are relieved by cracking. In log homes, this cracking is called checking.

Logs typically develop a single, primary check, along with other smaller secondary checks. The location of this main check can be controlled by kerfing, which is cutting a slot, much like establishing control joints in concrete and serving the same purpose.

Although the primary check may penetrate 50% of a log diameter, this check is not a structural concern. Logs will retain most of their strength both in compression (used vertically as a post) and in tension (used horizontally, bending).

Logs in the photo at the above right have had a kerf cut along the bottom. Logs in the photo to the left have been allowed to check naturally.

Logs in the photo to the right have been kerfed long the tops of the logs. Kerfing along the bottom of the log can cause more settling than top kerfing because kerfing in the long groove at the bottom of the log will cause the long groove to open as the log checks.

As the long groove in the bottom of a log becomes wider that log slides down further over the log in the round below.

If an 8 ft. tall wall is constructed of 16 6-inch logs and each log settles 1/16th inch as the log groove opens, kerfing the bottom instead of the top will have created 1" of settling in the wall.

The logs in the photo to the right have been top-kerfed. If this were an exterior corner these primary checks would catch and possibly trap water and the recommendation would be for filling with an appropriate sealant.

Checking on the upper surface of logs exposed to weather will trap moisture from rain and snowmelt, In addition to freeze damage, this trapped moisture will cause wood to decay. Check decay is often not visible until it has done considerable damage.

INSPECTION CONSIDERATIONS

Inspectors should recommend filling any checks on upper surfaces of logs which are ¼-inch wide or larger with an appropriate sealant. Filling checks smaller than ¼-inch is not recommended because once applied, the sealant becomes a permanent maintenance item, adding to the total maintenance load without providing much benefit.

Advanced decay from unfilled checks

Filling Checks

Checks should be filled using backer rod (compressable foam bead) to provide an adhesive break. This will leave the sealant attaced only at the two check outer edges and will allow the sealant to expand and contract effectively. This condition will be difficult to confirm and is shown here for educational purposes.

Courtesy of Permachink Inc.

Contracted chinking                                 Expanded chinking

All photos not cited are courtesy of Kenton Shepard