Lower Wall Logs

SILL LOGS

Sill logs are the very bottom round (course) of logs. Typically, they rest on a supporting structure such as a floor assembly or foundation. To allow corner notching to work out correctly, some sill logs will be ripped down their length (visible in the photos below) to an extent whcih will vary with log diameter and taper. Good building practice requires leaving enough log to provide sufficient strength, but removing enough so that  logs have an adequately-sized flat area where they rest upon the supporting structure. The ILBA Log Building Standards call for a minimum flat area of of 4 inches.

SILL LOGS/FOUNDATION JOINT

The joint created where sill logs rest on the floor assembly or foundation is often poorly protected from moisture intrusion and air leakage(see photos below).

SILL SEAL

An good method for sealing sill logs is a moisture-impermeable, compressible foam strip (commonly called "Sill Seal") laid beneath the sill logs as they are installed. Sill Seal will help seal the gap beneath sill logs against air leakage and moisture intrusion.

OVERHANG

sill log overhang at the foundation. A drip kerf cut into the underside of the sill log will prevent moisture from running back to the foundation. Verifying the presence of a foam strip will be difficult. Foam may be visible from the interior.

FLASHING

This joint may be protected with flashing. Flashing should fit into a slot cut into the sill log exterior and extend down to cover the sill log/floor assembly joint.

SEALANT

An appropriate sealant may also be used to protect this joint. An appropriate sealant will be one designed for that purpose, and will be resistant to ultraviolet UV damage and flexible enough to absorb the difference in expansion and contraction rates of the logs and materials upon which they rest.

Inappropriate sealants will dry, shrink, crack and may allow moisture intrusion unless diligently maintained.

Identifying sealant types and commenting on their appropriate use or expected lifetime lies beyond the standards of practice of log home inspection.

PROBLEMS WITH LOWER LOGS

Logs located lower in walls are less protected by roof overhangs and closer to the ground. As a result, they’re more likely to suffer deterioration of the exterior finish, damage from impact, abrasion or insects, and decay from fungal activity than logs located higher in walls. Sill logs, as the logs lowest in the wall, are especially vulnerable.

The following are areas of special concern in lower wall logs:

• Moisture damage- Logs lower in the wall are those least protected from rain by roof overhangs. Rain driven by wind against log wall exteriors drains to the lower logs, increasing their risk of moisture-related damage.
• Splashback- Logs should have 12" clearance from grade. Water from rain or snowmelt pooling on the ground near foundations contains fungal spores. As droplets of water from rain and roof drainage strike the ground, spore-containing water is splashed onto logs, inoculating them with fungal spores and providing spores with the moisture they need to germinate. Smoother logs will shed spores more readily and allow easier cleaning and removal of fungal colonies. Logs with an intact finish will help prevent fungi from penetrating the finish coat and damaging or discoloring wood.
• UV damage from direct sunlight. UV breaks down wood cells. These damaged wood cells deteriorate the bond between wood and the finish coat, increasing the chances for moisture penetration of the finish coat. Damaged cells also provide food for various kinds of fungi and enough moisture intrusion of the finish coat will allow these fungi to become active.
• Foundation movement- As the lowest logs in the wall, the sill logs carry more weight than any other course of wall logs and support all log courses in the wall above. They're also the logs closest to the foundation. For these reasons they may be the first to reflect foundation movement or failure.
• Wood-destroying insects- logs closer to the ground are more likely to suffer damage from wood-destroying insects. The types of insects likely to be a problem will vary with region and climate. Inspectors should familiarize themselves with insect species likely to be found locally and learn to recognize evidence of infestation.
• Contact with vegetation- vegetation will hold moisture against logs and provide access for insects. Vegetation should be cut back.
• Transition areas- transition areas are areas in which dissimilar materials meet, such as where sill logs rest upon a floor assembly or foundation. Transition areas are always more vulnerable to moisture intrusion than other parts of the home because dissimilar materials have dissimilar properties such as expansion and contraction rates, texture and shape. Assembling different materials in a manner which prevents moisture intrusion may require proper flashing installation, sealant application or special building methods. In an effort to reduce costs, these seemingly-simple procedures are sometimes performed by non-professionals, often with unsatisfactory results.

A stepped foundation in which no sill logs are ripped. Also…

1. Sill logs lack flashing or a notched overhang to route runoff to the outside of the foundation. Runoff will drain behind mortar at the foundation and be trapped.
2.  Note heartwood decay of middle log indicating that fungi were present in the tree before it was chosen as a log. Sapwood typically decays first in seasoned logs of softwood species.                     

Sill logs rest on a conventional floor assembly. This home has flashing protecting the floor assembly/foundation joint but nothing protects the sill log/floor assembly joint.

This home has sealant visible where the 1/2 sill log rests on the floor assembly (covered with log veneer). Flashing or an overhang would have been a longer-lasting choice.    

These wall logs overhung the foundation until the foundation was built out with stucco and stone veneer. In the future the home will depend on sealant (none applied here) to keep moisture out of the sill log/floor assembly joint.

To protect the sill log/floor assembly joint, this home uses sill log overhang at the left side. Flashing at the right side is installed in a manner which will prevent moisture from draining behind stone tiles, but the sill log/floor assembly joint is protected by sealant only.

Mold damage from contact with snow drift (downwind side). This home has poor clearance from grade.

The long-term effects of lower log exposure can be seen in this photo of a Colorado hay barn built in 1861. No visible foundation existed to keep logs from contacting soil. The lower logs have decayed and crushed and the structure above has sagged.

Overly-long lower log extensions are less protected by roof overhangs and are more likely to suffer damage from sun, moisture and decay. 

Courtesy of Tom Tydeman, Log Options LLC

Splashback damage

INSPECTION CONCERNS

While inspecting lower wall logs, Inspectors should look for...

• 12" minimum clearance from grade
• evidence of structural failure
• connection to supporting structure (floor assembly or foundation)
  • gaps allowing air leakage or insect infiltratrion
  • method for preventing moisture intrusion
  • mechanical fasteners
• finish coat deterioration or failure
• evidence of fungal staining
• evidence of wood decay
• visible UV damage
• evidence of wood-destroying insect infestation (exit holes)
• logs in contact with vegetation