Log homes. They spark the imagination with images of pioneers, ax in hand, clearing the land and using the very trees they fell to craft the DIY cabin of their dreams. As romantic as that notion is, in the 21st century, things are a bit different.
So it often comes as a shock to builders and buyers alike that when we’re talking about modern log home construction, there’s a standard – a code – by which they must be built. And that’s a good thing. The codes found in the ICC 400 are in place to help you build a quality house that everyone is happy with and proud of.
What is the ICC 400?
The 2012 ICC Standard on the Design and Construction of Log Structures (ICC 400-2012) was first published in December 2011 by the International Codes Council (ICC). It is the only document regarding log structures that has been approved by the American National Standard Institute (ANSI) as a consensus standard. This means that the ICC 400 is applicable to all "types of construction whose primary structural elements are formed by a system of logs." All proprietary methods and materials of construction must demonstrate compliance with sections of the standard as required by the jurisdiction having authority. The standard applies to all new log construction with the following scope:
"This standard establishes the minimum requirements for log structures to safeguard the public health, safety and welfare through structural, thermal, and settling provisions."
ICC 400 consists of five chapters (1-Administrative Provisions, 2-Definitions, 3-General Requirements, 4-Structural Provisions and 5-Referenced Standards). It is referenced in other 2012 ICC Codes (IBC-building, IRC-residential, IECC-energy conservation), therefore adoption of these codes in a jurisdiction includes adoption of ICC 400. It is important to note that non-log portions of the building (e.g., foundation, roofing, plumbing, mechanical, electrical, etc.) must comply with applicable adopted codes.
The National Association of Home Builders’ Log Homes Council was a key participant in the development of the ICC 400. It addresses many of the unique attributes of log home construction that diverge from the codes and standards written for non-log structures, such as:
All logs used in a structural capacity are required to be visually stress graded. Such grading must be performed under the auspices of an accredited grading agency. Two agencies are accredited as rules-writing grading agencies and have provided their design values for inclusion in the standard. Therefore, ICC 400 gives additional value to design professionals to engineer log structures rather than adapting data that is published on other structural wood products. Log grading programs are tasked by ICC 400 to certify moisture content of any log product that claims to be dried to any extent.
Prior to ICC 400, log wall systems were regularly challenged by code officials where fire resistive construction was required. ICC 400 sets a prescriptive requirement that a log wall used for 1-hour fire separation have a minimum dimension of 6" at the narrowest width of the log profile. For other fire resistance, the log wall can follow calculations in the AF&PA National Design Specification (NDS) for Wood Construction, Chapter 16 or tested per ASTM E119 by an accredited laboratory.
Solid wood walls have a dynamic relationship that tempers the temperature and relative humidity of the interior climate. They do not have tremendous R-values, a static measure of heat transfer using standards developed to measure insulation products. Closely tied to the IECC, log walls benefit as a mass wall and two methods are provided to facilitate compliance with the energy code - a prescriptive U-factor for logs of particular wood species and average log width or a prescriptive minimum requirement for the overall thermal envelope. Additionally, ICC 400 provides a calculation and test method to demonstrate thermal performance.
In the 2012 edition, the forecasted requirements to minimize air exchange rates (e.g., verified via blower door testing), Section 306 was added to provide guidance on infiltration. Section 305 Thermal Envelope and 306 combine to address energy conservation.
The development of ICC 400-2012 permitted the list above to be expanded to include allowances for settling and maximizing durability.
Provision for Settling
ICC 400-2012 expanded the evaluation of log structures to include provisions for settling, which encompasses log grade, moisture content and shape. It governs how joints are managed with sealant systems and establishes minimum requirements for how the change in log wall height is accommodated in other aspects of the building.
Wall Protection Using Roof Projection
Roof overhang minimum requirements are designed to minimize repeated wetting of lower log courses that generate deteriorization of the finish and wood surface. Options are presented to eliminate splash back on the wall from lower horizontal surfaces (e.g., a porch roof, balcony, deck, or any individual log member). The extension of the roof overhang shall be measured horizontally from the face of the exterior wall to the drip line at the edge of the overhang.
By Rob Pickett, Rob Pickett & Associates
Reprinted from The National Association of Home Builders