(Originally published in the July 2007 Housing Journal)
International Code Council Clarifies Framing Method for Monolithic Slab-on-Grade Foundations
During the recent review of the 2006 International Residential Code (IRC) and 2006 International Energy Conservation Code (IECC) prior to adoption by the state, a question came up concerning the practice of many builders to cantilever the sole plate an inch to bring the framed wall flush with the foundation insulation. The practice of cantilevering the sole plate and the wall studs above it was begun to eliminate a “bump” at the bottom of the framing, presenting a smooth, flush appearance to the ground.
However, some designers and architects felt the practice of cantilevering the sole plate and wall studs weakened the structural integrity of the framing since the foam provided no support for the outermost inch of the wall system. The 2006 IECC increases the foam requirement to two inches in many parts of New Mexico. Having a 2 X 4 wall system cantilever two inches from the concrete foundation over foam was seen as a severe compromise of the wall system. A 2 X 6 wall system would be reduced to having the structural equivalent of a 2 X 4 wall if it were cantilevered two inches.
After discussing options such as outlawing monolithic pours, reducing the energy efficiency requirement back to one-inch foam, or possibly forbidding 2 X 4 wall systems, the Code Review Committee decided to have the Construction Industries Division (CID) ask the International Code Council (ICC) for a clarification of the framing requirements in the IRC. Below are the question sent by CID and the ICC response.
I am requesting an interpretation of IRC Section R602.3.4.
Currently and in the past we have allowed for a 2×6 inch bottom sole plate to overhang over the edge of a monolithically poured slab on grade foundation to allow for the required foundation perimeter insulation to be set even with the wall sheathing. The 2×6 inch bottom sole plate bears on the slab on grade foundation a minimum of 3-1/2 inches depending on the thickness of the perimeter insulation.
The question has been brought up that IRC Section 602.3.4 indicates that the bottom sole plate must have full bearing on the slab on grade foundation to meet code requirements.
Does IRC Section 602.3.4 indicate that the studs must have full bearing on the bottom sole plate or does it indicate that the bottom sole plate must have full bearing on the slab on grade foundation?
Do we still meet code requirements in allowing for the 2×6 inch bottom sole plate to overhang over the edge of a monolithically poured slab on grade foundation to allow for the perimeter foundation insulation?
From: Larry D. Franks, PE, CBO
Senior Staff Engineer
Codes and Standards Development — International Code Council, Inc.
The IRC is a prescriptive code and as such the text and figures are the prescriptive requirements. Section R602.3 and Figure R602.3(1) require the sole plate on a monolithic slab on grade to have full bearing on the foundation. Section R602.3.4 requires the sole plate to be the same width as the stud and the stud to have full bearing on the sole plate.
Overhanging the sole plate over the edge of the monolithic slab on grade will require an engineered design in accordance with Section R301.1.3
This opinion is based on the information you have provided. We have made no independent effort to verify the accuracy of this information nor have we conducted a review beyond the scope of your question. As this opinion is only advisory, the final decision is the responsibility of the designated authority charged with the administration and enforcement of this code.
What this means for residential builders in New Mexico:
It appears 2 X 4 wall systems can no longer be cantilevered beyond the foundation wall. Homebuilders will have to find a way to deal with the “bump” where the sole plate meets the foundation insulation. Some builders in the state have been using a “Z-shape” galvanized flashing below the weep screed. Others have begun carrying this flashing down to the ground, and painted/textured it to match the wall finish.
The local building official has the option to allow cantilevering of 2 X 6 wall systems under Section R104.11. Alternative Materials, Design and Methods of Construction and Equipment, or require the wall system to be engineered under Section R301.1.3. A design professional should be consulted if the 2 X 6 wall system is to support a second story, as a 2 X 8 wall stud may be required to achieve a full six inches of support by the foundation.
CID will be issuing a memo to its field offices and local building officials soon, so check when you pull a residential permit to see what will be permissible in your jurisdiction.