Dear Inspector,

This month I would like to publish a few answers to questions about I-Joist framing that I have received over the past few weeks. I figured the answers might assist others as well.

Question: What is the difference between a "squash block" and a "web stiffener" in I-Joist (some call them TJI) engineered truss floor framing?

Answer: Squash blocks are installed in I-joist framing where there are point loads from above, such as posts under headers, posts under roof or other loads, etc. Squash blocks are usually 2x4 (many times multiple) blocks cut about 1/16-inch longer than the depth of the I-Joist (for settlement/shrinkage). Squash blocks extend the full height of the joist from flange to flange. Web stiffeners are usually required over less than 5" wide intermediate bearing points and some other locations. Web stiffeners help reinforce the plywood or oriented strand board webbing in the truss where it is stressed at a bearing point to prevent deformation. Web stiffeners should be attached with at least three clinched nails and extend from the top of the bottom flange to about 1/8-inch from the bottom of the top flange. The size of the material for the web stiffener depends upon the style of joist. Typically at least ½-inch by 3-inch material is required.
Web stiffeners are different than "backer" or "filler" blocks. These backer blocks are typically installed at hangers, and are larger to accommodate at least ten clinched nails.

I would recommend all inspectors obtain installation instructions for I-joists, particularly if you are inspecting newer homes and/or new construction.

A good field reference is the Truss Joist Framer's Pocket Guide to the Frameworks ® Building System by Weyerhaeuser (check out www.trusjoist.com).

Mike Casey
Kaplan Professional Schools
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