I-Joist Filler Blocks by John Bouldin

Let’s review the importance of filler blocks in cases where a perpendicular joist attaches to the side of a doubled I-joist via a joist hanger. This common connection is often found at headers around stairways and around the opening for a masonry fireplace. When two I-joists are specified to carry the displaced load around this type of opening, the intent is for the two I-joists to act as a single beam. Filler blocks are required to tie the webs together so that there is a sharing of the load between the two I-joists.

A few years ago, truss manufacturers re-designed I-joists to have wider and slightly shallower flanges. This requires a full 2 X member plus a .5” OSB or plywood panel to be used as filler blocking. If the .5” panel is left out, the 2 X stock alone will not properly secure the webs together, and the closest I-joist to the connection will bear a disproportionate load. This is especially a problem when face-mounted hangers are used, and can result in unacceptable deflection in the closest member. The net effect can result in a header that does not perform up to specifications.

For more information about I-joists and the required blocking, check out the website of the Wood I-joist Manufacturers Association (WIJMA) at www.i-joist.org.

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