Q: I subbed out a deck frame and wasn’t too thrilled with the sloppy way the crew installed the joist hangers. There are gaps—some almost 1/4 inch—between the seats on some hangers and the bottom of the joists. All the joists are flush with the top of the ledger, but I’m afraid the joists will sink down to the hanger-seat level eventually and the decking will telegraph the resulting wave. The framer offered to install shims to fill the gaps between the hanger seats and joist bottoms, but I’m wondering if there may be a problem doing so? Or should I have him pull the hangers off and install them properly?
A: David Finkenbinder, a branch engineer with Simpson Strong-Tie, responds: The bulk of a hanger’s download capacity is by way of the joist bearing on the seat of the hanger, with that force then being transferred through the steel and into the header via the hanger fasteners installed into the header face. Without full bearing, the hanger joist fasteners would still transfer some download to the header, but there would be a significant reduction in capacity from our published download values. We do not have recommendations for shimming because it’s difficult to do properly. Most materials on a jobsite that might be used for shims—such as cedar shingles or scrap lumber or decking ripped to size—have insufficient strength.
Another consideration would be the code requirements for bearing of joists and beams. Section R502.6 in the 2015 IRC calls for joists, beams, and headers to have no less than 1 1/2 inches of bearing on wood or metal, and similar wording has existed in the code for some time now. I believe the common interpretation from a building official would be that the gap you describe does not meet this section. This is also why most building departments would not accept framing angles (such as the A35Z or ML26Z) to attach joists to a header, though some might. Your framer isn’t going to like this, but the best fix is to remove the hangers that show gaps and install them properly.