Continuously spanning joists across an intermediate support provide the most efficient framing and best floor performance. But there are scenarios where the joists on either side of a bearing must end over that bearing. Spans where double joists are needed on one side of the bearing, or where a different on center joist spacing is desired for one side are two such instances.
When these situations occur, the question arises of whether to butt the joist ends against each other or to lap the ends beside each other over the bearing below. The choice of detail can lead to blocking installation irregularities and potential performance problems.
Generally, opposing joist members should butt against each other with their ends centered over the bearing below. Assuming a 3 ½″ wide typical bearing, this provides the 1 ¾″ minimum bearing length each TJI joist requires for end bearing. Then the required blocking panels can be installed along the bearing, to brace the top flanges of the abutting joists.
Lapping the joists should only be contemplated if one of the opposing joist spans requires a longer bearing length than can be provided when centering the joist ends on the bearing. Lapping the joist ends requires the floor decking panels to be offset at the lap since the joists are now offset from one side of bearing to the other.
Should the joists supplied be longer than truly needed, lapping allows framers to install them without taking the time to cut to exact length (running them ‘long’ over the bearing). This is poor field practice and effectively creates a short cantilevered end which can create uplift issues (seen as high spots) in the finished floor above.
Weyerhaeuser’s “Floor Framing Standards” document (#9030) shows several blocking conditions for joists butting above a bearing (see below). Scenarios such as different joist series with same spacing may require two rows of blocking panels. Other cases, like opposing joists with different spacings or double joists abutting single joists may only need one row of blocking panels. The blocking is needed to temporarily stabilize the joists until decking is attached, so face/toe-nailing the blocking into the joist ends is generally effective.
Additional joist details can be found in the TJI Specifier’s Guide (TJ-4000).