Wood Flooring Gaps: The Rest of the Story

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Floor Truss Butt Joints

In my last post, I mentioned a house where a wet crawl space dried such that the beams shrank. But the beams were held in place by hurricane straps, so the beam shrinkage created a gap under the beam. In this house, it was structurally interesting, but the gaps under the beams didn't affect the floor. The outside ends of the floor trusses were set on the foundation wall, so there is no beam to shrink on that end. The result is that you can get different settlement between outside edges and interiors. The different settlement can lead to humps and dips in the floor.

There is another twist or two to the situation in this house that did affect the floor. One of the larger gaps in the floor was at a transition between the tiled foyer and the hardwood floor. In looking at this from the underside, I observed a butt joint between two floor trusses that was aligned with a joint in the subfloor panels:

Floor Truss Butt Joints

...and all that aligned with the gap in the hardwood floor.

The situation here is that the subflooring (or even hardwood flooring) gets wet after installation, causing it to expand. The expansion pushes the joint apart, since nothing is really holding it together. Then when things dry, a gap forms, as nothing can pull the joint back together. You end up with a permanent large gap at this kind of aligned joint. Building codes actually require an overlap of joists or some kind of overlapping splice. Even so, some floor system manufacturers do not require an overlapping joint or splice.

And there's more. Cracks in the two tiled floors in his house align with joints in the subflooring. I know it's not a hardwood floor, but the situation is similar: cracks aligned with joints. A wet floor system before, and a dry floor system now. Sometimes we get pieces of the story in places other than just the hardwood floor.

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