Plan to shore up floor joists may not pay off — SFGate

Plan to shore up floor joists may not pay off - SFGate

Plan to shore up floor joists may not pay off

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Q: I am strengthening my house in the crawl space by putting oriented strand board (OSB) shear panels on the cripple wall from the mudsill to the floor joists. I am wondering if the same application would work horizontally, by nailing or screwing panels to the bottoms of the floor joists. I would do this at the corners of the house, and «sprinkle» them throughout the field of floor joists.

I think the floor-joist spacing may be too wide, so I thought maybe I could do that shear scheme.

My reasoning is that in a severe quake, that a «right-angled» structure might become a parallelogram. It seems it would help stiffen the joists and corners against flexing against and away from each other and thus guard the understructure against folding and crumpling.

Is there a good reason not to try this?

A: First off, kudos to you for installing shear panels on the cripple walls, the short wall between the concrete foundation and the floor structure. Make sure to connect the mudsill, studs and top plates so the panel forms a monolith and resists being racked in a quake. Also remember to drill ventilation holes in the panels at the top and bottom of each stud bay. As far as nailing panels to the bottom of your floor joists, we can’t think of a good reason not to do it — other than it’s a lot of work and will take a fair amount of time. But we also wonder how effective such a project would be.

Assuming you have a plywood or OSB subfloor, there already is shear protection for the floor joists. If you live in a relatively newer house, (around 1990 forward) your subfloor is probably affixed with construction adhesive and nails. This process creates a monolith that is highly resistant to racking — or as you say «folding and crumbling.»

If you live in an older home, the subfloor may be constructed of boards. The boards are most likely placed diagonally across the floor joists. Diagonal placement provides protection against racking and allows the finished flooring to be run in any direction. This is important if the finished flooring is hardwood.

Joist spacing depends on the width of the floor joists combined with the thickness of the subfloor and the distance to be spanned. Floor joists are usually set either 16 or 24 inches on center. But 36 inches on center is OK with a subfloor of 1 1/2-inch tongue-and-groove boards. If the floor joists are conventional lumber they should have blocking or bridging every 8 feet or so to prevent the joist from rolling.

Blocking is a solid piece of lumber nailed perpendicular to the run of the floor joists. Bridging consists of two pieces of wood, usually 1-by-4s, nailed diagonally between two floor joists. One end of the first piece is nailed to the top of the joist where it intersects with the subfloor. The other end of that piece is nailed to the bottom of the adjacent floor joist. The second piece is nailed to the joists in reverse order. When viewed from the side it forms a cross. The purpose of blocking and bridging is to prevent the joists from warping.

This being said, we can’t see a compelling reason not to do as you suggest. You’ll do no harm, but we can’t say that this will do much good either. If the floor framing system is like we described, it won’t turn into a parallelogram in the next quake. Seems to us if the quake were that strong, shifting of the floor would be a minor problem, considering that the entire house might collapse.

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