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Question DetailsAsked on 5/15/2015

The tear out in my bath/shower exposed subfloor damage and one of the floor joist. how should repair be done?

I am having my shower redone. The tear out exposed subfloor damage and one of the floor joist. The joist is a 4"x6" beam. In two places the rot is at least 1.5" deep. Widest across the beam is is one half to almost the 4" witdth on the beam. They are plan inning to sister pressure treated boards. Treat with chemical and not replace the beam. Is this the best practice to repair?

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Probably not best practice, but common. Right way would be to cut out all the deteriorated wood as a rectangle - up to and including an entire piece of the beam if necessaruy to get all the rot, treat the beam in that area with a rot preventative, replace with good treated wood (as a whole section if needed, or as a glued and nailed/bolted in repair to bring the damaged beam up to dimension, then sister it (with properly designed sistering or alternatively rust-resistant painted steel plating if needed for load) with proper overlap and fasteners. And of course supporting it from below with stulls (temporary posts) while doing this.

Best practice would usually be total replacement of the bad beam, but that involves a lot more opening up of the floor or underyling ceiling, and having to resecure the flooring attached to it, so the sistering method if commonly used. However, to work right, the length of the sistered wood (plywood or joists) and the fastener pattern has to be designed, not just guessed at - so either using a standard design table like from one of the American Wood Council manuals, or designed by a structural engineer. Typically would involve overlapping the sisters about 3-5 feet to each side of the bad zone and fastening with through carriage bolts or a tight pattern with large framing nails or screws. A typical installation for this size beam might need 10-20 bolts or about 30-40 nails all told, both ends combined - not just a couple here and there, because it is necessary to transfer the load from the bad beam to the sisters through the damaged area, then back to the bad beam again - and the sisters have to be fastened tight enough that they are in full friction with the original beam so they deflect as one - hence, usually bolts or wrap-around metal splice plates with large nails are used for this.

Answered 5 years ago by LCD

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