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I'm going to stiffen up my main floor joists by sistering then bridging them. In the past a friend put something in the middle, is that standard? Should I put 1/2" plywood or OSB in between?

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up vote 4 down vote accepted

There is no real reason to put a spacer between the joists when sistering them. Usually using some 1/2 plywood between dimensional lumber is to correct the thickness to vertical lumber dimensions, like on a header over a door. If you were to add a spacer, I would use plywood as OSB has much less sheer strength. Adding some plywood will add a bit more strength, but in your case, may not make a difference.

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spacer not needed, it doesn't upgrade header/beam span limits when used, just as shirlock has mentioned –  HerrBag Feb 27 '13 at 0:08

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