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?

  • Sometimes a plywood spacer is used when making a door or window header so that the thicknesses line up. E.g. a double 2x header with 1/2" spacer is the same thickness as a 2x4 stud wall. Maybe that's what you're thinking of? – Hank Feb 27 '15 at 1:20
  • The spacing reason could be why I saw it was done once. – RLZaleski Feb 27 '15 at 18:33

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

Both posters above are wrong. OSB has roughly double the shear strength of plywood, according to Umass Amherst and every other piece of info available easily available on the web. It also does contribute to structural strength and span limits, although there are more effective ways to increase the span strength using the same amount of cellulose.

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  • You may be overstating OSB strength "The OSB in I-joist webs is not the garden-variety OSB used for sheathing products. It has more wax and about 15 percent more resin, making it a very stable material. In fact, instead of being a cheaper substitute for plywood, I-joists made with OSB webs mark a significant improvement in the structural floor-framing industry." jlconline.com/how-to/framing/plywood-vs-osb-i-joists_o – HerrBag Feb 6 '19 at 12:45
  • Shear is different than bending strength: "Strong and stiff. Plywood’s main attribute is that it’s stiff, strong, and durable. Citing an APA technical document, Georgia Pacific says “plywood panel bending stiffness is 10% greater than OSB at equal joist spacing.” This, the company says, results in stiffer floors that are less likely to be soft and springy." builderonline.com/products/… – HerrBag Feb 6 '19 at 13:03

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