OSB versus 2x lumber strength?

[Joe Pack]

I need to reinforce two floor joists. I can use 2x10 material, or I can use one or more layers of OSB sandwiched with construction adhesive. The OSB is more flexible put in place in 1/2" layers, then glued/screwed together and into the original joist, which makes the installation easier.

How would the on-edge strength of two layers of 1/2" OSB glued together compare to the strength of one 2x10 SPF?

[Mark Ashmeade]

Depends on the span...

I'm no civil engineer, but given the I beams in my house are made from what seems like 3/8" OSB, I'd say they're pretty strong on edge. Two plies of 1/2" would be considerably more ridgid. I think I'd still try and face the edges with 2x.

[Rod Sheridan]

Depends on the span...

I'm no civil engineer, but given the I beams in my house are made from what seems like 3/8" OSB, I'd say they're pretty strong on edge. Two plies of 1/2" would be considerably more ridgid. I think I'd still try and face the edges with 2x.

Mark, that's because the load is carried by the flanges, not the web. I wouldn't expect the OSB to have as much strength as a solid timber............Rod.

[Ryan Dyer]

Mark, that's because the load is carried by the flanges, not the web. I wouldn't expect the OSB to have as much strength as a solid timber............Rod.

Rods correct, the more material on the top and bottom provide more compression strength on the top and more pulling strength on the bottom.

[phil harold]

I would use plywood on both sides if repairing a cracked joist, glue and nails/staples (hardened screws have very little shear strength also elmers/wood glue, construction adhesives are pliable thus not as strong in this application )
if,
going for a stronger/stiffer floor I would double the floor joists to increase you load rating

[Joe Fabbri]

Joe,

I just repaired a few joist ends a couple months ago, where the joists and the sill rotted under the front door. For what I needed to do, regular lumber was the only choice, since the ends sitting on the sill were basically gone. I simply installed two half-inch carriage bolts per joist, and nailed it.

For you, if it's only cracked, then maybe the plywood is good enough. If something's rotted, I think I'd sister another joist to it with bolts and nails (maybe with construciton adhesive also).

If you're worried about getting the joist in place, you will probably have to roll it in, by rounding the opposite edges of the joist, so it can be slid into place. A handplane works well for that.

[Mark Ashmeade]

Well I did say I was no civil engineer...

[Stan Krupowies]

Rods correct, the more material on the top and bottom provide more compression strength on the top and more pulling strength on the bottom.

That's correct. The flanges (top and bottom lumber) provide bending strength while the web (the material in between) provides shear strength. That applies for both wood and steel I-beams. So especially if you are looking to reinforce a long span where bending would be great in the middle, you'd be better off going with 2x lumber.

[johnny means]

As I learned it, the height of the web is the, determines the stiffness of a beam. A 10" strip of OSB on edge will resist sagging better than a 2x10 laid flat. Of course there are many other types of stress put on structure that may not be addressed by the height of the webbing.

[Glen Butler]

Everyone is correct in one form or another here. The flanges are the supporting members in an I-joist. So the beefier they are the more they can support. But for any given series of joist, spread the flanges apart (using a taller web) and they will also get stiffer. (I knew I shouldn't have thrown out my old tables).

Ultimately to the OP, whether you should use 2x10 or OSB is really a matter of the reason you need to stiffen them up. But in any case 2x10 can be used and will be a stronger method over OSB. If you have a sagging joist the best thing to do is sister another joist to it. Filling the web in a sagging joist will be of little benefit.

[Gary Kman]

Engineered beams are marketed that look to be made like OSB. I wouldn’t expect OSB to have the same properties.
You’d be limited to doublers of only 8’ in length with OSB or plywood.
With plywood only the plys with grain longways are adding strength. It is used in framing because plywood is useful to make beams a convienient thickness.
I’ve seen carpenters using buckets of glue and zillions of nails laminating beams. Most loads are straight down bearing on the beam not applied to the sides and needing to be transfered sideways into the center of the beam.
If it is stiffness you are looking for, use lumber with a high modulus of elasticity (the ratio of load applied to resulting movement). In the following chart Southern yellow pine is about 50% stiffer than Spruce-Pine-Fir.


Grade Designation
Species
SG
Shear parallel to grain
Fv
Compression perpendicular to grain
Fc


1650f-1.5E
SPF
0.42
135
425


1650f-1.5E
HF
0.43
145
405


2400f-2.0E
SPF
0.50
170
615


2400f-2.0E
SYP
0.57
190
805