framingjoistsstructural
I have pre made truss ceiling joist in the garage. The joists are 2x4x21'.
I want to use space above the joists for storage. I want to sister a 2x6x21' or 2x8x21' next to 2x4x21' is this ok?
If it is ok how do I do this: nails, bolts? Any other recommendations? Or should I put the new lumber in between existing joists?
None of the three options you've shown are likely to add substantial strength or stiffness to your floor. To add either, you'll need a single, continuous additional member to span the majority of the joist length.
The blocking is probably not critical, especially if you're doubling the joists for most of their length. It's typically installed to prevent deflection, and that's not a significant concern where you have doubled joists.
My suggestion would be to remove blocking from every second joist space and double for 80% of the joist's length in those spaces. The remaining blocking will serve to stabilize all joists.
If the original joists are sound (not showing rot), compression is probably not a concern. Even a tub of water doesn't add up to enough weight to compress healthy lumber.
Regarding your comments on the original ideas, you will see some gains if 1) you fit your additional blocking extremely well, and 2) you fasten it all together with screws and construction adhesive. The result will essentially be a load-bridging honeycomb that will improve rigidity and strength. However, the flooring already provides substantial load-sharing, so gains will be minimal.
The primary reason that it's not widely hailed is that it's a very putzy, time-intensive plan that's ultimately inferior to a proper joist sistering. You'll spend more time cutting and fitting all those blocks than you would tearing out half the blocking and slapping new lumber alongside the joists.
Ended up creating a bridge under the subfloor gap using a 2x8 that sits on a 2x4 sistered to the nearest joist. I used construction adhesive and screws to attach both subfloors to the bridge. There is no longer any bounce in the floor.
Best Answer
First you need to determine the weight of the items you plan to store. If the trusses were designed under the IBC/IRC or the UBC (and maybe the BOCA and SBCCI) codes, then they should of been designed with a 10 psf live load on the bottom chord. If you keep below this loading you should be okay, but you might want to check out the code requirements for your building. 1/2" plywood will add a 1.5 psf load. A 2x10 at 24" o.c. will add 2 psf (but a 2x4 will add .75 psf) to the ceiling. A 2x12 at 24" o.c. will add 2.5 psf. Subtract the plywood and add-on from the 10 psf to get what load you can add. The safe bet is to consult with a structural engineer familiar with your truss assembly. Could prove to be the lesser alternative to a truss collapse.