I have not done a lot of flooring, and when I have I’ve always run 4×8 sheets of ply perpendicular to the joists….but have an app that the reverse might make sense and am looking for a reality check.
Space is almost exactly 12 x 8. Joists are 9.5 inch TJI 230’s spanning ~7′ 11″; 16″ OC. Interior space where none of the walls are load-bearing, so minimal weight.
Basicall stiff as…you fill in the metaphor here.
Subfloor is to be 3/4 T&G ply….running parallel allows me to have fewer seams and those seams would be all t&G…running perp means more seams, including butt joints that don’t have T&G connections.
Thoughts?
Replies
Running paralell only allows you to tie a maximum of four joists together at a time.
Even with the joints, perpendicular is a stronger floor.
J. D. Reynolds
Home Improvements
The plywood should be installed perpendicular to the joists. However, in this case, I would not lose sleep if the plywood was installed parallel to the joists.
The room is small and I'm assuming your not putting a hot-tub or any severe loading on the floor. We are talking normal floor loads.
You may have an issue with the TJI manufacturer should there ever be an instance where the warrantee is called into play. They may decline any guarantees as the plywood was not installed according to recommendations. So you'll have to ask yourself if that is an important consideration.
Would I accept this installation method on something I designed? Most likely not. The plywood glued and screwed to the TJI act a single unit. And the manufacturer specifies direction for a reason. But based on the brief explanation of the layout, I would not think twice about it.
I could be wrong here, but I believe the layers of plywood are laminated in a specific orientation for strength. Not a big deal on walls, but could be a big deal on floors.
If you take, say, a 4' X 10" cross cut of 3/4ply and step on it, it'll break much easier than a 4' x 10" rip of the same stock. We'll sometimes use scraps of 3/4 decking as temporary stair treads on our frames. We never use a cross-cut because someone inevitably puts their foot through it with a pile of 2x4's on their shoulder. The rips last through finish.
Maybe it's my imagination, and you'll be just fine. But I'm fairly certain that the strength of the plys is specific to their orientation on the joists.
I'd stick with perpendicular.
Remember that T&G plywood isn't exactly 4' wide, so it wouldn't break on an I-joist every time. So almost all of your edges would be unsupported. Not a good situation.
Butt joints should fall over an I-joist so they're supported.
I once cut a corner, by installing a 1/2" sheet of ply parallel to the trusses on a roof. After seeing how it bowed when my roofer walked across it. I removed it and replaced it with two pieces in the right orietation.
I'd go perpendicular.
Thanks everyone; glad I asked.
Perpendicular it is.