Hi Guys,

I think I know what everyone will say but just thought I'd survey the peanut gallery.

I'm going to install some 5/16" Bruce Natural Choice flooring in my Dad's house. His existing wood subfloors are a style not used for over 40 years. They consist of 4 x 6 wood girders (beams) supported by 4 x 4 posts every 6' or so. The posts are supported by concrete piers into the ground. Now these beams are spaced about 4.5 feet apart and over them is installed 2 x 6 T&G solid fir planking. The nominal size is actually 1.5" x 5". These are just spiked to the beams. There is some flex between the girders so I am going to install plywood over these. Right now, I'm thinking 3/8 but may go to 1/2". At any rate, the question is: lay the plywood parallel, at 90 degrees or diagonally? I think diagonally would be the best but also would take the longest time and cost the most (more waste). What would you do? Just wondering. Please, everyone chime in.

45º for the strength, if anything.

I have honestly never run into that subfloor situation..

Running it parralel or even perpindicular just doesnt seem like it would hold any better. So I would have to go with 45 degree.

Possibly 2 layers 45 in different directions like a 45 basket weave? Or am I just being too enginery(sp?)

Naw, makes sense. Here in CA. during the post WW11 housing boom, they were trying all kinds of ways to build these houses. Prior to WW11, the only wood subfloors were 1 x 6 diagonal over joists 16" oc. OR 3/4" x 3.5" T&G doug fir perpendicular over the joists, 16 to 20" OC. Then they came up with this bogus system, which sucks, IMO. The shallow piers sink and swell due to the expansive soil and the T&G planks were installed green and after shrinking, some of the T&G's are no longer engaged. This died out in the early 60's then they went to 1&1/8" plywood over these girders spaced 4' apart. That lasted another 25 years. It was better but still has too much flex between the girders. They tried concrete slabs but didn't know enough about re-enforcing them and they'd shift and crack and have all sorts of problems. Today, it's much different. Engineered trusses with either AdvanTech OSB or 3/4" Plytanium. No more shallow piers. Now it's all grade beams with piers at least 12' deep and 10" in diameter. And sometimes much more. Slabs are post tension and 10" thick throughout; not just at bearing points. All in all, much better. But I think I will go the diagonal route. This subfloor can use all the help it can get.

I think the diagonal would be the best choice. I have not run into a situation like this either.