We have a room undergoing renovation - we took up the carpet and the disintegrating 5/8" particle board that was under it to prepare for a new hardwood floor. The 1/2" plywood subfloor we were left with is useless. Between the water damaged areas and the broken areas we have plenty of opportunities for a quick ride right through to the basement.

We can't just put another 1/2" or 3/4" plywood or OSB subfloor over it because with 3/4 hardwood on top of that the floor would be a big step up from adjoining rooms. (not to mention the squeeks we would likely have).

We need to take up the 1/2" plywood and put down a new proper subfloor right on the joists.

Two problems I need help with:

1. The 1/2" plywood is under the walls since the house was "decked" w/plywood before the walls were ever in place. How can I cut very close to the bottom plate of the wall to remove as much 1/2" as possible? Is there a special saw other than a typical circular saw?

2. The floor joists run parallel to the 2 longest walls. The walls sit over joists. If I cut out the 1/2" just along the walls, leaving whatever is sandwiched under the walls intact, the new sub floor will hang in air unsupported along those 2 walls. Is there a technique or something to support the new subfloor from hanging (which is obviously no good)?

All help is very appreciated!

 

Get your new plywood first of all (3/4" T&G), and some 2x6, 2x4 or whatever you can fit to support the new plywood edges where they don't centre on existing joists. This blocking is cut very slightly longer than the space between joists, hammered into position, and nailed fast however you can. If you guess the joists are uneven in height, get some more 2x4s to clap on the sides of low joists (this is better than shimming the joist tops).

Cut off the old ply, all around with a circular saw. So your new ply will not go right up to the bottom plate or even the wall covering, but neither will the new floor boards (remember why we have baseboards) - so it shouldn't matter as long as it supports every board well, and the flooring nails hit it. Check with the flooring people as they may know something I don't. You can use a hand saw to get into corners where your circular saw stops, or use it to cut as close as the wall covering if you wish.

Flop down new sheets as you pry up old ones, so you have something to walk on. Use a straight 2x4 variously to check the joist heights. You may have to plane down a joist if it's an odd one up, otherwise raise the dips by nailing 2x4s to the offending joists, slightly higher. Lay the new sheets perpendicular to the joists, shortening them as necessary so their ends centre on wood. They will lock together along their sides but should have blocking there anyway.

To fasten a subfloor I prefer to use gazillions of galvanised spiral nails (galvanised holds better, if it doesn't bend when you hammer it), but this is unusual.

If there is a finished ceiling under these joists remember now is the time to run wires even ones you may not want now. Just in case. It will cost you little at this stage. I like to take digital photos of all cavities before closing them off, for future reference.

 

Thanks Kobuchi for the very thorough information! One thing I need clarified though:

I think you are saying that the blocking is placed perpendicularly to the joists and fitted across the span (approx. 13-1/2" long). Or are you saying to nail it "siamesed" along the outer joists? If perpendicular, would that be standard 16" on center spacing between each?

 

First, you may need to "Siamese" some old joists that are lower than their neighbours, with wood nailed on that much higher.

Next, at 90 degrees to these joists, you will likely need to nail in spans to support plywood edges running across the joists, not along them. All plywood edges should be supported. It's unlikely the new plywood will fall just where the old plywood did. Lay the plywood lengths across the joists and trim the ends to centre on joists as necessary. This orientation because the tongue and groove plywood edges run only along the long sides; avoid trimming that off.

I can only guess the joist spacing in this house. Using the above approach will take any irregularities in stride.

 

I would block under that spanned wall you were taliking about!!!

 

Thanks again for all the info!

Last issues: the floor joists are 2x10's 16" on center - blocking with 2x4's or 2x6's rather than 2x10's is definitely easier to deal with for a lot of reasons, but is that sufficiently strong? What spacing should they have?

 

2x10 joists, you're good. I've walked on floors with just 2x4 joists, 24" inch centre (!) in finished attics... with 3/4" ply and some layers of what-have-you the bounce is hardly noticeable (not that I recommend this). Those 2x10 joists don't need any help.

The blocking is just to support plywood edges which don't fall along a 2x10 joist. Much of its value is in keeping both edges of abutting sheets secured to each other, effectively, at the same height, and sharing load between them. You could join adjacent plywood edges underneath with screws through old scraps *which don't even touch the joists* and gain most of this effect (just for illustration). Of course everything will benefit if the blocking is nailed to the joists - the ply will be even better supported and the joists will be better kept in parallel, which doesn't hurt.

So, as far as plywood stability is concerned, if it's laid over 2x10 @16", a 2x4 block at every edge is all we need. Blocks perpendicular to the joists may be spaced every 48", where plywood ends or meets, no more.

I think you'd also have a plywood edge running parallel to the joists and between them, floating near the wall. For this I think 2x4 will be fine but I'm unsure how to set that up because it depends on which way the hardwood flooring will run and what the floorers expect to nail into - ply, or (better) through the ply and into solid wood. If they run boards across the joists and nail them every 16" through plywood and into joist (ideally), then I guess some 4' long 2x4 "joists" each pre-built like the letter "I" would be the way I'd do it. The new "joist" should be about 3" - 6" from the hardwood flooring ends, where floorers want to put a nail. If the flooring runs parallel to the joists, then all you have to worry about is supporting the plywood - in that case I would just nail 2x4s every 16" between those two joists that run beside and under the wall.

You could block with 2x6, 2x8, 2x10, and this would help keep the joists in alignment, but as it's a settled house I guess the joists have already warped all they ever will. Wider blocking will help divide load between joists somewhat too, but for 3/4" ply on 2x10... that's overkill. The only difference you'll notice is it costs more, is harder to put in, and it gets in the way of pipes, ducts, and wire fishing between joists.

Don't forget to mark joist locations on the wall before you put the plywood down - so you know where the joists "were".

 

Kobuchi - thanks very much for all the help!! I think the only way you can help me more is to come do the job...