24" OC with unconventional subfloor?


Old 06-07-08, 06:27 PM
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Join Date: May 2008
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24" OC with unconventional subfloor?

I recently purchased a house; an A-frame built in the 60s with a two-story addition added during the mid-to-late 70s. The entire structure seems to use the same joist and flooring scheme:

24" OC 2x6 joists covered with what appears to be 1/2-inch or 5/8ths- inch plywood, and then a staggered layer of 5/8ths-inch "versaboard" particleboard.

As part of my pre-move-in fixup, I've ripped up the top layer of subflooring in the bathrooms on account of water damage. In some places, particularly around the toilet in one bathroom and the tub in the other bathroom, the rot extended down into the plywood. I want to cut out these sections and screw down new pressure-treated lumber to replace it, but in both cases, the rot is against an outside or interior wall frame immediately above joist support, and the 24" OC is complicating this further.

I'm presently considering cutting out the rotten flooring and nailing in strategically-placed additional joists, perpendicular to the existing 24" OC joists, possibly using metal so-called 'joist hangers' or 'slant nail hangers' to provide additional support, particularly below the toilets and bathtub.

Finally, I (carefully) ripped out the particle-board, exposing the plywood in one of the living rooms on account of the particleboard being covered with very old 12" vinyl tile and mastic. I'm thinking about putting down plank hardwood here. Should I build up with another sheet of plywood, or would the hardwood planks be OK to put over the 24" OC joists and ~5/8" plywood subfloor directly?

Thoughts? Would more info be helpful?

Last edited by chromal; 06-07-08 at 06:32 PM. Reason: correction
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Old 06-08-08, 01:04 PM
mikeTN's Avatar
Join Date: Dec 2007
Location: Nashville
Posts: 635

24" OC for floor joist is not suitable. and ESPECIALLY 2x6!!! nowhere NEAR enough support for a one story let alone two.
someone was trying to save money!!! on a two story i want 2x10 16" OC minimum. second floor 2x8,16" OC. nothing less for me!!!

particle board for me is a wast of money. it is used a lot in kit. and other applications. a little water and it is done! osb is a good substitute i'm told. too many new products to go with particle board.

i'm sure you will get more(and better) advice. consider the support before the finishing. it needs to hold up and support the weight. good luck!
Old 06-08-08, 10:32 PM
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Join Date: May 2008
Posts: 74
MikeTN, yeah, I tend to feel the same way about the OC. I think they got around the 2x6 limitation by placing perpendicular paired 2x12's at 1/3rd and 2/3rds across the 2x6 runs, so there are beams supporting joists below the 1st and 2nd floor. I suppose I should have mentioned that.

But, and you'll love this, the flooring arrangement was the very same in the bathrooms; plywood, particle board, foam, carpet. x_x Naturally the particle board is hand-friable below the toilet and tub after two or three decades. Over the existing plywood subfloor, I'll probably put down pressure-treated plywood or OSB, cement backerboard, and ceramic tile. c_c
Old 06-12-08, 09:19 AM
Join Date: Oct 2004
Location: New Jersey
Posts: 1,132
Hold on there:

I'll probably put down pressure-treated plywood
You never want anything pressure treated going under a tiled floor.

There are methods for tile over 24" oc framing and believe it or not, with support beams at 1/3 and 2/3 the span, your joists can span 6' between beams and be a sufficient supporting structure for tile. Now the subfloor and underlayment requirements before a tile membrane or cbu are what needs to be taken care of before the span between the joists can support ceramic.

Why not post this part of your project over in flooring under ceramic.
Old 06-12-08, 10:22 AM
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Join Date: May 2008
Posts: 74
I'll plan on posting over in the tiling forum for certain as I get closer.

At this point, I'm still doing build up, essentially, I used a circular saw and cut out rotten subfloor in the downstairs bathroom. It's 5/8ths sheathing on 24" OC 2x8 joists. I've done some cross-structural 2x4s under where the toilet will be, as well as along plywood edges, hung perpendicular to the floor joists using shear hangers. I'll do this with the upstairs bathrooms, as well. After that, I'll replace the 5/8ths ply, and then replace the 5/8ths? 3/4? particleboard with more ply. I'm thinking 1/2" ply and then cement backerboard over that. But, we're drifting into a conversation about tiling best continued over there.
Old 06-12-08, 09:56 PM
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Join Date: May 2008
Posts: 74
It looks like I do need to make one correction. The upstairs floor joists are 24" OC 2x8's. I'm new to some of this, and didn't realize there was such a large difference between the nominal and actual sizes of dimensional lumber...

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