Proper framing and subfloor for bath


Old 04-07-05, 01:09 PM
trance's Avatar
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Join Date: Aug 2003
Location: usa
Posts: 406
Proper framing and subfloor for bath

hello all - I'm getting ready to remodel my bathroom. It has 2x8 joists, 16" o.c. - about a 6' span for the joists.

I'm tearing out the existing subfloor - it's shot. The joists are in good shape

My question(s):

1) What should I do, if anything, to properly frame for a bathtub? (regular bathtub, not a jacuzzi or anything) There isn't any blocking between joists now.

2) What would be the proper construction for the subfloor, if I want to install tile? I'd PREFER to just use 1 layer of 3/4" plywood (easy and cheap) but I could also use 1 layer of 1-1/8" plywood or 2 layers of 3/4" plywood (with 1 recessed down to be level w/the tops of the joists - a big pain in the butt).

3) some numbskull cut a big hole through one of the joists to pass a drain pipe through. It's too close to the edge of the joist right now. (but joist is not failing-yet) Should I sister on a new joist & cut the hole at the right spot or should I just reuse the existing hole? Or should I sister on a new joist & cut back through that spot again? Seems like regardless of what I do, it will never be as strong as it once was...


Last edited by trance; 04-07-05 at 01:38 PM.
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Old 04-08-05, 03:56 AM
awesomedell's Avatar
Join Date: Mar 2003
Location: USA
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Hi trance,

As long as everything else is in good shape, your floor should be able to handle a tile or stone floor. If you're wanting to tile you need to sister up that bad joist and then lay 2 layers of 5/8" plywood, use a good grade of plywood, something rated for exterior is the best. This should be glue & screw down and don't overlap the seams.

Then you can lay a cbu backer, durock is what I generally use, this should be laid over thinset, I like Versabond, and also scerw down. When you pick up the backer board get the proper mesh tape designed for it, should be located close by in the store. Might want to pick up a pair of kneepads while you're at it The seams of the backer should be taped using thinset as well. Once that's all installed, you're ready to spread your thinset and lay some tile.

Can you post a pic of the bad joist with the cut? Could probably advise better if I can see what you're working with.
Old 04-08-05, 07:21 AM
trance's Avatar
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Join Date: Aug 2003
Location: usa
Posts: 406
Hi - thanks for the great advice - very clear.

I can't post a pic, since my camera is dead

What I'm reading is that I need to end up with 1-1/4" thick of plywood + backer.

I think I'll recess the first layer, then lay the second layer continuous.

Should I use 1/2" thick backer, or can I use the thinner 1/4" material? Can I use the wonderboard, or should I stick with the durock? How close should the screws be?
Old 04-09-05, 09:58 AM
Join Date: Oct 2004
Location: New Jersey
Posts: 1,132
Since you have access to it, avoid problems with the cut joist by adding cleats of 2x whatever will fit along side it, plenty of pl adhesive and screws. As for your ply, make sure it's bc or better grade. The ends of the first layer will end on the joists. Blocking should be installed between the joists to support the cut edges of the joists. Face grain perpendicular to the joists. The second layer should overlap the first , also perpendicular to the joists, but no glue between the plywood layers. The short end should end 4" past the joists, not on it like the first. 1/8" gap between sheets, and 1/4" gap around the perimeter. Crew into the new subfloor only. Don't screw the second layer to the joists. You can use 1/4" cement board, thicker cbu's don't make for a stronger floor, only a higher one. For all backers except wonder, use an unmodifed thinset to set the backer (Masterblend @ HD) but versabond is great for filling/taping the joints as well as setting your tile. If you want to recess the first layer, install 1x3 cleats along side the joists 5/8" below the top of the joists and cut the ply to fit between 1/8" shy of the joist sapcing. Then install the ply (cross cut so you have a bunch of 4' pieces) with a 1/16" space on either side and screw it into the cleats. The second layer should only be screwed into the recessed ply and not to the joists. (I reread your post about recessing the first layer) This isolates your floor from the seasonal adjustment of the framing.

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