I have been hired to tile a bathroom floor with a 1/2″ plywood sub-floor.
All of my previous experience has been with at least 3/4″ sub-floors. Then I usually use 1/4″ HB. Would 1/2″ Durrock be stiff enough or should I add a 5-ply 1/4″ baltic birch ply then 1/4″ HB?
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Replies
You'll get a variety of opinions on this.
I'd add 5/8 to 3/4 AC plywood and thinset to that. My tile installer say adhesion to plywood is fine if the proper thinset is used. Others say you should never set tile directly on plywood, but use a membrane or backer board. Unfortunately, backer board doesn't add much stiffness, and there are usually limits to how much thickness you can add to the floor.
Al Mollitor, Sharon MA
One of the best tile setters I've known always uses plywood. Had been doing it that way for 20 years before cement board or modified thinsets without problems. Of course, I also know a tile setter who mixes his own old fashioned mortar and does thickbed mud setting over "chicken wire," with success (he's a 70-something chain-.smoker, who looks 90-something, but is still at it everyday).I think this homeowner wants cement board though. I think I prefer cement board too, partly because of the way it is now percieved as better (or maybe just more fault tolerant).
I would agree, that CBU is better than plywood as a surface for tile, but it is not a substitute for plywood regarding structural stiffness. If you can afford the floor thickness, go with both more plywood and cement board. If floor height is an issue, go with the plywood.
Al Molitor, Sharon MA
Thank you for the good input.I am aiming for 3/4" thickness on top of the subfloor. That is why I posted the 1/2" cement board + tile or 1/4" ply + 1/4" cement board + tile proposal.The 5-ply baltic birch ply (actually 5mm not 1/4") is actually quite stiff and added to the 1/2" ply, I think it might make a sturdy 3/4" sub-floor for the cement board and tile. This does mean underlaying the floor twice, but might be a reasonable compomise vs. just 5/8 ply or just 1/2" cement board?
I'm guessing getting the 1/2" plywood up isn't an option?
It would be stiffer with a single layer of 3/4 then a layer of 1/2 + 1/4?
If you go to the work of taking up the 1/2", maybe keep on working hard and glue/nail 1x3 cleats to the sides of the joists 1 1/2" down from the top, and cut and drop in two layers of 3/4" ply. That's your strength and stiffness, choose the thickness of backer board on top of that to bring the finished tile surface close to the level of the floor in the adjacent room or hallway.
-- J.S.
Pieces of plywood dropped between joists will give you zilch for strength. You are looking for the "T" beam effect over the joists. Also cement board only gives you a bonding surface. You still need the strength under it. If you pay the money for any of the better Thinsets with the word "Flex" on it, you have already paid for additives, don't wast your money on more by mixing it in. That is for adding to basic plain thinset.
> Pieces of plywood dropped between joists will give you zilch for strength.
They don't help with the stiffness of the joists, which have to be adequate for deflection in the first place, like L/480. They are plenty strong for deflection of the subfloor between joists. Water barrier and backer board over that to isolate the tile from expansion and contraction should yield a reliable floor.
The same thing except with solid sawn one by instead of ply is what supports the mortar beds under my bathrooms. It's working fine after 79 years, except where they screwed up headering around the toilets.
-- J.S.
How about this scenario? We have 15" thick Trusjoists spaced 24" oc. On top of that we have 3/4 subfloor. Would just placing a layer of 1/2" Denshield make the floor stiff enough for 1" ceramic tile? We have the same situation in our master bath with 12" granite tile on top, and no problems.
However, in our downstairs bath (also 15" trusjoist, 24"oc), we have 2 layers of 3/4" plywood under 12" marble tiles, and we have had problems with cracking grout (no tile failure).
I really can't say anything about trusjoists. What I work on is old, solid sawn lumber. Two by's are really 2" thick, give or take 1/4".
I would speculate that the cleats on the sides of the joists trick wouldn't work with trusjoists, because there aren't any sides where you'd need them to be.
-- J.S.
John, I didn't understand that you were using it for backer under the other board. That sound better to me now. Thanks
I think a minimum of 1-1/8" plywood is desireable. That means glue and plenty of screws or ring-shank nails between layers. If the subfloor is 1/2", I'd go with at least 5/8" AC. If you still have room for 1/4 backer board, all the better.
Al
what is the joist span and what are the joists?
and 1/2" durock isn't going to add any noticable stiffness to the floor, it's non structural.
2x10 joists, 12' span, 16" oc.I think the floor framing system is stiff enough, I just wonder about the 1/2" plywood deflecting bt/wn the joists.
I would put another layer of 5/8 ply down with polyurethane glue and a row of screws every 8 inches. Then thinset to that. It has worked fine for me a load of times.
Have a good day
Cliffy
I second what Cliffy wrote... with one add'l note.Use the flex-type thinset. It bonds well with wood and has enough elasticity to absorb some floor deflection. Any decent supplier will carry it. The big boxes carry it too.F
Ditto Cliffy on this one. Did one 2 weeks ago, and have another in progress with exactly the same situation - 16" joist centers w/ 1/2" subfloor.
I'm adding 5/8" ply with adhesive and 8" screw schedule. Will follow with 15# felt, thinset, 5/16" cement board with 8" screw schedule, tape and thinset the seams, more thinset and tile. Lotta steps - makes a great floor!
Jim
Never underestimate the value of a sharp pencil and good light!
Hmmm...two 1/2" sub-floors in two weeks. Do you run into that all the time or are you just lucky? Where do you work? I seldom see 1/2" sub-floors (the reason for the thread). Just curious.
Basswood,
Wilmington, Delaware -- both houses by the same builder in the late 60's - next door to each other.
Jim
Never underestimate the value of a sharp pencil and good light.
I see.Just worked on that 1/2" subfloor hourse today, it was built in the 60's too.
add a layer of 1/2" Advantech...its seriously stiff and stable..glue with PL Premium (polyurethane) to the existing subfloor....screw the pizz outta it through the existing....Use Wonderboard...I don't think any other CBU is as sturdy.....thinset it down.
All your thinset should be mixed with a liquid latex mortar addititve...not water...its much more resiliant.....on the other hand you could also use Ditra but thats a whole other thread.
Be floored???
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