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Old 12-24-2010, 12:15 PM   #1
Bryan the Amateur
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tiling with 1/2 inch plywood subfloor

I wish to put 6x6 porcelain tile on a bathroom floor. The floor is only 6.5 ft by 5 ft with an additional 2 ft by 3 ft closet (no standing in the closet; it is full of shelves). I removed the previous flooring and the 3/4 inch particle board underlayment. I had planned to use Hardiebacker 500 board (1/2 inch cement board) over the plywood subfloor, but discovered that the plywood subfloor is only 1/2 inch thick, and the manufacturer's instructions require a minimum of 5/8 inch plywood. I decided to go under the house and block the floor underneath. The existing floor joists are 2x8, unknown wood, 16'' on center, 10 ft span. One of the beams under the home to which the joists attach runs along the edge of the tub in the bathroom, the other runs along the hallway underneath or very close to the wall across the hall opposite the wall of the bathroom. The joists appear to be in perfect condition (the house was built in spring 1984 and is therefore 26.5 years old). For the blocking I used 2x6 pieces put flush to the plywood floor and used two exterior screws on each side to attach them to the joists, plus liquid nail on the top of the board where it meets the plywood. I placed these pieces on average 11'' on center, although because of pipes, duct work, etc. the spacing varies from 9'' to possibly 14''. I did this between the joists that run under the room; I did not do it between the joists where it would run under the wall. That made 6 blocks in three rows for 18 blocks under the bathroom floor, plus two under the closet door area and two more under the closet for a total of 22 blocks. Afterward I had a 190 lb person step on the 1/2'' plywood to see what happened, and when that person stepped in the middle of a 14'' by 10'' box, the plywood floor still flexed noticeably (I was in the crawlspace below looking at the floor). That flex looked like it may have been as much as 1/4'', but it is hard to tell; I did not measure and therefore I may be exaggerating. My question is this - under these conditions, will the 1/2 concrete board, adhered to the plywood with thinset, be sufficient support to lay the 6x6 porcelain tile?
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Old 12-24-2010, 12:33 PM   #2
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Cement board doesn't add any structural strength. You'd be much better off adding some good 3/4" plywood and then a thinner substrate for your tile: 1/4" cement board or 1/8" Schluter ditra. The pros here can show you how to do either one correctly so your floor won't wiggle.
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Old 12-24-2010, 01:02 PM   #3
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Welcome, Bryan.

I'm sorry to hear of all the hard work you put into blocking between the joists. Sorry, because it probably did little or nothing to improve the deflection.

Like Wendy says you would be much better off removing the 1/2" plywood and install new 3/4" CC or better exterior glue plywood. Make sure you install blocking to support the edges between sheets (unless you use T&G plywood).

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Old 12-24-2010, 01:34 PM   #4
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Question

Do I remove the existing plywood up to the edge of the stud walls? If so, what is the best method for such removal?
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Old 12-24-2010, 01:38 PM   #5
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Quote:
Do I remove the existing plywood up to the edge of the stud walls? If so, what is the best method for such removal?
Yes, right up to the edges of the sole plates. Some will use a sawzall with a long flexible blade.
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Old 12-24-2010, 02:21 PM   #6
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Question

If there is no joist immediately below the sole plates, should I put something in underneath to support the joint in the plywood subfloor that will be created?
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Old 12-24-2010, 02:25 PM   #7
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yes, most definitely
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Old 12-24-2010, 02:39 PM   #8
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Thanks, everyone, so much for your replies! It sounds like I've got more work ahead of me, but I've told my wife that this is the last time I'll redo that room, so I am bound and determined to do it right. Your help has been greatly appreciated!

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Old 12-25-2010, 07:49 AM   #9
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Bryan, let me offer a different solution: instead of ripping up the old subfloor, why not glue down a layer of 3/8" plywood, then use either 1/4" backerboard or a membrane for your underlayment? To do this without making a real mess of things, you need to pre-drill and countersink the top (3/8") layer and use a full-spread waterproof carpenter's glue, like Titebond II. Make sure the existing subfloor is clean and the underside of the 3/8" plywood is free of any splinters from drilling the screw holes. Also, spread only as much glue as needed for a single sheet, then get it covered before it skins over. Drive the screws immediately, but take care not to strip them. Place your body weight directly over the area you are driving the screws in to prevent "screw jacking." Space the new panels 1/8" apart at the seams and 1/4" along the perimeter, and stagger all the joints from the ones below.

This new layer, combined with all your blocking, should make for a substantial subfloor for your tile.
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Old 12-25-2010, 09:56 PM   #10
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That solution certainly sounds like a lot less work than removing the old plywood and putting in new. Two questions: (1) Would it help to go under the house and push up on the old plywood while putting in the screws from above? (I'm hoping you will say that's not necessary.) (2) There are two spots where the plywood was gouged when removing the previous underlayment. Should I do something to level those spots before putting the new 3/8'' plywood over it?
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Old 12-26-2010, 07:56 AM   #11
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1) No. Pre-drilling and countersinking the new plywood will allow the screws to bite into the lower layer and pull the two sheets together. All you need is for the screw to reach the bottom layer and dig in, the threads do the rest.

2) Depends on how gouged. Minor dings and splinters in the top ply are not a problem. Big (say wider than 3 fingers) or deep (the top ply and some of the next layer) sections may warrant some filling. However, you could do as much harm with a bad fill job as by leaving things as they are. The filler won't add any lost strength, so I'd not worry about it, or replace that section and worry even less.
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Old 09-19-2011, 01:49 PM   #12
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Same problem but one more question

I am having the same dilemma I ripped out all of the rile and top layers of old subfloor and have 1/2 inch ply attached to the joists on a second floor bathroom.

I intend to screw and glue 3/4 inch BC ply on top of the 1/2 base ply.

1) Does the 3/4 inch have to be screwed directly into the joists through the 1/2 ply?

2) What glue wood you recommend. plain old elmers wood glue or a glue rated for subfloors?
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Old 09-19-2011, 02:08 PM   #13
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If you want to use 3/4" ply, then you could simply ignore the fact that the 1/2" ply is there and screw directly into the joists every 6-8" on center. Use T&G ply for this.

I think Bob's method was to basically laminate the two layers of 1/2" ply together for strength. You don't need that if you're using 1/4" ply, although you can if you want. Make sure you have enough room to add 3/4" ply though. You'll probably have to raise the toilet flange, and it will affect the amount of space you currently have for the toe-kick at the vanity.
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Old 09-19-2011, 02:32 PM   #14
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Thanks for the quick reply

That will save me a lot of work if I didn't have to rip out the subfloor to the joists. I did that on my wife's bathroom. The bigger one of course. What a pain.

So essentially I'm ignoring the 1/2 ply

This bathroom is tiny. About 8 x 4. I will take the toilet flange into consideration, the transfer into the bedroom, and the shower drain. I will change to PVC if need be with a coupling. Bathrooms are such a can of worms.

1) so I am assuming no glue needed if I screw through the 3/4 ply, 1/2 inch ply right into the joists.

2) are pilot holes and counter sinking

Ditra is going over the 3/4 inch then either a porcelain or ceramic tile.
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Last edited by Mike2187; 09-20-2011 at 08:52 PM.
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