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Unread 01-26-2005, 02:50 PM   #1
RobC
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Plywood Underlay Install - Doh!

I am already afraid of what CX is going to say...

I am just 'finishing' putting down the plywood underlay that I was going to put my tiles directly over. I had planned on sealing the tiles & grout with a teflon HD product that has a lifetime warrantee. Almost all the sheets are already cut and tacked into place perpendicular to the joists, with a gap of 1/8" between each side. (Can someone confirm if the gap is okay?)

Details: I have 9.5" engineered I joists @ 16"o.c. The maximum span is 15'. The subfloor is 5/8" TG and I have been adding 3/4" ext.grade plywood on top. The area to be tiled is continuous from my kitchen 15x18', through a 3' door to a foyer 15x15, that is connected to a hallway 4x15'; at the end of which is a powder room 7x5'. I plan on screwing the underlay to the subfloor using 1 1/2" lubricized floor screws @ 4" o.c. The tiles I have already purchased are 3/8" thick by 12.5x17.75", which I plan to lay diagonally with staggered joints (kind of looks like a staircase pattern if that makes sense?).

I was not going to use Ditra because of the cost. It is $6 CDN/linear foot (from a 3 foot roll), or $2/sq.ft which would be around $1200.

I knew to install the underlay in a staggered fashion over the subfloor so that none of my joints fall over one another. I had planned on screwing the underlay to the joists - which I will not do now after reading threads here all afternoon.

Every single butt joint (the 4' side of a 4x8) falls on top of a joist. I did this thinking that I would get less heaving or shear between the underlay sheets if they could be screwed to the joist itself. The sheets themselves are staggered so that no two rows of butt joints are in line with each other.

Are these butt joints on top of the joists going to crack my tiles/grout? Will I be okay if I dont screw those edges into the joists, just backing the screws off say 3" from the seam?

Thanks very much.
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Unread 01-26-2005, 02:58 PM   #2
bbcamp
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The TCA addresses a plywood subfloor and underlay like you are doing. On the issue of staggering the sheets of plwyood, here's what they say:

offset end and edge joints of the underlayment panels by at least two
inches from the joints of subfloor panels; they should not coincide with framing below.


I'd re-do this, if I were you. No sense tempting the tile gods!
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Unread 01-26-2005, 09:52 PM   #3
cx
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How's come you're worried what poor ol' mild-mannered CX is fixin' to say? It's obviously Injineer Bob what's the real hard-ass here.

I can't tell if this is something you are planning to do, or something already done.

Since you are planning to tile directly to the plywood (something we don't recommend for our DIYers), I would want your subfloor and underlayment layers to be installed exactly by the book. We have one pro hereabouts, Bill Vincent, who tiles over plywood alla time. I'll ping him to have a look at this.

That said, I can tell you that a lot of floors have been installed over the years exactly like you have yours laid out. It's been only a few years since I learned about the quarter-span spacing for the second layer myownself. But I never tried to lay tile directly onto such a floor, either, nor have I ever known any installers who did. What will happen if you do that? Dunno. But as Bob said, the Tile Gods don't like it when you triffle with them.

My opinion; worth price charged.
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Unread 01-26-2005, 10:03 PM   #4
Bill Vincent
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Rob-- What grade and species of plywood do you have for your top layer?

Thanks, Cx
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Unread 01-27-2005, 09:18 AM   #5
RobC
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I bought all the plywood from HD.
The grade is standard exterior grade, R2S, 4x8' x 3/4"
The species is Spruce.

I think the mortar I am using is Chembond 8840. Have paid for, but not picked up the grout yet - Mapei, sanded.

CX - the underlay is almost all done. I have it tacked down with about 10 screws per sheet. I have not yet gone around and put the screws in at 4"o.c.

Bob - I have called and ordered the TCA Ceramic Installation Handbook... for future reference.

Bill - not sure if this is important, but using a 4' level I have spots that drop down 1/8 - 3/16" but they seem to level out within a few feet. I plan to level the tiles during installation. Should I think about a levelling compound instead - and if so, won't it run into the gaps I have between each sheet for expansion?

Thanks everyone!
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Unread 01-27-2005, 03:16 PM   #6
RobC
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Forgot to mention that my subfloor is glued and nailed to the joists. I then went around and screwed the subfloor to the joists, one screw every 6" along each joist throughout the entire house.

My joists are manufactured by AllJoist with a flange 2.5" wide.
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Unread 01-27-2005, 05:27 PM   #7
Bill Vincent
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Rob-- as for the dips in the floor, rather than using slc, just use thinset instead. As for the plywood you used, it should be fine. The problem is that if I understand you correctly, you screwed the second layer of plywood to the joists? If so, bad move. Pull those screws back out. Especially if you're going right over the plywood, that's instant death to that tile floor. Also, were you talking about the bottom or top layer when you said it got glued?
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Unread 01-27-2005, 05:55 PM   #8
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Bill:

I think the biggest question for him is that he laid out his second layer such that his end joints fall over joists, rather than at 1/4 joist bay. Given that, and the fact that he has 2 1/2 inch joist chords, he'd need to back off about 1 1/4 inch from the edge of his sheet to miss screwing into the joist. I'd rather see him re-cut the second layer so he has the joints in the proper place.

Other than that, it looks to me like he's done everything correctly with the subfloor/substrate system (It's the subfloor that he's glued and screwed to the joists).

You ever lay your substrate (second) layer with the joints over the joists and get away with tiling directly to it?
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Unread 01-27-2005, 09:07 PM   #9
Bill Vincent
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Actually, I've paid particular attention NOT to lay the joints over the joists, only because it seemed to me (even before reading about it) that that would cause a stress point that could cause cracking afterward. It's not something I would recommend, but I'm not sure how much it would play into the scheme of things. I guess it would all depend on how much seasonal movement there is in the house. I don't think I'd chance it. One other thing I just noticed--

Rob-- If you DO leave the sheets where they are, I would strongly recommend once you have a couple of screws into each sheet at 4"OC, pull the rest of the screws from around the sheet that you've used to tack it down. You always want to work your way from one corner out, or the possibility exists to end up with a slight "roll" or "bubble", for lack of a better term, in the plywood which could come back to haunt you after.
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Unread 01-27-2005, 09:25 PM   #10
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Wow $2/sf?
I think Ditra round here is no more than $1.30/sf or so. You may be able to find it cheaper somewhere online, I dunno.
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Unread 01-27-2005, 09:27 PM   #11
Bill Vincent
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Pete-- Canadian prices are alot steeper. Even ordering it online-- tariffs still have to be paid on the shipping.
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Unread 01-27-2005, 09:40 PM   #12
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I thought there would be extra costs but that suff is pretty light.
Yeah, tariffs would probably eat up alot of the cost.
Maybe it chaeper to get it staight from Germany.
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Unread 01-27-2005, 09:46 PM   #13
Bill Vincent
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Hey Pete-- you goin to Dal tuesday for the seminar? Big name from Schluter gonna be there.
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Unread 01-27-2005, 09:56 PM   #14
Bri
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Don't you fellars know that the Ditra you get is made in Canada.


HD's prices are stupid. Check around, you can get it for less.
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Unread 01-27-2005, 09:56 PM   #15
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Yeah I'm going.
I looked at the list today, maybe 45 or so. Gonna be a big crowd. I might have seen the name of the other Peter you confuused me for earlier.
I wanted my helper to go but haven't seen hide of hair of him for more htan a week.
Who's the presenter?Big name?
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