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Old 12-03-2005, 01:58 AM   #1
dwb1957
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What about my subfloor, with Ditra, heating mats and SLC?

Man, a guy like me (accused over-analyzer) can get his head spinning listening to so many different "expert" opinions...The subfloor requirement issue...for my new 12'X6' kid's bathroom addition.

3/4" plywood on 9.5" "silent floor" engineered floor joist (16" centers)...
Longest span is 10.5', but the furthest tile from foundation wall is only 6' away. 12" ceramic tiles w\ warmly yours heat mats (ordered).

This particular bathroom butts up to 9/16" engineered Mannington Hardwood floor. I really do not want to add MORE plywood + backerboard etc...Most of you mud-guru's would say I need to add the plywood(?). The cement Backer Company's say I do not...Ditra actually specs that I can install the heat matt on the existing plywood (w\ Latex p.c. mortar between), + un-modified thin set on top, + Ditra,+ unmodified thin set, + tile (OR stone)... The Ditra design thickness is lookin best! Lay it on me, gentlemen.

Oh, & the next (2) bathrooms & (maybe) kitchen tile projects are on the same thickness plywood but 9.5" TJI's on 19.2" centers (after I remove the particle-board underlayment). Ditra site says that's ok too!

What is the best product to use, to save tile height & installer friendly...I don't think the price difference is that big of an issue.

Dave
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Old 12-03-2005, 05:40 AM   #2
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your plan with the ditra sounds fine, don't cheap out on the thinset you use underneath, use a good quality unmodified/liquid additive combo and not just a bagged modifiedtginset, or lay the mats down, prime and pour self leveling cement (after damming the perimeter) as it will be the easiest way to get the floor nice and even as the heat mats won't be covering the entire floor. Then Ditra, then tile. Kids bath is a good place to seal the Ditra seams with Kerdiband and maybe even flash it up the walls a bit.

At 19.2" oc for the other areas, another layer of ply will be a safer install, especially in high trafic areas like a kitchen. A single layer of ply at 19.2 is stretxhed well to it's max and your home has many years of both settling and materials fatigue to come. Even at 18" oc I'm not a big fan of a single layer of ply, but yours is a bathroom so it's fine

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Old 12-03-2005, 09:52 AM   #3
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If the ditra is fine as they spec, would you also find that wonderboard, hardi-backer or durock be ok as well (without additional layer of ply).

Where does a guy find ditra? I did see I can order it online, does any bigbox store carry it?

I'd also like to add that everyone I talk to in North Idaho, scoffs @ adding plastic or felt under tub\shower backerboard (that blows me away)...This is why I typically do my own projects, taking forever, but would rather o-kill than regret...except when it comes down to a 1" trip lip between flooring materials.

Also, the thinset you are talking about (non-bagged), do you mean in buckets? Any specific recommendations from Lowes or Home Depot?

Last edited by dwb1957; 12-03-2005 at 09:55 AM. Reason: additiaonal thought
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Old 12-03-2005, 02:02 PM   #4
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First, let me apologize for all the typos, I'm doing the responding while flat on my back, one handedand at times drifting in and out of sleep from medication. If anything seems incoherent, there was really a coherant thought behind it, just the hand did not let it come out that way.

Let's talk first of the bathroom. Your tji's are at 16" oc. Tile needs a deflection rating of L/360 for tile, both in the supporting joist structure and in the subfloor between the joists. 3/4" t-g plywood (not square edge plywood) over joists at 16" oc in a newer construction job shoud be fine, minimum deflection for most living areas is L/360 by code. This means the floor cannot give more than 1/360th of an inch for every inch of unsupported span of the joists. It also means the subfloor, measured between the joists cannot sag/give/bounce, however you want to phrase it, more than .04 inches. It's been decided by the powers that be, through testing, that 1 layer, properly instaaled 3/4" t-g subfloor panels is sufficient. So I am saying your bathroom with the tji's at 16" oc should be fine. Self leveler, poured over installed heat mats, with the use of the slc primer will give you a perfect floor to set on in your bath. The webbing of the mats will take the place of the diamond lath you would otherwise need when pouring slc over ply. Troweling a thinset over them is also acceptable, but it is sticky and not as easy for you to get flat, especially in areas where you don't have the mats to act as screed guides. Slc will eliminate that problem. You need to maintain the expansion joint around all fixed objects and walls, so damming the perimeter, along a tub, flange, etc, becomes a neccessity. The Ditra serves as an uncoupling system over top, allowing for the different rates of expansion/contraction of the tile field and the substrate. This makes it a better application for you.

Thinsets, there are buckets calling themselves "premixed thinset" which should not even be on the market at all. Avoid at all costs. There is unmodified thinsets, modified thinsets, (sometimes refered to as premixed, because dry additives are already in the portland mix, sometimes refered to as premodified, sometimes refered to as fortified) and I wanted to make sure we are on the same page with that. Going directly over the heat mats with an unmodified thinset, mixed with liquid additive instead of water would give you a better shear strength with the plywood. Still think you are better off with the SLC and Ditra as your floor will be flatter and while backers offer a minor amount of uncoupling, they do not offer the performance of Ditra. Hieght differences between floors can be made up with thresholds at the doorway. The last thing to consider when doing tile is the height build up. The right materials for a lasting install are the most important considerations. Aside from Ditra, which can be ordered through HD for much more money than it would cost you through www.tile-experts.com a Schluter retailer who also offers real advice along with the product, you could also use Noble CIS, which is only 1/16" thick installed. Ditra is 1/8" installed and 1/4" cbu is 3/8" thick installed.

Onto the other rooms. At 19.2" oc, the joist system will still be stronger than L/360, so that when the average deflection of the room is considered, the room as a whole meets L/360. This means between the joists is less than L/360 which you need stronger for tile. Adding plywood will do that, adding backer will not make the floor stronger. Time for my unproven rant...Companies keep coming out with lower and lower requirements for their product. It's called calculated risk. They know how long to expect a floor to last under certain minimum conditions, in combination with the average turn in the housing market, the non transferable warranty will more likely be gone before a claim needs to be paid out. If company "A" will pay 1 million in claims by lowering thier standards, but gain $1.5 million in market share with the reduced standards, then it's in company "A's" best interest to reduce their minimum standard. Warranties are written to make a product apeal to the consumer while protecting the company. In your situation, being a high traffic area like a kitchen, it would be foolhardy to worry about bare minimums and be more concerned with how will your install last the test of time. Rant over.

Brian
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Old 12-03-2005, 02:31 PM   #5
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When damming for expansion joint of the slc, beneath drywall bottom edge (@ bottom plate) will a bead of expandable foam work? In front of tub edge what would work (not sure how thick of expansion joint is desirable)? I do have one plywood edge between plywood that is not t&g perpindicular to joist that has an edge that deflects a bit...I was going to mary (7) strips of plywood from the crawl space (bottom side) to be safe.

Also when ordering the ditra, are there any other supplies that relate to that product that also needs to be purchased?

Get well, thanks.
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Old 12-03-2005, 03:48 PM   #6
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If you want to make the floor waterproof (it won't matter unless you run the stuff up the edges of the room, though) you'd want to waterproof the seams. Schluter makes Kerdiband for that.
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Old 12-03-2005, 04:09 PM   #7
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Deflection

DWB1957,
For the type of joists that you have you need to contact the manf. for the deflection. They will have deflection tables for the size, joist spacing and span of the your joist. Contact your manf. for that and if is not at least L360 then you must add at least 3/8" of plywood.

For the expansion joint if you pour SLC you get this stuff called seam sill @ HD, Lowes or anyother big box that sells building supplies. Then you tack it to any hard surfaces in the room the SLC will be poured in.

Also, check out the liberry link for other info about pouring SLC.
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Old 12-03-2005, 08:29 PM   #8
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Where to buy Ditra online.

Is "tile-experts.com" the most cost effective place to buy Ditra?

Does the tub-shower enclosure tile go on after the floor tile?
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Old 12-03-2005, 08:55 PM   #9
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Yes, and the friendliest, with fantastic technical support. As to which goes first, either can go first, just don't be sloppy with the bucket of thinset for your tub enclosure if you do the floor first. Tub first means you can be a bit messier as you work, but leave the last trim tile coming down along the face of the tub out until last so you can cut it to the right height.

So, from an earlier post, I see that you will use poly over your studs and then 1/2" backer. On exterior walls, cut the facing on on your insulation so you don't create a double vapor barrier situation. Use bagged modified thinset for setting your wall tile as well. Don't be tempted to use mastic, which is what those big box folks and those "never heard of a VB tile setters" you've got in ND will tell you to use.
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Old 12-03-2005, 10:16 PM   #10
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Welcome, Dave.
Quote:
Ditra actually specs that I can install the heat matt on the existing plywood (w\ Latex p.c. mortar between), + un-modified thin set on top, + Ditra,+ unmodified thin set, + tile (OR stone)..
You gotta be careful and read the fine print there, Dave. You must have double-layer plywood floor for natural stone tiles over Ditra, regardless the joist spacing. Go back where you read the spec you quoted and read the line under REQUIREMENTS. Big print giveth, small print taketh away.
Quote:
Originally Posted by pitterpat
Contact your manf. for that and if is not at least L360 then you must add at least 3/8" of plywood.
Looks like Brian (bljack) researched the engineered joists, Pat, and found them adequate, as did I. But if they didn't meet L/360 standards, the addition of 3/8ths plywood wouldn't help. It would, of course, help the deflection between joists, but not the joist deflection.

And I'd suggest you do not try to use expanding foam to seal under the sole plates for pouring SLC, Dave - too difficult to control. Use caulk there, and then tack up some of the Sill-Seal stuff Pat recommended to make an expansion joint. You can just cut it off when you finish.

Tile-Experts (our own David Taylor) is by far the best place to get your Schluter stuff, especially in small quantities. He'll take good care of you.

My opinion; worth price charged.
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Old 12-03-2005, 10:31 PM   #11
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Quote:
Originally Posted by cx
Looks like Brian (bljack) researched the engineered joists, Pat, and found them adequate, as did I. But if they didn't meet L/360 standards, the addition of 3/8ths plywood wouldn't help. It would, of course, help the deflection between joists, but not the joist deflection.
CX, I just meant to use at least that since his floor seems to be very touchy about height. I was pretty sure it would not be enough but some of our DIY'ers are like that quote of "asking for direction but following their own roadmap" . Not that I was infering that Dave is that way.
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Old 12-03-2005, 10:51 PM   #12
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Not to worry, it'll be ceramic tiles (not stone). Just returned from HD w\ a 50' roll of sill seal when I read Pat's reply...freaky.
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Old 12-03-2005, 10:52 PM   #13
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Do you folks think I actually have to seal the joints of Ditra?
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Old 12-03-2005, 11:00 PM   #14
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Only if you want/need a waterproof installation.
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Old 12-03-2005, 11:07 PM   #15
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If you want waterproof, yes. If you don't expect your kids to be flooding the bathroom, then no. It's not worth it to seal the seams with Kerdiband and then not flash it up the walls a couple inches. Otherwise water will just find it's way down under the baseboards. Ofcourse you would beed to caulk real well along the toilet base as well.

While you are researching your projects, here's a great article on addimg the second plywood layer to those 19.2" oc areas you are considerimg for later. Underlayment Plywood Installation
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