Flooring Tile - Ditra and underlay

First off i should point out that we are having our kitchen, den and entrance way tiled by a local tile installer.Our subfloor is only 5/8" BC grade exterior installed perpendicular to the floor joists but the installer say's it's ok because he's using ditra and the floor joists etc are fine.My thinking though it's costing me $4700.00 Canadian for everthing thats floor leveling, tile, ditra etc etc.Ater reading this forum for the last 2 months i'm thinking about adding another 3/8" underlament myself and my question is do i install the 3/8 perpendicular to the joists as the subfloor already is?I always read you want to install underlay the opposite as your subfloor for more strength? Sorry for rambling on.

5/8" plywood is the absolute minimum required by Schluter Systems for their Ditra. The item that the installer is missing is that the plywood has to be a T&G plywood. The reason is that without the T&G the edge of the plywood between the joists will deflect. (Move up and down) Schluter Systems requires (as does the TCNA) that there be another layer of plywood before the Ditra. The sheets should be 3/8" minimum and placed with the 8' side 90 degrees to the joists and staggered so that all seams are overlapped. The WWW.Schluter.COM site has a installation manual that is downloadable. This shows the placement of the sheets of plywood.
Hope this helped.

Thanks for the help Jim.The subfloor is T+G however.I just don't want to spend all the $ and have problems a couple years down the road.I'll check out the Schluter site.

Grant

Grant - The 5/8" t&g is the bare minimum you can get away with. More plywood is always better and I'd recommend it here. 3/8" exterior bc or better plywood would be the absolute minimum and if you could afford the height thicker would be better. Install the plywood perpendicular to the joists. Leave 1/8" between the sheets for expansion and leave some space around the perimeter of the room as well. Use lots of screws and no need for glue. Screw every 8" or so in the field and every 6" at the ply edges. Stagger the seams of the plywood so they dont line up with the seams of the subfloor. Avoid screwing into the floor joists if possible. This helps to isolate any floor joist movement from the tile above. If your going with ditra, do any leveling that needs to be done directly over the second layer of plywood, then thinset, then ditra. My opinion here is the additional layer of plywood is good cheap insurance (well maybe not cheap).

Tiling over a 5/8" is very scary to me. I did it once about 10-12 years ago, didn't know it was 5/8" until after I removed the old flooring. It was a cheaply build apartment building converted to condos. Even after re-screwing every 6-8" and then thinset and Durock, I didn't have a confident feeling as compare with the typical 3/4" ply.

Although many manufacturers of tiling products give their blessings with 5/8" ply, they always state that the subfloor has to meet L360 max deflection with a 300 lb. load per test ASTM C627.

So, the more the better, and the floor will sound better too.

At least your tile setter is using Ditra and not the scratchcoat method that seems to be widely used around there.

How about your joists? Tell us the size, spacing and the unsupported span.

Jaz

At least your tile setter is using Ditra and not the scratchcoat method that seems to be widely used around there. Jaz

Jaz when you say around there. Where are you referring to? I cant tell from his posts where RCFlyer is from. What am I missing?

Thanks for the help everyone.The house is 25 years old with a 3 foot crawl space.The joists are 2x10 at 16" centers.The entrance is 14ft long so i built a pony wall with 2x6 at around 7' thank god for a crawl space. I live British Columbia canada.Of the 5 tiling company outfits in my area this is the only 1 that uses Ditra thats why i picked them.But like you i'm very leary of only a 5/8" subfloor.Unsupported span is 11ft hope this helps.

Johnny,

Flyer mentioned Canadian $$ as you now know. Actually I thought he might have been in the Toronto area. I guess they do that cut-rate method all over. You of course would refer to it as a "Jersey-Job".

Ok 11 ft. unsupported span with 2x10 should be just fine. What the hey...add more plywood....eh.....?

Jaz

I'll let you guys know if i'll add the wood myself or get a contractor, i don't have much confidence in doing it myself.I almost forgot total area to be tiled is 450 sq ft.

Agreed, add more plywood. Jaz-good detective work there with the Canadian $$. Yeah on occasion we still see some jersey mud jobs on tearouts.

Well i just got off the ph with the tile installer he say's adding more plywood is not necessary 5/8" is fine with using ditra.He also said if i add another underlayment it must also be glued and screwed to the subfloor.What do you guys think?He's been using Ditra for over 15 years and has never had a problem on a 5/8 subfloor as long as it meets the 360 code.

Why don't you call Schluter direct since it has to be done to their specs. if you use their product. There 800 # is on the site.

Your contractor is wrong about gluing the additional underlayment. He could glue it, but only if it's glue 100%, no construction adhesive. Having said that it is recommended that the underlayment NOT be glued at all. The reason as I just hinted is that most people would simply squirt some stuff from a tube and then nail/screw. This method however creates slight humps where the glue is and so many hollow spots next to the adhesive beads.

How does he know whether the subfloor meets L360 and ASTM C627 over every sq. ft. of your floor? Has/will he test it? Of course not. Calling Schluter won't answer those questions either. The Schluter tech dept. won't know either. They will only tell you that from their experience it should work.

I think your tileman is trying to save you $$$. If you don't want to take chances and are unwilling to accept the minimum, spend the $$$, it's your house.

Jaz

Thanks for the help.I'm not sure what to do now this is getting more complicated.When i mentioned adding underlay he said i could just add another 1/2" and forget about using Ditra all trogether?Anyways i have a month before he shows up to do the tile job to decide.He said the best glue to use is the white carpenters glue it has a longer set up time.

Ok good, white glue spread real fast and fastened as quick as possible. Two guys on a sheet. I don't understand why he is reluctant to add more plywood, he has to know it'll be a better job. He knows you're gonna pay him more $$$ right?

Now when he says skip the Ditra....that is not a rational comment.

Jaz

For years I glued and screwed the second layer of ply to the subfloor. I havent used glue though in about 3 years now. Theres been a lot of research thats gone into this by many people smarter than me, and the concensus is no glue. Biggest reason as Jazman pointed out is the fact that the gluing is generally not done right and creates voids between the two layers of ply. Its also thought that not gluing may aid in isolating joist movement from the tile above it (maybe, maybe not). I wouldnt give up the ditra as your contractor suggested. Why not do the plywood work yourself and have him do the installation just as he planned. Part of the issue here is that 5/8" t&g plywood is ok according to schluter. Its bare minimum and not something I'm comfortable with but it is acceptable. Your joist system is more than adequate, and ditra is a good plan. The single layer of plywood is really your only weak link here. How long will the installer warranty the work for?

Johnny and JazMan, What he said was if i wanted to add another layer of plywood be it 1/2" he wouldn't use Ditra.According to him 5/8" and using Ditra is fine with our subfloor of only 5/8".I think it's a 5 year warranty.I mentioned to him that Schluter does not recommened glueing between sheets of plywood and according to him it's a must on underlay to the subfloor.I just got back from the lumber store i can get 3/8" exterior fir good on 1 side for $20.00 a sheet.I gather the bad side gets glued to the subfloor corrrect?

Grant

What grade is it? I think you want B/C or better.

Wow that tile guy is a hard headed fella! I have no idea why he wouldn't want to do anything better than the minimum. Maybe he's the type that says...."good enough" when he does his work? He wouldn't have a chance to do work for me.

Forget the warranty, means nothing.

Jaz

I've been busy this last week 14 sheets of 3/8 fir exterior G1S plywood,12 pounds of decking screws,6 gallons of capenters glue and 3000 staples and alot of sweat.The tilers will be here in 8 days, their going to freak when they see i added another layer of plywood.:) Thanks everyone for your input the added plywood is a great idea.:D

Today the tile installers came. They even jumped up and down on the floor to make sure everything is ok and recommended not to install the ditra saying it wasn't necessary.The one guy said he never seen a home owner do so much prep such as adding an underlay with so many screws and staples and glue I even gluded and screwed a 2x6 to every floor joist.The only complaint was they recommend counter sinking all the screws in the underlay, mine didn't suck in very well and they have to hammer some in.Also they mentioned that you don't need good one side underlay as it has less sticking power with the thinset.

Flyer

Dont let them set the tile directly to the plywood. You still need cement board or a membrane. Setting tile to plywood is risky business.:wall:

OK great, you now have a very solid floor, that is a good thing. :thumbup: Why does your installer think otherwise?

Neither Ditra or any cement backer board would have compensated for a weak subfloor. By that I mean a subfloor that had deflection more than the max allowed of L360. Which means 1 inch in a 360" span, or any fraction thereof. That is NOT the purpose of a CBU or membrane. The purpose of CBU's is to give the tiles a cementitious surface to bond to. Membranes also give a good surface and also allow for shrinkage and expansion of the subfloor. You could have 5 inches of plywood and it'll still shrink and expand.

The minimum grade of plywood recommended is C/C plugged underlaymend grade - exterior glue. I supposed these guys would have put in something like CDX sheathing instead?

I don't know what else to tell you? You are supposed to be in charge of who does the work. But, maybe this is the only crew within hundreds of miles?

Jaz

One of the 2 guys that came was the owner of the tile company and he has 11 installers.There's atleast 15 or so companys in town that install tile, i picked this one because it was one of the only compamys ones that use ditra.I'll have more info tomorrow.