I will be tiling a 5-foot by 11-foot mudroom floor with 12 x 12 ceramic tiles. The plan is to offset each row by 6 inches.
My question is about the substrate. The subfloor is 3/4-inch plywood, on 2 x 8 joists, 16-inch on center, over a dirt-floor crawlspace. The joist run lengthwise, and are supported at mid-span. They are also blocked every four feet, so the floor is very rigid.
I was planning on laying felt paper on the plywood, then screwing down half-inch backer board, then thinset and tile.
But upon researching my FHB archives, I have one article (FHB #124) by Dennis Hourany that advises attaching backerboard with mastic and fasteners directly to the subfloor.
Another article (FHB #173) by Tom Meehan warns that it’s important to isolate the subfloor from the tiles, either with felt paper followed by wire lath and mortar (instead of backerboard) or by using a newfangled “uncoupling membrane.”
For my small floor area, can I get away with fastening the backerboard to the subfloor? And is it better to have felt paper or not? I’m hoping to get away with using materials I already have on hand (which does NOT include wire lath, mortar or an uncoupling membrane).
Thanks.
Allen
Replies
I think you'll be fine screwing down the CBU, but how much more would it cost you to trowel a coat of thinset down on the plywood before you do that? Not enuff $$ to be worth worrying about IMO and it will make the two layers of subfloor into one unit.
I also recommend you use a DITRA membrane, especially as this is a mudroom over a crawl space, thus subject to lots of different kinds of temperature fluctuations. 5x11 is only 55 sq. ft, not a big deal; even tho they don't give Ditra away it's not going to cost you the crown jewels to add it to your BM. Up here I can get it for under $2/sf and I'm fairly certain it'll be cheaper down your way. Prices for materials in Canada tend to be anywhere from 25-50% higher unless its something we make ourselves.
Dinosaur
How now, Mighty Sauron, that thou art not brought
low by this? For thine evil pales before that which
foolish men call Justice....
Thanks for the insight. I'll call around for DITRA.
Allen
What Scooter said about modified thinset is gospel. Shluter, the company that makes Ditra (and Kerdi and other products), recommends unmodified thinset between the membrane and the tile for those reasons, as Mongo also pointed out.
Before I learned this, I did a few jobs using membrane with modified thinset and did not have a problem. But that's known as 'getting away with it.'
Generally, when you use Ditra, you should buy three times the amount of thinset you would normally use for the square footage you have to cover. If you're laying thinset under the CBU as well, make that four times.
I'm not sure the CBU is actually necessary if you use the Ditra. I think you might be okay with ½" ply glued/screwed to the existing ¾" and then lay the Ditra on top of that. The Ditra will waterproof the installation and protect the ply subflooring. I wouldn't mind getting Scooter's take on that myself.
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
Plywood
Thinset
Backerboard
Thinset
Tile
The thinset under the board is to take out voids, and reduce the possibility that it might crack under weight.
Where the substrate is plywood and it is nailed directly to joists, you are correct that it is nice to isolate the backerboard. I would add another sheet of ply, as thin as a half inch, nailed to the ply not the joists.
But I think for your application it would not be necessary. Desireable, yes, necessary no.
Regards,
Scooter
"I may be drunk, but you're crazy, and I'll be sober tomorrow." WC Fields, "Its a Gift" 1934
Scooter, I'll likely go with your formula. I don't have enough depth to add another layer of plywood, but is there any advantage to avoiding joists when screwing down the backerboard?
Thanks for the reply.
Allen
Yes, the joists will move more than the ply, and you want the ply to "float". Since you can't do that you'll nail the CBU per manufacturers instructions, usually every 4-6 inches and stagger the seams.You can also use Ditra, an orange waffle membrane designed to go over plywood and it acts as an isolation membrane, too. It is pricey and is applied with thinset.Do not use mastic or pre-mixed thinset which is just mastic with some sand in it.Regards, Scooter"I may be drunk, but you're crazy, and I'll be sober tomorrow." WC Fields, "Its a Gift" 1934
I'll use thinset. Modified or unmodified? Assuming I'm not using Ditra.
(Tom Meehan's article specs latex-modified under the Ditra, and non-modified between the Ditra and the tile.)
Thanks, again.
Allen
Allen, the latex forming the basis of "modified" thinset extends the curing time. Thats great under tile, because a longer cure time, means a stronger bond. But under buried under backerboard, modified thinset will take forever to cure. So I'd use unmodified under the CBU and modified on top of the CBU.Regards, Scooter"I may be drunk, but you're crazy, and I'll be sober tomorrow." WC Fields, "Its a Gift" 1934
If NOT using Ditra, I recommend using modified thinset on the ply. Then SCREW the CBU to the ply. As others have mentioned, the thinset will fill any voids between the CBU and the ply.Nailing the CBU can sometimes displace the thinset in the area around the nails, which is why I prefer screws for the CBU.Set the tile on the CBU with a modified thinset.If using Ditra, you can use a modified thinset between the ply and the Ditra.Use an unmodified between the porcelain tile and the Ditra.Mongo
May as well throw in my two cents,
subfloor
30# felt paper (you can bend it up along the wall, bend a tight 90` and cover what's left exposed above the finished tile with your baseboard)
thinset (non- modified)
backerboard screwed to subfloor but not to joists
modified thinset
tile and grout
This acomplishes your uncoupling the tile from the subfloor and gives a solid surface to set tile on.
Geoff
Thanks, everyone, for the feedback.
Allen
And like Ed Mcmahn used to say on Jonny Carson
Right you are sir! Hoooooooooooo.
#30 lb felt is used as a moisture barrier ONLY!!
IMHO
When the 1/2" cement board is attached to the subfloor with Modified thinset and 1 1/4" cement board screws ( with the all joints staggered) the floor will achieve the 1 1/4" base recommended by ANSI. If felt is used it CANNOT act as an isolation membrane by simply a moisture barrier. (similar to red rosin for hardwood floors)
Gestalt, welcome to Breaktime. Could you elaborate on your response, please?
Are you saying that in achieving the 1 1/4-inch base, an isolation membrane cannot be included (3/4 plywood plus 1/2 cement board)? Or that felt paper, by its nature, will not act as an isolation membrane?
Also, is the inclusion of the felt paper a good idea -- as a moisture barrier?
And, finally, you mention modified thinset under the cement board, while others suggest that the curing/drying time would be too lengthy in that application. In your opinion, is this a concern, or not?
Thanks,
Allen
I would not use the felt paper at all.
Gestalt,
you're right, felt paper is intended as a moisture barrier only, but the reality is that it does act as an uncoupler (not an isolation membrane) for the backer board and tile assembly while still allowing the subfloor to be a part of the structural assembly.
This helps prevent tile cracking due to movement of the subfloor/framing assembly.
If the backer is thin-setted to the subfloor and screwed to the joists, it is tied directly to any movement of the subfloor/framing assembly, whereas the felt helps in un- coupling the two assemblies, although it is truly not an isolation membrane, but it doesn't hurt to use this method.
Could you elaborate on your rosin paper analogy, I didn't quite understand it? TIA
Geoff
P.S. why wouldn't you use the felt paper?
Edited 8/10/2007 1:07 pm ET by Geoffrey
I'm with you on the felt.
And to whoever is counting 1/2" cement backer board as part of the 1-1/4" thickness of the stuctural floor - sorry, the 1/2" backer has no structural strength.
The purpose of backer boards, Hardi, Wonder, Dens, etc in a flooring installation is:
1) elevational control ( that's why they make 1/4" and 1/2", and all of the manufacturers approve use of either on a floor) and
2) bonding aid for the tile.
Jim
Never underestimate the value of a sharp pencil or good light.
Jim, good point about backer board not being structural, I was a little tunnel visioned on the felt question. Probably not the first or last time that'll happen! :)
Geoff