DIY Chatroom Home Improvement Forum

DIY Chatroom Home Improvement Forum (
-   Flooring (
-   -   Subfloor/underlayment question (

tmbrink 11-09-2008 03:39 PM

Subfloor/underlayment question
I am remodeling a 5x8 ft.bathroom. I have removed a ceramic tile floor laid in 3/4 in. mortar mud. The remaining subfloor is only 1/2 in. ply over 2x10 floor joists, 16 in.on center. As i question my ability to lay a level 3/4 in. mortar base, what other options would i have regarding a suitable ceramic tile underlayment. I only have 3/4 to 1in. to work with as the room transitions to a 3/4 in. hardwood floor. If the mortar base would be the best, how hard would it be to do? Any suggestions would be appreciated.

JazMan 11-09-2008 04:23 PM


Are you positive the subfloor is only 1/2" thick? Half-inch plywood measures just under 1/2", could it be 5/8"? All plywood measures 1/32" under the stated thickness, so 1/2"=15/32", 5/8"=19/32.

The easiest method for a novice is to install a sheet of 5/8" underlayment grade plywood and then either Ditra membrane or tile backer board. Ditra is a great product and only adds 1/8" total, tile backer boards for floors are 1/4" thick and total about 5/16" installed including the thinset under it.

Be sure to re fasten the old subfloor before you install the underlayment. The underlayment is fastened only to the subfloor, avoid hitting the joists and use screws that are about 1 1/4" long.


tmbrink 11-09-2008 05:27 PM

Jaz, Thanks for the quick reply. Yes, it is 1/2 in. ( 16/32 ) interior/exterior glue C/D plywood. Not the best choice by the builder ! I could easily add the 5/8 ply and have already investigated the Ditra. As the floor does have a little movement, do you think the 5/8 glued and screwed with Ditra over it would suffice ? Tmbrink

JazMan 11-09-2008 07:39 PM

If you can go thicker go ahead....the more the better. BUT, do NOT glue the new plywood to the subfloor. Ditra is supposed to work well with even just one layer of 5/8", kinda scarry to me though.:eek:

I should have asked where the house is located and when it was built. In rooms where there is no tile or hardwood, like the original kitchen, they probably added a 5/8" layer to the 1/2" plywood subfloor.

While we're at it, please let me know the type, size, grade, species of the joists. Also the unsupported span of the joists. You will probably be ok for ceramic tiles, but we need to make sure the joists are stiff enough for tile.


tmbrink 11-09-2008 09:35 PM

Jaz, The house is in Cleveland, Ohio, built 30 years ago. The 1/2 " ply is supported by 2x10 joists. Unknown species. The room is at the end of the appx. 13' run of joists, so is well supported. So i should not glue or thinset between ply layers, just screws. Tim.

JazMan 11-09-2008 10:07 PM


Only glue 2 sheets of plywood if you can do a 100% lamination. If you use construction adhesive from a tube, you will create voids where there is no glue because the glue is thick.

Your old subfloor with 5/8" + Ditra will give you a very good floor. :thumbsup:

Because your subfloor is ONLY 1/2" thick, I recommend you fasten the new underlayment to the joists as if the subfloor wasn't there. Also fasten between the joists too. Use screws of two different lengths. Be sure to offset the seams both ways, and gap the sheets per normal.


tmbrink 11-17-2008 06:27 PM

JazMan, Thanks for your input. I just found out the 1/2"plywood rolls down about 3/8 to 1/2 " at the transition to the hardwood in the other room. What can I do to level this out before adding the 5/8" plywood?

JazMan 11-17-2008 09:22 PM

Go ahead and install the new underlayment then use a Portland patching product made for over wood. Then install Ditra per directions. Alternatively you could patch the floor before installing the new plywood, if you have time to allow the patch to dry long enough, and you are very careful when fastening the new ply that you don't destroy the patch. If dry it should matter much?


tmbrink 11-18-2008 09:37 AM

Thanks JazMan, I think i will use the leveler before adding the plywood ,unless you suggest otherwise . Hopefully it will protect the leveler from chipping or releasing. What about screwing or gluing it down ? Is it possible or ok to screw thru the leveling compound ? Do you recommend predrilling ? Screws into joist/plywood or plywood only ? Maybe i'm making a bigger project out of this than i should, but want to do a quality job with this much time and money involved.

JazMan 11-18-2008 07:24 PM

I have no problem leveling the subfloor before adding more plywood. Just make sure it hard enough. Pre-drill if it's on the thick side, but screws shouldn't hurt anything if cured and covered with plywood.

Underlayment is fastened to the subfloor, not to the joists in addition to the subfloor.


tmbrink 11-18-2008 09:44 PM

Thanks Jaz. Thats what i will do.

tmbrink 11-22-2008 09:34 AM

Jaz, In post number 6 you recommended attaching the underlayment to the subfloor and the joists, because the subfloor is only 1/2 ". Your most recent post says to attach the 5/8 underlayment to the 1/2" ply subfloor only. Which do you think will give the best resistance to cracking grout?

JazMan 11-22-2008 09:54 AM

Normally you are supposed to fasten only to the subfloor trying to avoid the joists and using short screws. Because your sub is so thin, I think you should also fasten into the joists. The Ditra will compensate for any possible lateral movement the joists might cause.


tmbrink 11-22-2008 01:17 PM

Jaz, Thanks for your help.

All times are GMT -5. The time now is 02:33 PM.

vBulletin Security provided by vBSecurity v2.2.2 (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
User Alert System provided by Advanced User Tagging (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.

Search Engine Friendly URLs by vBSEO 3.6.1