Flooring Tile - Subfloor question (Concrete underlayment board)

Hi,
First time DIYer (out of necessity, kids in daycare) but have some experience. I'm prepping the Master Bath for a new tile floor and shower (down to studs and subfloor now). The tile center I purchased the tiles from also sold me a Schulter Ditra underlayment.
1. The store said I don't need to put down backerboard on the floor but everything I read says to (although they don't specifically address this system). Thoughts?

2. If I put down the backerboard what type of thinset do I need? Went to HD and Lowes (probably a rookie mistake right there) and looked at premix - but they all say NOT to use them with backerboard on floors. Any advice on quality non-premixed thinset to use in this case? If so, why those brands?

3. This may be a dumb one. If I use backerboard on the floor, should I level it first then apply the backerboard or backerboard first, then use the leveling compound?

I appreciate you insights. This newbie can use all your help.

-MZ

Ditra is your underlayment. You use it instead of backers. Before we progress any further on thinset choices and discuss any leveling needs, a few questions so we can help you best without backtracking.

Shower...tiled custom pan/walls or a prefab stall?

Floor...Tile only needs flat, not "level" How out is it, humps/valleys, etc. Lay a straight edge across and what do you see?

What is the make up of your subfloor? Every layer, thickness, and can you determine joist size and spacing (sometimes a small hole drilled through the subfloor is needed to figure out those details like subfloor thickness and joist height. Just shove a piece of coat hangar wire down there) Joist spacing can be determined from the nailing rows. Also post the unsupported span of the joists as measured from supporting walls or beams below.

Shower is another kit from Schulter as well. Pre-fab tray with waterproof membrame for shower floor, walls and seams. Once the floor pan and membrame is up, tile goes over everything - mosaic on floor, larger tiles on walls up to ceiling.

Floor is actually fairly flat, some dperessions, nothing serious. Leveling was intended to even out the 1/2" to 3/4" grade of the floor from one corner to the opposite.

Subfloor is plywood approximately 1/2" to 5/8" (I'll need to verify). I'll need to verify the joint spacing. How do I verify the joist size? Nearly all other joists throughout the house are either 2x8 or 2x10 (per my estimate). Is it safe to assume the joists in the basement are the same kind supporting the bathroom floor?

The bathroom is rectangular and fairly narrow. Unsupported span is ~ 8 ft. -- widest points (shower wall to opposite wall (behind vanity)) is ~8ft. Room is ~9ft long.

I'll provide the add'l info in a few hours. Thanks for your help!!

-MZ

Matt

Good move with the kerdi shower kit. Take the time to read the instructions and ask questions here or contact Schluter directly.

The floor doesnt have to be level, it just has to be flat. Why the 1/2" to 3/4" drop from one side of the room to the other? Its a small room so that seems like alot. Any structural issues to deal with here?

If the floor is 5/8" t&g plywood it may be ok. If its 1/2" then no good. Let us know for sure. Drill a hole in the subfloor, and stick something straight into the whole until it bottoms out on the drywall ceiling below. Mark it, and then measure it. Subtract off the height of the subfloor and you'll know your joist size. You can verify from the nailing pattern of the subfloor if the joists are 16" on center or something else. For the joists that are supporting the bathroom floor you need to go to the floor below and see what the unsupported span is from load bearing wall to exterior wall etc.

Ask lots of questions as you go. Less chance of serious mistakes that way.

Hi all,

Correction on the slope of the floor; off by 1/4". Plywood subfloor is 5/8". Joists are 2x8s. Joists are 16" on center, except under the shower. Seems to be a little unique there. Wall to first joist = 22", Next two joists are 12" on center, thereafter 16" on center (Starting from inside shower area and moving out to the opposite wall)

I'm not sure I'm understanding the unsupported span concept. Bathroom is literally in the dead center of the house. From what I can tell, the exterior wall to the innermost bathroom wall (farthest away from that exterior wall) is 15". There is a double joist (2 2x8s) under one side of the bathroom (in between toilet and shower). Not sure if any of that gives you the info you need.

Thanks again for the help so far. I appreciate it!

-MZ

What they mean by unsupported is how long are your floor joists.

The floor joists supporting your bathroom are supported from the walls etc on the floor below the bathroom. Those joists are supported by exterior walls, interior load bearing walls etc. How long is the span between these supports.

Thanks for the clarification. I'll have to get back under the floor and double check.

One other question. Does it matter which side of the backerboard facees the studs and which side is used to attach the tiles? Just asking because one side of the Durock is smooth and other is rough.

Thanks again for all your help! I GREATLY appreciate the advice.

Smooth side is for mastic, rough side is for thinset. You want the rough side out. Either side will work though.

I finally got back under the bathroom floor. The unsupported span is approximately 12' from the back exterior wall to the first load bearing interior wall. This interior l.d. wall is directly under the shower. From that point, the next unsupported span is approximately 9' to another interior load bearing wall.

Another question for y'all.

I put up the backerboard in the shower. Everything appears to be square and plumb (checked with 4' level and T square in several locations), some very slight deviations but nothing I would consider serious. Some of the screws though did not go in perfectly straight, they are tilted slightly and therefore not perfectly flush with the backerboard. If I'm putting on the thinset and then Schulter waterproof membrame and then the thinset for the tiles, do those application generally work together to eliminate most minor surface imperfections or do I need everything (screws, taped joints, etc) to be absolutely perfect and flush. I don't mind doing the workas I want to do it right, but also don't want to adjust something which is actually okay. Thanks.

-MZ

Matt

It best to fix the screws now. Back them out a little and then drive them in again. You should be able to set them flush with the board surface. If the screw head is stripped, replace the screw. Tape, mud, kerdi will all make it worse, not better. Its too late now but they make square drive screws for backer board. They are easier to drive than the phillips heads.

Thanks HJ. I'll take care of the screws before continuing.

Question regarding thinset mortar.

The tile store delivered two different types of mortar -- (1) an unmodified portland cement thinset with an ANSI designation which I'll use to apply the Ditra to the floor and Schulter waterproof membrane to the cement backerboard in the shower, (2) Kerabond brand mortar I'll use to set the tiles everywhere (floor and shower).

This may be a stupid question but can I use the unmodified thinset mortar to seal the joints AS WELL AS apply the Ditra? Or, do I need to use the Kerabond to seal the joints, the unmod thinset to apply the Ditra and then the Kerabond for the tiles?

Also, how long can I keep mortar once mixed. My concern was if I mix too much (for the time I have to work) and don't use it all can I seal the bucket to prevent it from hardening (like spackle?). Or because it's been in contact with water is it just a matter of time before it hardens regardless of whether or not the container is sealed?

Thanks!

Matt

I'm confused here. Are you saying you put cement board on the floor and now you're putting ditra over that? If thats the case, why? Either use the cement board or the ditra. Theres no need for both. If you wanted to strengthen the floor, plywood would have been better than the cement board.

No, sorry for the confusion. I put cement backerboard in the shower. I need to seal those joints before applying the waterproof membrame provided in the Shulter shower kit.