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Unread 06-01-2020, 09:30 PM   #1
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New construction subfloor question

Thank you for reading this. I'm putting a modest one-floor addition on the back of my house this summer and I'm just now learning that tiling on a slab is way different than on wood! (Previously I only tiled on slabs.)

My foundation contractor needs to know where exactly to hang the joist brackets so that my existing house floor and the addition floor will be the same height. (This is very important to me.)

I'm GCing the project myself and feel foolish at the moment as I thought I could save money by tiling myself as I've enjoyed it previously.

I used the Deflecto and just found out that 2x8 by 12 ft joists are not strong enough for tile! 16" O.C. This was a shock to me. The 2x10 12 ft joists seem to fine though. I assume I will change to 2x10.

I'm a teacher, so please pardon that I'm long-winded...My framing contractor is going to install a 3/4 in OSB subfloor. Is that all I need? Can I install Ditra membrane directly over the OSB? Or is another layer of plywood required?

I just need to know all my "thickness" dimensions to know where the joists are to be hung. Thank you for any advice.

Last edited by cookiehead; 06-01-2020 at 09:46 PM. Reason: missing detail
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Unread 06-01-2020, 10:24 PM   #2
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Welcome back, Victor.

The surest way to get exactly the height you want in a finished tile installation over a wood framed floor is to set the tile over a reinforced mud bed. The minimum thickness is 1 1/4 inches, but I'd allow a bit more for adjustment. You can then make the substrate exactly the height you want to accommodate whatever tile you elect to use.

You can, of course, try to calculate the height with your substrate material and tile (you have the tile in hand, right?) and mortar and get pretty close, but if pretty close isn't enough for you, I'd recommend the mud bed.

You can use Ditra over a single layer of nominal 3/4" OSB over 16" joist spacing, but in new construction I'd always give myself the second layer of subflooring. Unless, of course, you elect to use the mud bed for a far flatter substrate.

For your joists I'd sure recommend you look into engineered wood. Sooo much straighter and more uniform for a flat floor. Tell the designer (place I purchased my engineered wood components was always happy to do the design work at no charge) exactly what you plan to do and get pieces made to order for your work.

My opinion; worth price charged.

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Unread 06-02-2020, 08:02 AM   #3
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Subfloor height in new addition - long plank tile

CX - You are the best! You're the 'Tile Yoda' as every answer leads to other questions.

I get that mudding the entire addition would give you the most control of the height of the new floor, but it's not worth that much effort for me. I can live with an unnoticeable floor height difference. Your suggestion about engineered wood is excellent and I'll look into that!

I see that you've recommended a 1/2" exterior glued ply in many posts. I can ask my framing contractor to add that over the 3/4" OSB. Would that 1/2" ply just be for stiffening or for a potential future demo? I think you suggested in other posts that BOTH OSB and ply should be perpendicular to the joists? Would the ply only be screwed into OSB and not the joists?

With some guidance, I'm just going to try to calculate the height of substrate material, tile, and mortar. The foundation contractor is going to use a laser level off my existing floor to calculate the joist bracket height, but this is all happening very quickly (so please pardon me as I didn't anticipate having to know so much so soon!)

Wife and I did look at tile this weekend for planning. I really like the wood plank tile. OF COURSE I like the longer planks. The tile I would like to install is 15x90 cm (about 6x35.5 in). This very long tile really needs a flat surface, so recommendations are welcome! I would spend extra for engineered joists to avoid mudding it all. I did use leveling compound for a portion of my basement slab tile job and I thought it was great. Would that be an option? Leveling compound is far easier than mudding in my opinion.

So many questions!

What added height does Ditra add? The membrane is 1/8" I'm told, but Schluter suggests a 3/16" trowel depth. (Those two numbers together = 5/16").

What would the recommended mortar height be for 6x35.5 inch plank tile? A half inch deep trowel? But I think they suggest back buttering each tile too?

The tile will be 1/4" height. These numbers all add up to 1 and 1/16", but that doesn't factor in back buttering. When you set a tile, you do lose some height by it setting into the mortar, so maybe don't factor in back buttering?

You can see that I'm losing my mind a little here!

I haven't yet researched this amazing forum for plank tile or leveling systems suggestions, but the tile shop recommended Raimondi. I've never seen these before. I guess they don't change the mortar bed.

Thank you very much for reading this. Any suggestions are very welcome!
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Unread 06-02-2020, 09:00 AM   #4
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Just a quick comment on trowel notch depth:

Many trowels have a notch pattern where the teeth are the same area as the notches. In that case, if you held the trowel perpendicular to the substrate and spread a full notch pattern scraping the substrate with the teeth, when you flatten it you get a thickness half the notch depth.

But usually the trowel is held at a 45 degree angle. That will further reduce the notch height by a factor of sqrt(2), so now the flattened mortar will be about 1/3 of the notch depth.

In other words, figure about 1/16" for mortar applied with a 3/16" notch trowel (as long as the teeth and the notches are the same area).

Cheers, Wayne
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Unread 06-02-2020, 04:52 PM   #5
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Project on hold now after all

Thanks for your reply. I sincerely appreciate this wonderful forum. CX - you are the very best! My wife has now decided a non-tlle option for the addition. Until my bathroom reno! Thanks!
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