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Old 12-06-2017, 09:16 PM   #1
JerseyDIYguy
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Minimizing subfloor height with Ditra Heat

My brother lives in Northern California and is having his bathroom redone. He contacted me (the family "expert" on all things home improvement) for advice on the floor. The bathroom will be about 10' x 7' with a doorway in the middle of one of the 7' walls.

Underneath he has 2x8 joists on 16" centers supported every 8 feet, so plenty sturdy structure. On top of the joists are 1x6 planks laid on a 45 degree diagonal. The 1x6 diagonal planks run continuously from the hallway outside the bathroom across/into the existing bathroom floor.

In the hallway outside the bathroom there's 3/8" thick hardwood laid directly on top of the 1x6 planks.

The joists in the bathroom area are not level. The low spot of the joists is at the door, and the high spot is 10.5' away on the opposite wall and is about 3/4" above the low spot by the door.

He wants to minimize any step up from the hallway floor to the bath floor, but could live with a 3/4" step up. He wants the floor to be extremely level due to wall tile and tub installation needs. He plans to use Ditra Heat mat over the entire floor which will be covered by 3/8" thick ceramic tiles.

What's the least possible floor thickness he can achieve?

I'm thinking he should
1. Cut all of the 1x6 diagonal planks around the perimeter of his floor.
2. Install blocking under the cut ends of the perimeter 1x6 diagonal planks.
3. Install 3/4" exterior grade Plywood directly to the joists.
4. Pour SLC over the entire floor with 7/8" maximum depth at the door and 1/8" minimum depth at the opposite wall.
5. Install Ditra Heat mat
5. Install tile on top of the Ditra.

He'd end up with a step of roughly 1-1/8" at the door along with a level and heated floor.

Any ways to reduce the step height?
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Old 12-06-2017, 09:22 PM   #2
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Warren, if he's willing to remove the existing subflooring, why would he not just sister his joists with some 2x6 material in a manner that would result in the new joist tops being both level and in plane (flat)?

Then a tiling substrate and his tiles.

My opinion; worth price charged.
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Old 12-07-2017, 04:16 AM   #3
JerseyDIYguy
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For some reason he's a bit hung up on using SLC. Probably because he's seen good results with it.

So:

1. Cut out all the 1x6 diagonal planking in the room.
2. Install blocking under the remaining 1x6 cut edges throughout.
3. Sister 2x6s to the existing 2x8s to create a flat and level surface.
4. Install 3/4" T&G plywood.
5. Install Ditra Heat Mat.
6. Install heating system.
7. Install Tile and finish.

Suggestions for mortar to use under the Ditra Heat? On top of the Ditra/under the tile?
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Old 12-07-2017, 09:02 AM   #4
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Warren, if you removed the existing subfloor - presumably with a sawzall along the wall sole plates, have you determined how to support the new subfloor edge along the walls that are parallel to the joists? Chances are that a joist won't conveniently be right where ya need it.
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Old 12-07-2017, 02:23 PM   #5
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Quote:
have you determined how to support the new subfloor edge along the walls that are parallel to the joists? Chances are that a joist won't conveniently be right where ya need it.
Yep. Step #2 is to install blocking at the cut edges of the 1x6 planks. The good news is that the room is over a crawl space so he can get underneath to install the blocking.
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