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Old 12-04-2010, 09:27 AM   #1
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Granite Floor Tile Help


Hi Everyone,

I'm installing granite floor tiles (3/8" x 12" x 12") in my bathroom.

My bathroom is 5 ft wide x ~10 ft long. My tub is against the back wall, so I'll only tile up to it (5 ft wide x 7.5 ft long).

My floor joists are 2" x 8" and 16" On Center. However, under the bathroom it goes, 16 OC then 8 or 9 OC then 16 OC then 16 OC (so four joists under the bathroom floor).

The main subfloor directly on the joists is 1/2" thick. I'm stuck on the correct underlayment and subfloor thickness and types required.

I have read that you need a strong floor to support granite tiles as its predominately a heavier tile. I was thinking of putting 1/4" ply on top of the 1/2" subfloor, followed by a 1/4" cement board for tile, totaling 1" support under the tiles.

Will this work for support? I also want to make sure my tile doesn't crack due to the house shifting also, and I don't know if this allows for it.

My other idea was 1/4" ply with a 3/8" ditra (D-W-S-10), however the ditra manual says that the 3/4" floor requirements include tongue and groove, and I'm not sure anyone in this city has 3/8" ditra.

Please keep in mind with all suggestions that we will be installing 3/4" solid hardwood in the hallway and living area outside the bathroom (which also has the 1/2" subfloor). I want it to be as close as possible, so unless I put plywood ontop of the subfloor in the hallway, I have to be around 3/4" height in the bathroom.

Please provide some suggestions.

Thanks,
Josh
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Old 12-05-2010, 09:48 AM   #2
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Granite Floor Tile Help


Neither the 1/4" nor the cement board will give you, IMO, enough support and dimensional stability and therefore I'd say take out the 1/2" existing, replace it with 3/4" plywood, put 1/2" plywood on top of that then Ditra, then tile.
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Old 12-05-2010, 09:06 PM   #3
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Granite Floor Tile Help


I already have 5/8" with 1/4" ontop (both plywood) = 7/8" total ply. I'm only tiling 38 sq ft of granite with the 4 floor joists spanning over 5 ft below. Is this a problem waiting to happen? I don't want to have issues, however I don't want to create too much extra work for nothing. I've decided to use ditra over the plywood, then the granite.

Would adding a few crossers perpendicular between the floor joists help me out? I have access to the floor joists from the basement. I prefer not to adjust the floor.

Thanks!

Last edited by trophywalleye; 12-05-2010 at 09:10 PM.
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Old 12-06-2010, 06:08 AM   #4
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Granite Floor Tile Help


No, I think you'll be fine with that set-up; the main reason tiles crack is because the longitudinal deflection of the floor joists, under load, was not taken sufficiently into consideration and thus not enough rigidity was created. Over long spans of 12'-20' this may be a serious problem, but with 5-7' of span, as I read it, you're good to go.

Cross-bracing i.e small 16" braces perpendicular to the direction of the joists never hurts...I wouldn't rip up a floor to put them in if they were missing, but would add them if the space was already open. Takes 5 minutes - but gives peace of mind.

The Ditra will do most of the work in preventing future cracking because it provides a cushion in all directions.
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Old 01-09-2011, 03:29 PM   #5
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Granite Floor Tile Help


I tried to cut a crossbrace but the joists are not perfectly perpendicular. Do I get it close and hammer it in? I don't want to add more stress to the joists. Or do I really need it for the granite tiles, as I have 5/8" subfloor and 1/8" ply ontop of that? Remember the joists are close together under the washroom.

Any other solution for crossbracing? It's tight in there to as theres gas lines and other crap in my way.
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