Shower - subfloor thickness question

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Old 07-10-09, 07:29 PM
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Shower - subfloor thickness question

Hello-

I have gutted my 5x8 bathroom down to studs.

Rather than having a small 30x30 shower basin, I decided to basically make the entire area a shower with the floor pitched towards the drain.

My original plan was:

1. Retain the existing 1/2" ply
2. Add another 1/2" ply with liquid nail between ply sheets and screw them down to floor joists
3. Lay down roofing felt paper with lathe
4. Pour pitched mud base
5. Lay down CPVC vinyl
6. Mud then lay down stone tiles with thinset

After doing some research, some people recommended Hardibacker, Durock, thicker ply, etc. so I was wondering what is the acceptable method?

The height of all these layers is a concern as I need to match to my existing hardwood floor in my bedroom.

This bathroom is directly above my remodeled kitchen so I want to do it right the first time.

Can you guys give me some recommendations?

Thanks!
 
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Old 07-10-09, 07:49 PM
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One more thing. Floor joists are 2x6 with 16" O.C. An exception is where the drain pipe is. at that particular space, it's actually 19" between joists
 
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Old 07-11-09, 08:56 AM
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Hi.

What is underneith the 1/2" ply? You probably do not have just a single layer of 1/2". If you do, I would rip it up and install at LEAST 5/8" ply. I would feel more comfortable with 3/4". If you can do that, then you can just install your felt and lath, and pour your mud.

What are the lengths of those joists until they are supported by a wall?

What are you using for tile? Ceramic or natural stone?
 
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Old 07-11-09, 12:14 PM
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Thanks for your feedback.

The floor amazingly has 1/2" ply only on top of 2x6 joists. I guess plywood was rare back in 1966?

Can I add another 1/2" plywood with grain running perpendicular to existing plywood grain? Would that provide same structural rigidity as 3/4"?

I only ask because the plywood seam on one side is not accessible and it would be a pain to install blocking due to plumbing.

Floor joists are 2x6 with a span of about 12 feet. Bathroom foot print (5' wide by 8' long) is touching the exterior wall, so I guess the right most side of the room is approximately 4' away from the center of the house where the load-bearing beam is.


As far as tiling goes, I just want to go with whichever one that will provide most rigidity and water-resistance.

Thanks again for your feedback!

JT


Originally Posted by HotinOKC View Post
Hi.

You probably do not have just a single layer of 1/2". If you do, I would rip it up and install at LEAST 5/8" ply. I would feel more comfortable with 3/4". If you can do that, then you can just install your felt and lath, and pour your mud.

What are the lengths of those joists until they are supported by a wall?

What are you using for tile? Ceramic or natural stone?
 
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Old 07-11-09, 06:56 PM
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JT,

With the information you provide:

2x6 joists centered 16"

12' unsupported span
Your floor is not strong enough to support any type of tile. According to the deflection calculator, your floor is only at L/144. In order to have at least ceramic tile, you should have at least L/360.

Without beefing up those joists and reducing that unsupported span, your current joists would 'bow' to much, causing the tile to crack. Adding thicker plywood would not help in this case.

If you could cut that unsupported span to less the 8', you would be alright.

If you choose to go ahead with your project despite this problem, I would take up that 1/2" you got now and start over. You can always take your circular saw and cut the ply where you can access.
 
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Old 07-12-09, 04:45 AM
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Hi Again-

Since the "old" bathroom had ceramic tile (plus terrazo shower basin) before, I decided to re-measure everything.

The lumber used is actually 2x8, but some of the spacings are actually narrower than 16". Red boxes are joist locations. please disregard the water pipes as they will be re-routed.

I know with 2x8 with 12' span, it's still no good for ceramic or natural stone, but since I have additional joists with narrower spacing, would it be okay to go with ceramic floor with 3/4" plywood (external sheathing grade)?


 
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Old 07-13-09, 05:53 AM
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You need to either shorten the 12' span with a support wall or sister the existing joists. Your 2x8's are definitely overspanned for ceramic tile. Forget about natural stone, that requires a floor thats twice as stiff.

A minimum of 5/8" t&g plywood is required as a subfloor for ceramic tile. 1/2" is not adequate. You'll need at a minimum, to add 5/8" plywood over the 1/2" that you have. Don't use construction adhesive, just use screws. Construction adhesive, unless it's full spread, will leave voids between the 2 layers of plywood.

If you are considering such a large shower floor, you should consider the kerdi shower system instead of a pvc pan liner. Kerdi requires only 1 sloped mud bed and will keep the height of the floor to a minimum. Remember, your slope must be minimum of 1/4" per foot from the drain to the furthest point in the shower (usually a corner).

Proceeding with the existing joist system and subfloor may very well cause problems for you in a short period of time.
 
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Old 07-18-09, 12:18 AM
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Should be good. As long as it is firm,you are good to go. The tile will sturdy it up too.
 
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Old 07-18-09, 07:49 AM
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Originally Posted by fredder View Post
Should be good. As long as it is firm,you are good to go. The tile will sturdy it up too.
Fredder...uh.....you want the floor to be sturdy BEFORE tile is installed.
 
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