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I'm framing in a 40" x 16" shower bench and I plan on using 3/4" plywood.

I plan on using granite to sit on top of the seat.

I would like to know what kind of plywood I should use for the seat?

Should I use pressure treated? I've heard that pressure treated wood is not recommended to use inside the home.

What about sheathing plywood?

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    What role do you intend for the plywood to play in this situation? This question about support for granite countertops has at least 1 answer which might be enlightening... – brhans Feb 7 '17 at 15:10
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    Yes, the larger design of your shower would seem to be relevant here. How you plan on managing moisture is important. – isherwood Feb 7 '17 at 15:16
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The answer is - it depends.

If you are using Kerdi membrane plywood would be acceptable. Actually Pine sheathing plywood is the common type I have seen used for benches and steps in showers by my tile guys.

I personally wouldn't use PT plywood. PT is perfectly acceptable to use in a home but its purpose is to handle water/moisture that should be common to the situation. Basement outer wall framing is a perfect example - and bottom plates is a requirement in most areas. However I have dealt with PT dimensional lumber and plywood and there is a significant chance of warping - even if you get your stuff from a good lumber yard. If big box you have a 80% chance it warps unless you secure its shape. The point is the last thing you want is a bow in the plywood on day 4.

The lumber used to frame your shower should never get wet. So you don't need PT. And even if your shower framing got wet, that would be the very least of your worries. Meaning get quality wood but not PT.

So I would answer that this should be plastic plus plywood with kerdi membrane over or it should be hardiboard plus some sort of roll-on waterproofing like redgard (I do not plastic behind hardiboard as I feel it doesn't weep so putting plastic would cause an area that retains water that doesn't dry which will weep down and destroy whatever is below it).

Note: Make sure you slope your bench in (doesn't need to be dramatic). The back of the bench is the most vulnerable part of the whole shower. I would make sure my waterproofing is perfect there. You don't know how many benches I have seen with no slope or sloped towards the wall.

  • Thank you for your detailed answer. I got a little confused when you said "plastic plus plywood". Are you saying I should use a plastic vapor barrier?Also, I have sloped the bench about 5/8" for the 16" bench. – bigmike7801 Feb 7 '17 at 15:54
  • Yep plastic vapor barrier. 5/8" is perfect. That will give you some leeway because the granite will want to sit straight. – DMoore Feb 7 '17 at 15:59
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I would skip the plywood and use Kerdi board. It will cost a bit more but it's very easy to cut, very light weight so it's easy to handle, and it is completley impervious to moisture.

You can get it 4x8 sheets and build a custom seat or one of their pre-made seats, if you can find one in a size you can work with.

Here's a video from the maker of Kerdi-Board about making a bench out of the material.

  • I would love to use Kerdi Board and all of the Schluter products, however there's nobody that carries them on-hand in my area. – bigmike7801 Feb 7 '17 at 15:59
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    @bigmike7801 You may need to look for tile shops. They mainly serve contractors and don't advertise to laypeople, making them harder to find. If you can'd find or don't have them, try Home Depot. They don't have much Schluter on hand (usually just Ditra and some edging) but then can order most Schluter products with a short turnaround - I got some Rondec from them not too long ago and it only took 2 days. You won't fnid it on their website - get a part or product number from the Schluter web and go to the customer service desk or pro desk. They should have a special order book there. – Sean Feb 7 '17 at 16:25
  • Holy jesus. That bench material is going overboard. I will make mine out of wood and waterproof. No way I am trusting foam. Other than just another thing to sell I am really missing the whole point. So if the membrane works well why do you need the foam? When I install the membrane I have less joints and my joints aren't at critical areas. This just makes my head explode. – DMoore Feb 8 '17 at 7:22
  • People use it all the time and it's been in use for years. It's a proven material with genuine advantages. It's lightweight, easy to cut, strong, and completely waterproof. It's not like XPS or Styrofoam, it's much more dense and stiff. – Sean Feb 8 '17 at 14:46
  • What I am saying is why wouldn't you use concrete board or plywood plus membrane vs the method on the video. All of the seams are on the edges of the bench which in my opinion is crap. I guess Kerdi doesn't care because under and behind is waterproofed. Just seems like overkill for no reason. – DMoore Feb 8 '17 at 16:13
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I would recommend kendi as well, but no matter what you do, you need to encapsulate it all with a membrane. RedGuard is a common product.

The ability for the material it to deal with moisture shouldn't be the issue, making sure moisture doesn't ever get to it should be the priority.

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I would consider using cement board.

We're looking for long answers that provide some explanation and context. Don't just give a one-line answer; explain why your answer is right, ideally with citations. Answers that don't include explanations may be removed.

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    What kind of framing under cement board would be required? If I were going to use plywood over the 2 x 4 framing, then cement board over that, I would use marine plywood. But maybe just heavy polyethylene or vinyl shower pan sheeting under the cement board would protect standard plywood. I wonder if the 2x lumber should be pressure treated. – Jim Stewart Feb 7 '17 at 15:06
  • You can use the 3/4" plywood. Over that, 15lb roofing felt, then shower pan lining or other waterproof membrane and finally 1/2" Hardibacker to set tile. And YES, a 1/4" slope from wall to shower floor. Proper caulking of bench-to-wall joint with wet-area silicone caulk and using a great sealer on the grout joints will make this virtually waterproof. – M.Mat Feb 7 '17 at 20:24

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