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I'm remodeling our bathroom because of some issues we're having with our floor. The kids weren't shutting the shower curtain when they decided to take a 30 minute shower. They would have a pile of wet clothes in the corner which made the bathroom floor sag. The edge by the tub is actually breaking in due to it being wet.

My question is, what type of wood should I use for a bathroom sub floor, and should I coat it with waterproof sealer?

Also I'm not sure what the best bathroom flooring options are to withstand water and moisture. We don't like the thought of tile.

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First of all, you cannot assume that any flooring you install is going to be "waterproof". That is an impossible standard in home construction, so the solution is to PREVENT water from getting to the floor surface and also to ACCOMMODATE any that does get there.

That being said, you should choose a flooring material that doesn't degrade when wet. Tile is one choice, vinyl flooring is another, also some engineered flooring products. Check with your local supplier for some good wet-location choices.

To prevent water from getting to the floor in the first place you want to fix the problem with your shower/bathtub so that the water is not able to be sprayed or splashed on it. Obviously shower curtains are one cheap solution but it seems in your case that's not being used. How about a surround that keeps the water contained in the shower/tub?

Finally as far as your underlayment, use a product that can withstand long-term water exposure. Cement-based backer board is a commonly used product for this application.

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    Good answer, would also include though that cement board (as thin as 1/4") can be used as the top layer before tile, but if the subfloor is rotted through you should cut out a large enough area to secure 3/4" plywood (match the thickness of the rest of your subfloor) to adjacent joists and apply the cement board over that. Don't put cement board directly on joists, of course. – Fred Shope Jan 24 at 14:21
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    Cement backer board is not used to withstand long term water exposure. It is used in showers and floors as a strong substrate to support tile, and when used in a shower it NEEDS a water proof membrane. Cement board is porous and will soak up water and possibly let it pass through. Cement board should be installed on a SOLID sub floor to prevent flexing,( important when flooring tile is used on top of it.) – Alaska Man Jan 24 at 17:53
  • @AlaskaMan I believe your information is dated: lowes.com/pd/… – jwh20 Jan 24 at 18:29
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    Exactly right. It's important to understand that while tile should be waterproof, the grout is not. If your grout is not sealed, it becomes a path for water to infiltrate the surface. Using a subfloor that can tolerate some water is in order to prevent infrequent accidents from requiring a tear out. If you really can't get the kids to stop, make sure the floor is sealed really well and you probably should caulk any gaps around the base of the wall. – JimmyJames Jan 24 at 20:34
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    If you really want to make the floor water-resistant, you could install a drain in the floor, but that's a much bigger job, since it involves plumbing, and you'd also have to slightly slope the floor towards the drain to avoid standing water. This is what you see in many public restrooms and showers, but it's not very common in home installations. – Darrel Hoffman Jan 24 at 20:54
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I've had lots of problems in rental properties with tile floors over "waterproof" wood-based subfloors (either "waterproof" plywood or "waterproof" particle board).

These subfloors always flex a little bit under load, which inevitably ends up cracking joints between tiles. Sometimes cracks are impossible to see. Then water seeps below the tiles and degrades thinset mortar and subfloor. Eventually tiles come loose. Even if the wooden subfloor is protected by a waterproof membrane, the subfloor might hold but the thinset will eventually fail if it stays soaked.

I've switched to vinyl flooring. It's easy to install, it comes in a roll, just put it down on the floor... and it is in one piece and watertight. Add silicone caulk on the edges and that's it. Silicone caulk must be checked for mold or degradation once in a while, but problems are easy to spot unlike hairline cracks in tile joints.

It is also more comfortable than tile, as it feels warm to naked feet. That's a nice perk.

If you don't want to use vinyl and you really want to use tiles, then I'd recommend using epoxy grout. It is awesome also for wet areas around showers and bathtubs. Unlike cement-based grout, it is non-porous, so if it gets moldy, it is very easy to clean. Also it's much harder to crack.

The edge by the tub is actually breaking in due to it being wet.

Looks like you have to really check the subfloor for rot. A full bathtub is very heavy, it needs a solid support. Also rotten wood will flex a lot and this will crack all the joints, and tear silicone caulk in the corners.

My question is, what type of wood should I use for a bathroom sub floor

If you use vinyl which is flexible and does not crack... then the floor is allowed to flex a bit. I'd go with 22mm marine plywood.

If you glue the vinyl to the subfloor, DO NOT put glue all the way to the edges! Leave about 100mm (4") unglued. So when you walk on the floor and it flexes, the subfloor will move down a bit, but it won't pull the vinyl down, so it will stay attached to the silicone corner joint and not tear it off.

If you take your current subfloor out, check under the tub for rot, also check the drain and all the fittings and joints while you're in there.

and should I coat it with waterproof sealer?

Not 100% necessary if you use vinyl floor, although it can't hurt. You should use plywood or OSB rated for "outdoors" or "marine" which means it's made with waterproof glue.

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First, why are you against tile? Tile/stone in general will be your best bet for the finished (top) layer - it is generally unaffected by water. Wood can swell (engineered is better, but still not great where bulk water is expected,) and even vinyl has problems if water gets underneath -- there were lux vinyl tiles in the laundry room of a house we purchased, which was all cupping because of water seeping underneath. Tile can be found these days w/ all different types of appearance, and can even be heated from underneath if desired - it really is ideal for a bathroom.

Second, assuming you are using tile, I would recommend a waterproof uncoupling membrane like Schluter Ditra. Proper installation of such a membrane (properly jointed and banded at the walls w/ e.g. kerdi band) will make sure that water has nowhere to go, while also protecting your tile field from cracking or separation due to natural flexing/expansion of the subfloor.

Btw, I am not a Kerdi employee, just a DIYer who has used this myself - it was quite simple to install. Options exist from other companies which may or may not be cheaper - e.g. Wedi or RedGard.

Add baseboards made of the right materials (ideally pvc, or at the very least real wood - no mdf,) the right adhesive (silane-polymer based like kerdi fix or sikaflex 505?) and sealant/joint filler (silicone if not using the adhesive as a sealant - which can be expensive and limited in colors,) and you should have a bathroom that could double as a swimming pool.

EDIT: Specifically for the subfloor question - you can't rely on a wood subfloor for water proofing. Your options are basically OSB or plywood, w/ the latter less likely to degrade under constant exposure to water. But really you should prevent the water exposure in the first place. Besides being a structural concern, wood + water = mold sooner or later.

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