subfloor question

[CharlieM]

I have a gutted 5x8 bathroom to put back together. The bottom layer of floor is 1/2" ply which I can't replace because there is no way to re-block edges, etc from the basement underneath (too much stuff in the way).

Anyway, I'm going to go over all of the 1/2 with 5/8 on which the new Sterling tub will set. Then one more layer of something on top of the rest for the finished floor.

The joists have settled enough over time that there is about 1/4" slope across the 1/2". It would be nice to have it level for the tub and stool foundation.

Is there any way to level out the 5/8" as it goes over the 1/2" ?

Ideally, I would shim or sister the joists underneath the 1/2", but that can't happen. Just thought I would ask.

Thanks.
Charlie

[jimbo]

Once you get your new plywood down, you could pour the entire floor with LevelQuick. This is a self leveling compound, which can go from feather edge to at least 1/2" thick Makes an excellent base for vinyl or ceramic. Make sure to plan in advance how you are going to get your closet flange to sit on top of the final finished floor.

[CharlieM]

Thanks Jimbo,

The flange is cast iron. Plumber is removing it and will re-install it when the flooring is done.

I tried some self levelling stuff once on a slab and it came out pretty wavy. On that project, I chipped it out and built a mud bed which wasn't easier to say the least, but at least I had control over the result. I didn't use Levelquick though. Who stocks that brand ?

Charlie

[jadnashua]

Self-leveling concretes (SLC) are made by a bunch of companies. Over plywood, most companies say you need to go a minimum of 1/2" thick, so that means it would be 1" thick at your low point. Some may allow a feather edge (typically on a slab, though), but you need to read the instructions carefully. Also, over a plywood floor, you should nail or staple down lath and MUST use the appropriate primer, or it will not adhere properly and is likely to delaminate.

The one recommended, may not be a concrete product, and may work fine on a plywood floor to a feather edge...what you decide to put on top of it may dictate the type you use.

If you are going to tile the floor, you'd need a decouping layer, and if you use a filler, can't use a cbu unless you do it over the top of the cbu, not underneath since screwing it down would likely compromise the filler. If you were going to tile, I'd consider Ditra from www.schluter.com as the decoupling layer.

It is important to use something like sill-seal foam around the edge of the room so that the filler can expand and contract, otherwise it can get tented off of the floor.

It is possible to do slc to a feather edge, but you'll find it is much easier to do it thicker rather than thinner, as it flows much easier when thicker depths. Especially in the summer when the heat will accellerate the cure, you don't have much time to mess with it before it creates big problems...best to mix and pour quickly, do a quick spread to even things out, then let gravity do its job. If you don't spread it quickly, sort of like honey, it won't spread to the edges and self-level. But, if you break the surface tension and spread it around, it does.

[CharlieM]

Thanks Jim,

I haven't decided whether to try to tile it or just put lino down.

Given that the bottom layer is 1/2", any suggestions for how best to fasten the 5/8" on top. Normally, I would keep the 5/8 joints and fasteners off of joists, but there's no meat with the 1/2 to fasten to really. Best just to fasten it to the joists ?

Charlie

[jadnashua]

1/2" is really tough! Especially if it has a D face. For maximum decoupling and to help prevent squeaks, you don't want it attached with joints lined up, into the joists, or with the ends over the joists. Maximum decoupling has the end at 1/4-span, or about 4" offset from the joist on 16" on center. It might be a pain, but deck screws, and possibly predrilling might be called for...a clearance hole in the top layer, and a pilot hole in the bottom and either a good clutch on the drill or a fine touch. One of those bits that does it all in one including the bevel for the flat-head screw might give you the best chance of getting any grip. You don't want to glue, since if you don't get good coverage and a tight bond, you'd be worse off as the sheets might be held apart.

You would need to know the joist deflection ratings to decide if you could use tile. There's a good calculator on www.johnbridge.com in the menu bar on the forum. They can help you decide on tile, too, if it comes to that.

[CharlieM]

Jim,

I'm familiar with the deflecto calculator. I'd be good since the joists are on about 12 inch centers under this bathroom. A couple even less than that.

I'll give the delicate touch a shot first. I actually have this bathroom in a thread over there somewhere. If I decide to tile, I'll probably use ditra just to keep the height down. We'll see.

Thanks for your help.

Charlie