underlayment under new tub

[lumsden]

Hi,

I'm in the midst of a bathroom remodel. We're planning on installing a vinyl floor under 1/4" underlayment. The question I have is should I run the underlayment under the new tub for extra support. The subfloor is 5/8" plywood and appears sound under the tub - is this sufficient support?. I'm putting in a Sterling Ensemble tub.

Any recommendations on underlayment as well - most of the flooring places around here seem to carry 1/4" exterior grade Luann.

Thanks,
Mark

 

[Nate R]

The underlayment is only 5/8 wood and that's it? What is the joist spacing? 12"? 16"? I'd be inclined to run the underpayment under the tub. I'd think I'd want at least 3/4" of support on 16" centers.

 

[Sponsor]

 

[Jadnashua]

1/4" ply has almost no structural strength...generally a minimum of 3/8" is required to gain anything useful. SO, running that under the tub probably won't make much of any difference other than adjust the height.

 

[lumsden]

The floor joists are 16" on center. Something I could do is add 3/8" over the whole floor and then add the 1/4" underlayment on the portion of the floor to be covered with vinyl. This should add some structural support.

Is there any problem with having 2 added layers of plywood? The underlayments usually recommend using staples to connect to the subfloor. How should I connect the 3/8" plywood to the 5/8" subfloor?

Thanks for the replies.

-Mark

 

[Jadnashua]

If you were preparing this for eventual tile, the procedure would be to add the ply in the same direction (across the joists), but with the end 1/4 span (i.e., not over the joist), then screw it down and not into the joists. This evens out the small (but potentially significant for a stone tile) possibility that the ends sitting on the joist could lever up and crack the tile. This is also how the I-beams are placed across something like a bridge. Screws are tough into only 5/8", there isn't that much to grab into. I know you weren't planning for tile, but the same rules would apply...you should use exposure I glue (almost all ply is these days) and C-face or better on both sides.

 

[lumsden]

How crucial is it that the screws don't penetrate through the bottom of the subfloor. If I buy 1" screws, it should pass right to the bottom of the subfloor (with 5/8" subfloor and 3/8" on top). It seems to me that the next size down from 1" would be rather short and wouldn't grab very well in the subfloor.

Also should I leave a small gap between the 3/8" boards to allow for expansion? If so, what gap is recommended?

Thanks,
Mark

 

[Jadnashua]

The screws can penetrate, just make sure if they have a smooth shank below the head, that all of the threads are actually holding in the bottom layer. The goal is to avoid screwing into the joists from the second layer...this provides some decoupling. An 1/8" gap should be fine between sheets. You don't want any of the edges or ends to align (unless it is at the edge of the room) between the different layers...offset placement of the sheets, if required to accomplish that and you gain max decoupling and strength.

There are two schools about using glue...since it is hard to achieve 100% coverage and no humps, not using it is the norm. If it is used, you must use something like Titebond II and spread a complete layer...no caulking guns, and no just squirting it on - a full spread; then, you have to get the sheet on and screwed down before it skins over. This is tough, so unless it is done right, it is rarely done. It also makes it almost impossible to rip up later if you need to do some remodeling or repair work. Another reason it is not done much.