Advice needed for bathroom project

[k.k.]

Hi everyone, I'm a new member here. I'm so glad I found this place, I have already learned a ton in the last few days checking out the forum.

I'm in the process of re-doing my bathroom. I checked out the deflectometer and my joists are ok for ceramic. I'm using 3/4" x 3/4" glass mosaic however (Casamood) so I believe that I should be ok as far as joist strength goes. I do have one area of concern though. There is a 36" wide alcove where the toilet will be where the joists are further apart (21"). This house is 100 yrs old and I don't think this room used to be a bathroom and they cut part of that joist away. So my issue is that the span may be too wide for the plywood to stretch across. Since the 4 inch cast iron runs under this area to the vent in the corner, there is no room to put backers across the joists or at either ends (brick wall and tub enclosure wall). What are the opinions on 3/4" plywood over this span? I also bought 3/8" ply to put over as additional strength....
I do have about 1.5" of clearance over the cast iron to the top of the joists, so if there is anything with that height that I could span across the joists, it would fit but I don't know what would be that thin and provide support for the subfloor.

On to my next question: If i'm screwing down 3/4" sub and then screwing 3/8" ply to that and then screwing permabase to that....that's too many screws, no?!!!

Any help is appreciated, thanks!

[Mike2]

If your 3/4" ply is clean and in good shape, I'd recommend gluing the 3/8" ply to it. Use a full spread liquid glue, like Titebond II.

Screw the 3/4" ply into the joists. Then screw the 3/8" to the 3/4", spacing them 6" OC along the edges, 8" OC in the field.

Then follow the installation instructions for the PermaBase. Here's a link to that if you need it. http://www.nationalgypsum.com/products/product68.aspx
Don't forget the thin-set mortar under the PermaBase. And tape those joints. All that very important.

Yep, that's a lot of screws. Nails are OK too, providing they are the right kind.

How about a first name, k.k.?

[k.k.]

Thanks Mike. The 3/4" would be new, as I'm about to install it. So you're saying that I would use titebond and then screws also?

Thanks for the instructions on the permabase. I already bought 1/2" so do you think I should do 1/4" instead? Or is permabase even a good choice? I can easily exchange it for the 1/4" if there is no added benefit of 1/2".

Anyone have any ideas about the issue with the 21" span between the 2 joists and any suggestions?

- Kendrick

[Mike2]

So you're saying that I would use titebond and then screws also?

Yes, you need both.

You can use 1/2" PermaBase if you want although over a floor it offers no advantage over 1/4". 1/2" is really made for walls where it does have an advantage.

There is no issue with that 21" joist space providing you have two layers of ply with a combined thickness of 1-1/8". Your plan gets you there. Problem solved.

[k.k.]

Thanks again Mike.

Just to clarify though...the alcove will have no backing spanning between the joists on the edges (or anywhere in between) of the plywood, so that 36" wide section will have no support on the edges of the plywood, just screwed into the joists.
Sorry if that's already how you understood it, i'm just nervous about it i guess!!!

- Kendrick

[Mike2]

Sorry Kendrick but I'm not following your concerns in that last post at all.

[k.k.]

Sorry, i didn't explain it clearly. I don't have a pic to post. Maybe I can explain it better...

Basically, there is a nook where the toilet will be. Exterior/window wall is on 1 side, wing wall/bath tub enclosure wall on the other side and wall behind the toilet of course. The measurement of this area is about 36" wide (window wall to bathtub wall, joists also run in that direction).
Due to the way the cast iron pipe & radiator pipes run between the 2 joists spaced 21 inches apart, I can't put support for the plywood between those 2 joists.
You said that the distance would be fine with the combination of plywoods that I'm planning on using, which is re-assuring but I'm concerned that the edges of plywood may cause the floor to be weak since there will be nothing supporting the floor on the sides (in between joists)

[Mike2]

I'm still not getting it, Kendrick, so lets wait for someone else to come by much smarter than moi.

Here are the phrases I don't understand.

I can't put support for the plywood between those 2 joists.

...there will be nothing supporting the floor on the sides (in between joists)

[k.k.]

I guess I'm trying to say the same thing with those 2 quotes. If you lay plywood across 2 joists, the more narrow you cut the plywood going across those 2 joist, the weaker it's going to be, right?
So wouldn't a typical plywood installation have support on all 4 edges of the plywood? If the plywood is only supported by the joists and not the other sides, I thought it would make the floor more prone to flex, but maybe not.

Thanks again for your help

- Kendrick

[k.k.]

bump for some morning advice!

Also, does anyone have a recommendation for a crack supression membrane to use under 3/4"x3/4" glass mosaic floor tile? Schluter told me that they can't recommend me using Ditra due to the need to use modified thinset on top and the glass being non porous. Are there any other options out there?

[Mike2]

If you lay plywood across 2 joists, the more narrow you cut the plywood going across those 2 joist, the weaker it's going to be, right?

Right. The rule of thumb in the construction industry up here is to have no plywood panels under 24" wide.

So wouldn't a typical plywood installation have support on all 4 edges of the plywood?

Yes, unless the panel is T&G, then only the ends need to be supported by the joists. This applies to the sub-floor layer but not the underlayment layer like the 3/8" in your case.

So, if you are not using 3/4" T&G and can't get blocking between those two joists, then yes that area will be weaker than otherwise would be the case. The 3/8" glued layer will help with that. The fact that it's in a low traffic alcove location makes is less important as well. Bottom line is, make the very best of it you can, then move on.

Does this address your concerns?

[Mike2]

NobelSeal CIS from one of our sponsors is a good one for your application. http://www.noblecompany.com/Products....aspx#products

If you poke around their website some, you should be able to find a distributor or retailer locator.

[k.k.]

Yes, that addresses my concerns, thanks! I was finally able to explain it well i guess!

It will be a low traffic area to some extent but that is where the toilet will be so I don't want someone plopping their fat butt on my toilet and cracking the grout on my beautiful tile!!! haha

I'm still going to try to find something to bridge between the joists in order to support the floor better in that section. Are you aware of anything that I can put there? I have about 1.25" depth clearance so I'm not sure if there's any wood options or if there is some sort of metal bracing available to span across the joists.

[Mike2]

Sure. Even laying 2x4's or 2x6's flat for blocking will help a lot. You'll have to notch them out a little to accommodate the 1.25" clearance needed. Try to nail through the joists into the blocking as opposed to toe-nailing them in place for maximum support.

Beyond that, keep your mother-in-law off the pot and you'll be OK.

[k.k.]

got it, thanks for sticking with me Mike!

- Kendrick