Doing a second bathroom floor

[JimR]

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I am working on tiling a second bathroom floor in a house built very different than my first. The house was built in the 50's with 2x8 floor joists that do not appear to be grade stamped anywhere and they span 12'- 2" over most of the room (less in a small area). The subfloor is 1x8 boards run at a 45 degree angle to the joists.

I have 16" porcelain tile to install but I'm not sure how to prepare the floor. The tile will butt to an oak floor that is 3/4" think and having nearly the same level would be great if possible. I'm thinking options would be ...

a) 1/2" T&G exterior plywood, glued and screwed (not sure if I want to screw to the joists or just the boards), then 1/4" Hardi with thinset and screws

b) 3/4" T&G exterior plywood, glued and screwed with Ditra

c) 1/2" T&G exterior plywood, glued and screwed with Ditra

Option c would match the existing floor surface best but mostly I want to get the subfloor right the first time. I could add wood to the floor framing (over a basement) ... either cross bracing or even additional joists (doubled or in between existing?).

I'm hoping for some good advice or pointers to other topics if the question has already been answered. Thanks very much in advance for any help figuring this out!

JimR

[JimR]

As I continue reading the posts related to tile floors it appears that a 2x8 on 12'-2" spans won't work. I'm not sure what the best option is for dealing with this because I don't know what stiffens the floor framing best. I think I could do one of these ...

a) add blocking between the joists that are 16" o.c.

b) sister the joists with a 2x6 ... not sure if this is even with the top of the joist or the bottom?

c) add a beam at least 2' from either end to cut the span to 10' max ... I assume this could be a 2x4 "wall" with a 2x4 stud under each joist?

Any thoughts on what works best???

JimR

[stullis]

How big is the bathroom?

As you surmise you do need to strengthen the joist what's best? Don't know but I'd go with either 2 or 3 most likley.

Then probably go with 1/2" ply screwed, no glue, and ditra.
Removing the 1x8's and adding new ply might even be better.

[JimR]

The bathroom is L-shaped ... 8'-3" at the widest and 10"-0" at the longest ... joists run the 8"-3" direction. I attached a text file (I think) with a crude layout.

According to the deflecto calculator referenced in another thread in this forum the joists are my issue. If tearing off the wood boards and putting down 3/4" T&G exterior ply with glue and screws in the joists (plus 1/2" ply on top I assume and then ditra) would stiffen the floor enough for 16" square porcelain tiles then that would be a reasonable option.

JimR

[Kman]

Just to confirm, the longest unsupported span of the joists is just over 12', correct? You need to shorten that span to 10' or less like you said, or use 2x6's or 2x8's to sister every joist. The idea would be to have the sisters at least 2/3 the span of the original joists, or 8', and have them centered along the original joists.

With the subfloor, adding 1/2" ply wood be the easiest and cheapest route for you to go. Screw it in to the subfloor only, not the joists, and as Scott said, no glue. If height is a problem, then your only solution is to remove the subfloor and install 3/4" T&G ply. Of course you'd then have to add some blocking around the perimeter of the room to support the edge of the subfloor. Lot of extra work to save 1/2" of height.

Either way, Ditra would keep the height to a minimum.

[JimR]

Thanks to all for the help ... I have a few follow-up questions ...

Kman said Just to confirm, the longest unsupported span of the joists is just over 12', correct?

It is a 12' span except fot maybe a 18" strip. Am I right that no matter what else I do with plywood/glue/screws these joists have to be beefed up or beamed?

Kman said ... or use 2x6's or 2x8's to sister every joist. The idea would be to have the sisters at least 2/3 the span of the original joists, or 8', and have them centered along the original joists.

So if I found the center of each joist and glued/screwed a 8' 2x6 to it on one side with the top of the 2x6 up tight against the underside of the floor boards this would be enough additional framing to meet the L/360 requirement for 16" square porcelain tile?

Kman said ... If height is a problem, then your only solution is to remove the subfloor and install 3/4" T&G ply. Of course you'd then have to add some blocking around the perimeter of the room to support the edge of the subfloor.

If I took up the floor boards, added cross-bracing and put down the extra-strong T&G plwood made for 24" o.c. over my 16" o.c. 2x8s (glued and screwed) would that be enough subfloor for Ditra ... or is another 1/2" plywood required?

Kman said ... Ditra would keep the height to a minimum.

I have done one tile floor and I used Hardi. It came out very nice (thanks to lots of info from the old site Bus and gang used to post on). Is Ditra DIY appropriate or would I be getting in over my head??? If anything I am very good at following instructions and not taking shortcuts.

JimR

[Kman]

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"Yes" to all your questions.

And Ditra is very user friendly. No nails or screws required, no taping and mudding of the seams, no carrying heavy backer board around, no difficult cutting.

Ditra is installed to your subfloor with modified thinset and it can be cut with a utility knife. The seams are simply butted together, and 323 sq. ft. of Ditra weighs just over 40 pounds, about the same as two sheets of Hardibacker. Ditra is only 1/8" in height, including the thinset used to bond it to the floor. Unmodified thinset is then used to install the tile on Ditra.

If you decide to go with Ditra, you can come back here for more advice, or you can go directly to Schluter's website.
Schluter-DITRA - Schluter-Systems