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rgb407 08-14-2008 11:49 AM

Underlayment over diagonal subfloor

I have a 5' x 5' bathroom floor and I just tore up several layers of old linoleum and the 5/8" underlayment. I currently have 1x6 boards (not T & G) running diagonally across 2x8 joists, 16" OC.

I am planning to put down a layer of tar paper then 5/8" plywood over the diagonals, then 1/4" tecply, then Vinyl sheet.

In the future I may expand the bathroom, tear up the vinyl and 1/4" and put down backer and ceramic tile, so using other postings, I have decided to screw the 5/8" ply into the diagonals but NOT into the joists (1 1/2" screws).

I know that you usually would install the plywood with the top grain perpendicular to the joists, but since the room is 5 x 5, there will be a seam somewhere and that seam COULD lie nicely over a joist if I run the 5/8" ply PARALLEL to the joists.

Does the diagonal subfloor and the small space allow me to "cheat" on direction and put the seam directly over a joist? Or is it better to have the plywood perpendicular to the joists?

On a related note, is 5/8" ply stiff enough over 1x subfloor to support 1/4" cement board and tile in the distant future? Should I go 3/4" now and just deal with the extra height?

Nestor_Kelebay 08-14-2008 10:01 PM

I just did exactly the same thing as you about a month ago, only I installed 5/16 inch fir underlayment over the 1X6 fir subfloor.

The original direction of the underlayment was with the top grain of the underlayment parallel to the bathtub, and that was also parallel to the direction of the floor joists. So, the original builders cheated on grain direction, and I did as well when installing the new underlayment. The reason was because the bathtub had a contoured side, and it was simply much easier to put down a 5 foot long and 4 foot wide piece of underlayment to cover most of the floor, and then scribe the contour of the tub into two 12 inch wide pieces to butt up against the tub.

I figured that if that building lasted nearly 50 years with the underlayment going that way, it would last another 50 if I put it the same way.

If you also have a toilet in that bathroom, and it hasn't already been done, now would be a good time to replace the toilet drain pipe with plastic (PVC or ABS). There's a good way of using plywood to replace the 1X6 boards you cut out, and I don't just mean replacing 3/4 inch thick boards with 11/16 inch thick plywood. Also, you need to be careful when cutting these boards because they go under your bathroom walls.

rgb407 08-16-2008 08:52 PM


It seemed like such a small space that I didn't really think it would matter.

I also remembered that I installed 3/4" ply underneath the subfloor when I had the downstairs ceiling out a few years ago. I think it's probably solid enough.

I thought about taking out the 4" iron pipe for the toilet, but for now I just wrestled the leaded flange off and will keep the iron. Haven't decided on the flange replacement yet...

There WAS some rot around the toilet, but that came from a tilted flange, not the pipe. Luckily, I was able to remove the rotten diagonals and replace them with a fairly normal looking piece of 3/4" ply since the diagonals all had seams in easy locations, and nothing went under any walls.

Thanks for your post.

Nestor_Kelebay 08-16-2008 10:31 PM


I thought about taking out the 4" iron pipe for the toilet, but for now I just wrestled the leaded flange off and will keep the iron. Haven't decided on the flange replacement yet...
Check these out:
As you twist the tapered thread on the flange, the external rubber ring tightens up against the ID of the cast iron pipe. It grips the ID of 4 inch pipe really solidly. The 4" PVC ones are very popular up here. Notice that you can also get these in cast iron.
With this one, you tighten three stainless bolts to compress the rubber ring against the ID of the pipe.

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