Flooring joist question [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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05-25-2010, 08:24 AM
My question relates to cutting and heading off a joist in the middle of a span.

I am working on two back-back bathrooms and want to tile the floors.

The bathrooms share a 2x6 wet wall and have back-back toilets connected to a 3" ABS waste line that runs underneath the wet wall.

The toilets are plumbed using a back-back fixture fitting and unfortunately experience blow-by with the newer gravity fed toilets I am installing. When one toilet flushes it will cause the water in the other toilet to rock and drops the water level slightly.

I would like to correct this and put each toilet on it's own fitting if possible. Unfortunately, there is very little room underneath as the 3" line cuts through joists. I've attached a rough picture.

The joists are 2x10 doug fir in great shape 16" OC with 13' span. The wet wall lands in the middle of the span.

Several questions..

1. Is it possible to cut the noted joist and head it off creating an open box for more plumbing room ? How do I do that and keep the floor integrity intact to prevent sagging subfloor and maintain ability for ceramic tile ?

2. Would such a box give enough room for two toilet feeds to the 3" waste ?


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05-25-2010, 09:14 AM
Charlie, it is no un-common that the water in one toilet "move" when another toilet on the drain line is flushed. The plumbing code requires that adequate venting be installed to ensure that no trap seal is subjected to more than 1" of water pressure differential. This keeps the water from being siphoned out of the toilet bowl. From your description, this appears to be OK. Also, the back-to-back drain fitting used to connect both toilets should be of the sweeping wye type, such that the actual flow of water is directed down the pipe and not across the tee. You could flush both toilets at the same time with no adverse effect. I would be surprised if the wrong fitting was installed in your piping. Looking at your plumbing diagram, I don't see an issue with venting (assuming the length of piping is reasonable).

To answer your question: you would have to install sisters on the joists to each side of the joist you intend to head off. Then, you'd install double blocking to support the ends of the cut joists. The blocking would have to be outside the toilet flanges, so the boxed area created by the sisters and the headers would be 32" by at least 24". The subfloor in that area would not have any support, so you would need a double thick subfloor (say 3/4" topped with 3/8" or two layers of 5/8" plywood) and an underlayment that allows 24" joist spacing. However, note that the plywood grain direction will be in the 32" direction. You may be able to help the subfloor with some 2x material laid on the flat against the subfloor and well attached to the joists, but that depends on the plumbing clearance you have.

All that said, I suggest you stop worrying about this. At least, stop looking in the toilet when someone flushes in the other room.:D

05-25-2010, 11:22 AM

You are correct - the double fixture fitting meets code and stuff heads in the right direction. The higher performance toilet just creates more turbulence. Thanks for convincing me to leave it alone.

Now to the subfloor itself ....

It is 3/4 TG OSB which I think is minimally good for tile. I was planning on adding 1/2" ply on top, then Ditra.

Should I bother with the 1/2" ?

05-25-2010, 11:26 AM
I'll never try to talk anyone out of more subfloor. :D However, unless you chopped up the existing subfloor during your reno, you don't really need it.