Hole in sub floor too big?

[Rmundo]

I am the middle of a bath renovation and getting ready to start tiling. I had a plumber complete the rough plumbing for the new shower, relocate the sink drains and remove the old toilet flange so a new on can be placed on top of the finished floor. I don't know what I measured when I cut the hole in the subfloor for the toilet drain, but you can see in the picture that it is off center, resulting in a big gap on the right. This is a full sheet of Advantec that goes uner the new shower pan. I marked all possible screw locations depending on the flange orientation and all on the right side fall 3/8" fron the edge to the center of the screw hole. The plUmber came back to finish some supply piping and didn't seem concerned. Is this sufficient for the flange screws to "bite" into? I plan on installing 1/2" backer board and 3/8" porcelain tile on top of this.

[hj]

The flange would need almost that much space around the pipe, and the riser can usually be "shifted" enough so you would not have to cut around the other side. I hope that "Advantek" sheet is NOT supposed to be a "safety liner" because it will not be watertight around the flange.

[jadnashua]

If you can gain access from below, you can reinforce around the hole. If you measure from the center of the pipe straight back to where the finished wall will be, is it at 12"? What about side-to-side? Code requires a minimum of 15" to each side of the toilet to any obstructions.

Also, unless you want to raise the floor, there is no advantage of using 1/2" cbu on the floor - 1/4" stuff is more than adequate, and if you really want to keep it low, a membrane like Ditra from www.schluter.com is barely 1/8" installed (and is easier, lighter, and I think quicker to install and carry). Because of the nature of the cement board (cbu), it really has little strength component to add to a floor as it will bend to match the substrate. Thicker works better on walls, since you might lean on it. The purpose of having it on the floor is to decouple it with the wood so the tile is attached to something that will move with it at the same rates rather than trying to attach it to wood which will change with the seasons and moisture content. Also, ensure you read and follow the cbu instructions - thinset underneath (this is not to adhere it to the floor, but to fill in any imperfections to ensure the board is 100% supported), and the proper screw or nail spacing. Hot dipped galvanized roofing nails is an approved fastener for the floor, but most DIY'ers use screws since they can be driven flush...the head of the nails will not be, and make troweling more of a pain.

[Rmundo]

Thanks for the quick response. I double checked the plumbers install and i have 12" to the back wall and 16" to the shower threshold on one side and 18" to the vanity on the other. The reason for the 1/2"CBU is because I had enough left from the shower walls to complete the 50 sq/ft floor. I figure it is easier to use then lug back to the store and the new tile floor will transition to carpet in the next room. I don't have access to the floor from below and there is enough clearance on the left side of the opening that I won't have to push the riser to the right, so I guess it comes down to is 3/8 from the opening to the center of the screw hole enough material to hold the flange screw? I would hate to have to patch a new subfloor just to move the hole a 1/2" or less, but I guess that's what i get for rushing to get it done.