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toilet flange install on concrete slab

texas_fire's picture


I am building a small guest house outbuilding.

I have a concrete slab with the 4 inch PVC pipe coming up from the exterior edge of the slab. Then it connects to a street 90. I tried to set it up to be at the proper final finished floor level height with tiles installed later on...I wrapped it with cardboard to allow the flange to slip down over the street 90. I test fit and it seemed OK.

But after the pour, the flange is not seating fully flat down on to the concrete slab.

Maybe the lower portion of the cardboard kind of crushed in and the space for the flange does not go down as low as I had planned. Maybe a quarter inch gap.

What should I do? Can I put a spacer in there and drill down through into the slab and screw down the flange with stainless steel screws?

Should I trim down the flange pipe length at its bottom end to allow the flange to seat on the slab?

Also do I use a masonry bit and pre-drill pilot holes and then epoxy in the screws?

    Thanks folks.

(post #105971, reply #1 of 14)

I have used a piece of galvanized steel as a spacer before for flanges in repair situations and it worked fine.

As far as screwing it down, I use tapcons and have never had a problem.

(post #105971, reply #2 of 14)

Figure out what part of the plastic is bottoming out & trim it off.

Another option is to use a 4x3 flange & glue it inside the 4" st90.

One more option is to use an instaset® closet flange that goes around the outside of the 4" & uses a compression style joint to make the seal.

If the 1/4" gap is going leave the top of flange even with the finish floor tiles then do nothing. The ring does not need to be secured to the structure it is designed to secure the toilet to the piping not to the floor.

“The world is a dangerous place, not because of those who do evil, but because of those who look on and do nothing.”Albert Einstein


(post #105971, reply #4 of 14)

+1 trim it and anchor the flange to the concrete.

(post #105971, reply #5 of 14)

I anchor some but not all.

The one piece systems I don't usually anchor to the floor, but the two piece that have a metal ring I do anchor.

“The world is a dangerous place, not because of those who do evil, but because of those who look on and do nothing.”Albert Einstein


(post #105971, reply #6 of 14)

on toilet flange here, we usally have a 4 inch pvc stem through the concrete, poured in place. once the concrete set, the pvc is cut flush and then a pvc toilet flange is glued in. this can be after the tile or before allowing for height, and thats all. what am I missing here.

Edited 3/30/2008 8:39 pm by brownbagg

(post #105971, reply #7 of 14)

What you are missing is he tried to set a street L flush with his concrete pour. Of all the things you could  do wrong this would be number 1.

(post #105971, reply #9 of 14)

This was the guest house. Stage 1. I still have the boys' bathroom and masterbathroom to do in the future. Also concrete slab.

Lots of lessons learned.

So next time I should just go a bit lower in altitude and above the street L, glue in a stub section of 4" up as high as safe/stable as needed? Wrap in cardboard to allow the flange to fit and then cut flush after the pour.


(post #105971, reply #10 of 14)

no need to wrap - cement around it and use a flange that glues in the ID of the pipe as suggested above.


(post #105971, reply #11 of 14)

Oh, the ID.

I had bought the "normal' type which went around the OD. 

I will get my hands on oen of these.

I have bought and am planning to use a wax-free toilet seal that slides down inside the flange and seals.

Maybe all these things that add up to restrict the ID will end up causing me flushing issues?

(post #105971, reply #12 of 14)

All these will still not choke it down as much as the usual diameter coming out of the toilet base.  I do not have a lot of faith in the wax free seal except one by Fernco that has knife edge seals and adheres to the bottom of the toilet.

For those who have fought for it Freedom has a flavor the protected will never know.

(post #105971, reply #14 of 14)

yes, i got the fernco one.

Trap diameter is already small. Yes, I see what you're saying.


Thanks folks.


(post #105971, reply #13 of 14)

It's easy to thiink that, but take a look at the hole on the toilet - it's pretty small, maybe
2-2 1/2" in diameter - so you're not restricting it anymore than it already is at the toilet itself.

I've used the o-ring seal kits and they are pretty slick, but I've just gone back to wax rings - done correctly, there's no reason not to use em.

For shimming rocking toilets, check out the plumbers shims at Lowes - they are about 1 1/4" x 2" and slightly rubbery, so if you've got a bit of rocking - yam a few shims in, trim flush and caulk.


(post #105971, reply #3 of 14)

Toilet flanges are actually supposed to sit on top of the finished floor, although few

plumbers will take the trouble to make the necessary spacers at rough-in. For a thin

(1/4-5/16 in.) tile finish this is not a problem.  For thicker finishes (say 3/4 in. wood)

this is a definite problem.

Toilet flanges need to be secured to the floor.  That is why they have the holes for

mounting screws. If your flange is the type with the movable outer ring, or if there

is any play in the drain pipe, your toilet will move around without them.

The area around toilets tends to be highly corrosive. Concrete screws (probably the

easiest way to go) are hardened steel, but usually have some sort of finish coat.

I would goop up the shanks with silicone or some such before screwing them in.

Some (poorly designed) plastic toilet flanges will not sit flush on the floor unless

you have a hole much bigger than necessary for the drain pipe. "Flush fit" flanges

are available that do not have this problem. If you find you need to use a spacer,

you can get 1/4 in. thick plastic flange shims. These are usually used to fix the

problem of recessed flanges described above, but you can just cut one in half and

slide it under your existing flange. The mounting screw holes may or may not line

up , so you might have to drill some holes in the shim.

(post #105971, reply #8 of 14)

If I am reading you right you used the hub of the street 90 to connect to the pipe so you have the equiv of 4" pipe coming up out of the floor.  If so you should be able to trim off 1/4" of the hub from the bottom of the flange.  There are flanges with knife edge rubber seals that would insert into the 90.  Also there are ones that have an expanding rubber seal tightened up with bolts to make it secure.  Cheaper at local big box or plumbing supply.

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For those who have fought for it Freedom has a flavor the protected will never know.