empty space around toilet pipe can't set screws in flange

Can anyone help? We are putting in a new toilet and have removed an old cast iron flange that was broken. We set the new plastic flange in and tried to bolt it down and there seems to be nothing but air below where the screws should go in. What should we do? Is this an unusual problem? Thanks - advice is greatly appreciated!

This is the same problem which was created for me. A person installed a new flange, new toilet and new floor. One of my screws go into the joist and the others go into the air space and dangle. How nice. I had an access panel put in underneath and can see them.

Another person came in ( when I requested the floor to be fixed correctly) and threw down 1/4 inch Luann board which sure, covers the gap but, a screw cannot bite into 1/4 Luann board and in time will warp and rot.

I am not a plumber but, if I were you, I would replace that floor where the gap is at and make it a nice, sturdy floor to work with. That is what I intend on doing. Myself. So far as the others, I sued them, if only they would had cooperated.

You can try to put boards up underneath if you can, but seriously, I don't think that is good either.

Thanks Cookie - I am trying to gather several opinions and figure out the best fix. I appreciate the advice.

Sorry - I should have added that this is on a slab. I am sure that changes things a bit.
Thanks for the reply.

That is what we are considering but did not know for sure if it would work. Thanks for the advice.

My favorite method of anchoring to concrete is to use lead ferrels and screws. You do have to drill holes in the concrete to accept to ferrels. I use a hammer drill for that. For flange anchoring, I use stainless steel because it's tronger than brass and will not corrode.

Thanks for the tips Gary. Hopefully we will be back in business soon.

flange

There is no way to make a leakproof seal between a plastic flange and a cast iron pipe. Do it the right way and install, or have installed, a new cast iron flange with a lead/oakum joint. Then you will not have to anchor the flange to the floor.