How to fill gap between waste pipe and concrete floor?

[Will_sy2002]

Hi Community,

I’m currently renovating a bathroom on the second floor. The waste pipe for the bath has been replaced by a plumber, but there are some gaps between the new waste pipe and the concrete floor — the largest being around 25mm.

I plan to tile the entire floor, so I’d like to fill these gaps before applying the waterproofing. Since there’s no easy access to the underside of the floor, what’s the best way to fill the gap between the waste pipe and the concrete?

From my online research, I’ve seen suggestions to use a backer rod and fast-setting cement, while others recommend expanding foam. Which method would be better in this situation? Or are there any other options you’d suggest?

Thanks in advance for your advice!

[EricL]

Hello @Will_sy2002 

I propose using the Parfix 340g Expanding Foam Filler, it will be a far more efficient filler than the gap filler rod as the foam will expand and conform to the irregular sides of the concrete. My best advice is to fill as close as possible to the bottom so that when the foam expands there will still be room for the concrete to sit in. If filled too close to the surface the expanding foam will occupy all the space.  

If you are quite handy with cement then using the fast-setting kind will be perfectly fine. My best advice is to give the cement enough time to fully cure before you start waterproofing.

If you need further assistance, please let us know.

Eric