Shuttering concrete with insulation

[23vc]

Hi
I think a builder friend once told me you can shutter areas like where a shower waste is going to be set into a concrete floor slab with a piece of foil insulation (eg celotex) then it can be removed after pouring.
However I may have made this up but on the face of it sounds like it should work and be easier than shuttering in ply (not much easier admittedly)
Anyway just wondered if anyone's done this / seen it done?
In my case I need to keep the concrete clear of where a wet room trap is going (and I'm not having a screed)
Cheers
John

 

[KenGMac]

23vc, good evening.

Short answer is yes, you can use any number of things to form a hole in a slab, many moons ago Plywood was used, but? the Ply was at times difficult to remove, especially when the Ply was used to form a gap in a large concrete pour and allow movement of say a Holding Down bolt.

Other materials used include, Polystyrene, specially made wire former's, where a material like Exmet was used especially when as above in a Holding Down bolt situation.

Indeed any material you can use and is simple and easy to remove, leaving as little residue as possible can be pressed into service.

Suggest you consider [obviously] running the under slab waste to the point where it is to pop up through the slab, fit a bend, cap it off, insert your preferred material to form the hole in the slab, pour the slab, remove the fill material and hey presto job done.

One other thing to consider is to leave sufficient space to allow you to reach the coupling connectors on the bend? Pointless getting scraped knuckles trying to get at the couplings?? Just a thought.

Ken.

 

[23vc]

Thanks Ken yes what you describe is what I'm planning to do. Good to have a sanity check thanks again

 

[^woody^]

Polystyrene and then petrol IIRC.

 

[23vc]

Petrol? Any chance you can elaborate?

 

[23vc]

Just found https://www.diynot.com/diy/threads/digging-into-semi-set-concrete.341518/ where you explained it - thanks
Only issue for me is there would be more foam below ie the under slab insulation so it may not work but thanks for the info.