# Forum Home Renovation Concreting  infill slab for bathroom

## Kieran989

Hi guys, 
I am planning to pour an infil slab for an ensuite renovation once plumber has roughed in. See photo below for layout. 
Current plan is:
- build up level with builders sand to allow for 100mm thk slab
- trench on open end near joists to thicken the slab. 300x300 trench
- form up doorway entry and open end
- layout reo mesh
- wheelbarrow concrete in, screed and finish 
Questions:
- Do i need to have a vapour barrier?
- Do i need to use a compressible filler strip where the slab contacts the limestone foundations/brick walls? 
Thanks in advance!

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## johnc

Don't trench, run 100mm all the way through and use reo. Make sure the slab is sitting on something, if it is the limestone that is fine, I would use the compressible filler around the three walls. Do use a vapour barrier although if you don't you are unlikely to encounter any problems. You just need to make sure the concrete can't drop or settle and remains stable with the walls, which means you need a connection to the foundations,  this need only be 100mm wide or less it isn't providing strength just a connection often that is an inner row of bricks in your case probably the limestone.

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## Kieran989

Hi John, 
Slab will be poured on compacted fill sand. The finished slab level will only be 40mm or so below the floorboards so it will cover the lime stone foundation edges but i'm not sure if I would consider this supported by the limestone?. Is this what you are referring to when you mention "connection to the foundations"? 
Why do you suggest not trenching on the open edge? There will be a stud wall built on this edge so part of the reason for trenching was to provide additional support for the wall by encasing the bearer in concrete. 
Also if I use compressible filler at the edges, how would I then water proof the wall/floor transition?

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## droog

As it is already mostly dug out, bearers, joists and cement sheet flooring. Easier that trying to pour a slab.

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## Kieran989

Hi droog, 
I did consider doing a new subfloor. But considering it would need to be built at a lower level than existing to maintain floor levels, I felt it was just as much effort and cost to do barrow in some concrete for a 100mm slab?

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## droog

Depending on room layout the floor could be set with slope inbuilt, then there is very little difference in floor level needed as there is no need for screed.

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