When to cast floor slab - before or after demolishing walls

[Adman101]

Hi,
I am looking at doing an extension next year. I’ve had the plans done and am looking at planning how the build will go. I am fairly au fait with basic building processes, but part of my build will involve demolishing two ground floor walls which meet at corner. You can see from the attached drawing that two RSJs (coloured cyan) will support the above walls of the existing and new walls. The magenta area shows the walls to be demolished. All of that I understand, it is how a builder would undertake the works with regard to the floor slab. Would they build the shell, put the RSJ in and then demolish walls and then cast the floor slab? Or cast the floor slab, build the shell, put RSJ in and then demolish walls and then patch up the footprint of the demolished walls to match the cast floor slab? My concern is there will be cracking in the tiles at the joint of the slab if the latter is used.
What are peoples thoughts?
Many thanks
Adam

Untitled

• Adman101
• 20 Sep 2013
• 1

 

[noseall]

We always pour a floor slab so that a floor screed can be added later on and finished to the desired height.

The height at which we position the slab depends upon the homeowners requirements, i.e. whether they are say having UFH or not, which in turn influences the position of the floor insulation.

In some instances we position the floor insulation under the concrete slab, in others we pour the slab and set the insulation on top or beneath the finishing screed, particularly relevant when having UFH fitted.

Regardless of floor insulation, the option to have a later addition of a floor screed is there. It is a convenient and seamless way of matching up all new floor surfaces to the existing.

A typical floor screed would be 75mm thick and would be laid once all knock-throughs are completed. Where the screed crosses over where the existing walls once stood, the walls are taken down below floor level and the trough lined with polythene and filled with screed.
A diligent builder may even remove enough masonry to add a bit of floor insulation also.

 

[Adman101]

Thanks Noseall, I am planning to have UFH so would have the insulation over the slab and then screeded.

Is it likely to see problems with differential settlement in the form of cracking between the new slab and where the wall once stood. We are planning to have stone tiles and i am a little concerned about them cracking thats all.

 

[noseall]

There is always a risk of differential cracking.

You could take measures to avoid it like removing enough existing masonry as to allow the flooring insulation to pass onto the masonry, rather than but up to it.