A reader recently sent in an interesting question regarding his house renovation; the flooring and what to do with the open space below the floor of his house.
I recently purchased a house that needs some renovation. My first task is fixing up the flooring. A section of the house has wooden floors, with 1-2 meters of open space below it. The previous owners didn't pour concrete at the bottom of the open space, and the wood started rotting from under the house.
I need to repair it now, but was thinking of losing the wood and trying for concrete. I had a builder come in, who advised me to not fill up and pour concrete in the empty space below the house, as this would cause moisture to build up and this would show on the outside walls. My second option would probably be to do concrete slabs, but I can't find an estimate of what it would cost. Is it possible to do concrete slabs, and what would I pay per square metre? I would also probably need to change the pipes, as certain pipes are currently running underneath the wood.
Les responds as follows:-
It is standard building practice to fill in under floors and to pour a concrete slab on top of it. Without seeing what is going on with the existing walls below floor level, there are a few precautions that I would advise as well as doing a few things during the process.
If you decide to go the route of filling the floor and casting a concrete surface bed, then I would advise that you get someone to assess the condition of the outer walls as far as their ability to retain 1 to 2m of soil. The big issue is that the fill must be compacted and if the walls are in a poor condition then they may not be able to retain the fill.
Proper compaction of the filling will ensure that the slab does not settle over time which will occur if the fill is poorly compacted. If the walls are in good condition then this is not a problem.
The next point would be to ensure that a damp proof plastic sheet, min 250 micron, is placed under the slab and that the edges are correctly sorted so that the moisture issue is taken care of. I would further advise that you place 25mm polystyrene sheets on top of the plastic layer to insulate the slab before casting the concrete. This will make a big difference to the temperature in the room in both summer and winter.
It would also be advisable to place a layer of damp proof plastic against the inside of the outer walls as you fill the void with sand in order to prevent moisture from the fill from tracking through the wall and showing on the outside of the wall.
Alternatively, installing a suspended concrete slab will require support around the edges which one can either achieve by cutting pockets into the existing walls (toothing) or by building an additional skin of brickwork next to the existing walls to support it. This also supposes that the foundations are wide enough to support the extra brickwork. The rib and block method will be the easiest in the suspended slab line and you can budget on between R700 and R800 per sqm. (Cape Town prices)
With either method, I would advise that you seek the opinion of someone who has no vested interest in either construction method and who can thus advise you on which one would best suit your application and your budget.
I trust that this will be of assistance to you.
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