Concrete slab on mud under suspended timber floor?

[Chris_123]

Hi,

I'm renovating an old 2 up 2 down terrace house and need to replace the downstairs timber floor. An old extension on the back blocked the rear bricks (this is going to be replaced) and as such under the floor the mud is damp and wet causing the old floorboards and joists to rot.

A builder has quoted to lay 100 mm of hardcore, followed by sand, membrane, and then 100 mm of concrete on the mud. Then rebuild the sleeper walls, new joists with insulation and then chipboard.

My question is if the concrete slab is required? Surely what is important is the membrane so can't this be placed without the concrete and hardcore?

The mud is about 500 mm below the floor level. Total ground floor area is about 28 m2, quoted cost of work is £ 3400 + VAT. Do people think this cost if fair?

Thanks for your help,

Chris

 

[JohnD]

You say the ground is wet. What are you going to do about the source of the water? Is it a leaking water pipe, or a leaking drain?

 

[Chris_123]

Its the ground water I think, not a leak.

 

[vinn]

Pics of the outside and inside of the old extension would help - so would pics of the main house floor work, why? Because the rear of the house is probably providing through ventilation to the main house suspended floors & anything you do to the extn might block the through ventilation.

Are there air bricks at the front and rear of the house?

On the face of it an oversite membrane (DPM) over the soil will work but its not always that straightforward.
As above, any source for excess ground water should be found - is the house on a hill or a slope for instance?

What you claim your builder proposes is a half solid and half suspended floor - a bit OTT.

 

[endecotp]

If it's ground water, you need enough weight of concrete to hold back the water pressure. Without it, the membrane, insulation and wood floor will float!

(Or something like that.)

 

[endecotp]

Note that in principle you also have the option of putting the insulation below the concrete slab - or possibly concrete then insulation then screed. This gives the room mkre thermal mass, i.e. it will warm up more slowly but then stay warm for longer. (Practical constraints may limit what you can do.)