Timber floor to Concrete

24 May 2019
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United Kingdom
Have a large rear extension which will be knocked through into existing rear rooms of house. New floor will be concrete, membrane insulation and screed. Existing is 1940's suspended timber floors. Although an older house it is cavity construction, brick outer and brick inner with a slate dpc. I'd like to remove the suspended timber floor and make a continuous concrete floor which lends itself well to underfloor heating for the whole room. It won't be possible to lap the membrane in the old part of the house into the brickwork with dpc. any other ways around this to ensure no moisture rises up walls from below membrane. Thanks in advance.
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So you want to remove an existing suspended timber floor.
Then put in concrete floor?

Just lay the DPM, allow it to run up the wall a bit.
Leave the excess until you have laid insulation screed and maybe even floor covering.
Then cut dpm so it finishes behind the skirting.

Between the new extension and original house you will have the to bottom of the wall running across.
Lower the brickwork to top of concrete, so the insulation runs over the brickwork.

The DPM can be lapped over the brickwork and taped to the other.
If the cavity is only 50mm or so, I would just run the insulation over it.

If you are building the extension now, make sure you deal with vertical damp arrangements and cavity trays as required
You need to consider through-ventilation to the rest of the timber floor void.

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