DIY Doctor

Replacing concrete slab due to damp in floor and walls

Postby zedox » Sat Aug 22, 2020 3:02 pm

Our house was built in the 1900s, with an extension added in 1990s. We've recently relocated our kitchen to the extension and are now renovating the old kitchen to become a downstairs WC and utility room.

The "old kitchen" is suffering from damp in the walls and concrete slab. The concrete slab is about 1" thick, is sitting directly on sand and earth and does not have a DPM. It is also bridging the old slate DPC which is causing rising damp in the walls. The old slate DPC has also failed in some places.

We had a damp survey done and have been advised to replace the concrete slab and to "Vandex" the walls up to around 1.5m height.

I'm leaning more towards replacing the concrete slab with a timber subfloor but have some questions...
1. How far down do we dig the earth and how wide a ventilation gap should there be?
2. The room is only 2.9m x 3.4m. Do we need to build intermediate walls to shorten the span?
3. Do we have to "oversite" the earth with a sand, hardocre DPM and concrete or could we just lay a DPM on the earth?

Thanks in advance!
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Postby stoneyboy » Sun Aug 23, 2020 10:05 pm

Hi zedox
A key element of a suspended floor will be whether you can ventilate the void. If you can instal air bricks on two sides then a depth of around 500mm below floor level will be more than enough. You will have to build sleeper walls on foundations on each side and incorporate a DPC on each one. Since the room will be a utility room a central sleeper wall is advisable to limit floor bounce from your washing machine.
Regards S
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Postby zedox » Fri Aug 28, 2020 8:05 pm

stoneyboy wrote:Hi zedox
A key element of a suspended floor will be whether you can ventilate the void. If you can instal air bricks on two sides then a depth of around 500mm below floor level will be more than enough. You will have to build sleeper walls on foundations on each side and incorporate a DPC on each one. Since the room will be a utility room a central sleeper wall is advisable to limit floor bounce from your washing machine.
Regards S


Thanks for your reply and advice
I have dug down 350 from the DPC and I’m at the base of the foundations. Assuming I shouldn't continue digging lower than the foundation?
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Postby zedox » Fri Aug 28, 2020 9:34 pm

We can only install airbricks on one side which is external facing. But could leave a small gap in the floor along the opposite wall to promote air flow.

We’ve dug down about 35cm so far from the DPC and are now at the bottom of the wall footings.

Great tip on the sleeper wall. Can use smaller timbers this way too!
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Postby stoneyboy » Sat Aug 29, 2020 9:33 am

Hi zedox
If the room the other side of "the opposite wall" is on a suspended floor this should provide adequate ventilation. Probably better not to dig lower than the bottom of the foundations, you can use face fix joist hangers if there is not enough width on the existing foundations for the outer sleeper walls.
Regards S
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Postby zedox » Sat Aug 29, 2020 5:36 pm

stoneyboy wrote:Hi zedox
If the room the other side of "the opposite wall" is on a suspended floor this should provide adequate ventilation. Probably better not to dig lower than the bottom of the foundations, you can use face fix joist hangers if there is not enough width on the existing foundations for the outer sleeper walls.
Regards S


Hi Stoneyboy
Thanks for taking the time to reply, I appreciate it! The opposite wall to the external wall is the party wall with my neighbour. Of the 4 walls, 1 is external and 3 are internal. The opposite wall to the external wall is a party wall with my neighbour. The other 2 walls are internal, 1 backs onto the dining room (suspended) and the cupboard under stairs (slab). The other internal wall backs onto an extension, also on a concrete slab. We could only get cross ventilation via the floor in one corner of the room. But I could disguise a gap along the floor behind cupboards etc to help?

We've dug down 400mm from desired floor height to the bottom of the wall footings. Not going any further. We're actually about 200mm below external ground level. The soil is very damp, you can see moisture on the surface.
I'm not sure what ground covering is best here? 100mm concrete oversite or 50mm of fine aggregate on a 1200dpm. I understand the floor covering should be above ground level but we don't have the height to do this.
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Postby stoneyboy » Sat Aug 29, 2020 9:03 pm

Hi zedox,
Suggest you cross ventilate using the corner location and lay 100mm concrete over site. This will allow you to build sleeper walls (these will probably be 150mm concrete blocks laid flat) direct on the over site.
Regards S
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Postby zedox » Sun Aug 30, 2020 10:20 pm

Thanks!
Should I be concerned that the top level of the floor covering concrete oversite will finish below the external ground level?
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Postby stoneyboy » Tue Sep 01, 2020 9:07 pm

Hi zedox
Provided your external paths/surround slope away .from the house there should not be any issues.
Regards S
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