Old concrete floor no DPM - options

[Stu666]

The garage in my 1936 build house has an old concrete floor with no sign of a DPM. It looks to have been extended and patched over the years, so it's a little rough and wonky in places. While I haven't ever seen any pooling water, there is a damp feel in general and anything that gets left on the floor seems to have a moist patch under it within a few days, which I'm sure is a sign of an absent or failed DPM.

I am planning on using it as a gym eventually, potentially with a rubber floor. However, I want to address the lack of DPM before continuing.

I've seen some suggestion to lay a 1200g DPM straight onto the concrete, lapped up over the existing DPC. But I am concerned this would lead to water pooling below the DPM with nowhere to go, potentially migrating where it's not wanted.

Another suggestion is to lay a screed on top of the concrete. I have at least a few centimetres to play with so this is a possibility, and it would hopefully iron out the imperfections. Only thing I'm not sure about is whether screed itself can be damp proof or if it would need a DPM below it?

I'd like to avoid digging up the existing concrete if possible.

 

[George_engineer]

If there's no route for water to get in, there shouldn't be any pooling just from dampness in the slab.

External ground levels should be 150mm below internal floor, gutters in good working order etc. Then laying a DPM on top should be fine.

 

[JohnD]

The garage in my 1936 build house .

At that age it is likely to be quite thin and poor quality. You say it is patched, rough and wonky.

With all the other work you mention it would be best to dig it up and lay new.

 

[Stu666]

If there's no route for water to get in, there shouldn't be any pooling just from dampness in the slab.

External ground levels should be 150mm below internal floor, gutters in good working order etc. Then laying a DPM on top should be fine.

It's a bit of a funny one, because to two sides of the building, the external ground level is below the slab. To the other sides, it is level. I suspect this is a hangover from when it was actually used as a garage and cars parked inside. For the level areas, there is a bit of a slope away, but not much. Not surprisingly, these are the areas registering highest when tested with a damp meter.

At that age it is likely to be quite thin and poor quality. You say it is patched, rough and wonky.

With all the other work you mention it would be best to dig it up and lay new.

On closer inspection, the areas I thought were patches are actually where foundations were put in around the perimeter when it was converted to an attached garage, probably during the 1980s. I'm told they were dug quite deep in order to support a future first floor extension, which hasn't yet materialised. To be fair, it seems to be in decent condition with no cracks, just the levels are a bit wonky in places and it's not as smooth as you'd expect a modern slab to be, as in lots of pebbles in the mix. There's a small hole in one corner which looks to have been made deliberately. From this, I estimate the original slab is 3.5" thick.