Minimal damage to a DPM

[i_am_fubar]

Ideal world, DPMs are a continuous, perfect sheets of plastic (or other polymer) and will stop everything.

But in reality, they are folded and rolled for packing.
They have creases in them that struggle to seal when jointed.
They get walked on.
They have concrete and screed poured on them.

I can't see a world where the 100% continuous sealing against water and gas can be guaranteed. Even the ISS leaks a bit of air, and the engineering there is a little better than most builders.

So, my question is.... What is a 'permitted' amount of damage/leak/discontinuity and how does it manifest?

Clearly no DPM = unrestricted moisture permeation. perfect DPM = completely dry (barring atmospheric moisture absorption).

But what about a pin-prick? a 1mm hole, 5mm? a 4" slash? Anyone seen the impact of small scale damage?

 

[jj4091]

Any moisture that got through a small hole in the DPM would permeate throughout the concrete and be absorbed by it then dry out so I think you are probably worrying needlessly. The concrete would have to be seriously wet before it had an effect on the surface.

 

[Ivor Windybottom]

If you were in a rowing boat in a lake then a small hole would be a major problem.

But for a DPM in the ground it's not. If 1% is punctured then it will still keep 99% of the damp out. Unless the ground becomes completely saturated up to the level of the DPM, which is very unlikely under a concrete slab surrounded by a wall.

 

[^woody^]

If 1% is punctured then it will still keep 99% of the damp out.

Brilliant. A bit like being shot at with 100 bullets and 99 of them miss.

Oh, hang on ....

 

[^woody^]

Any DPC or DPM must be continuous. Any gap or hole is a risk.
You won't know the impact of the risk until you see the damp patch.

Rising and penetrating damp can be either immediate or more long-term depending on the site conditions and seasonal or other changes.

BTW, the sheets of plastic you mention are graded to be resistant to being walked on and having concrete poured on. The material is flexible too, to cope with movement. And for creases, plastics regain their shape if allowed to or with a little heat. So there is no excuse.

 

[Ivor Windybottom]

SImple in theory, but the reality is that there are holes for pipes and other obstacles, these are (hopefully) taped around but in reality if it was a boat then it would sink.

Also there are seams - between adjacent sheets and wall DPCs. All overlapped but not impermeable.

Even DPCs in walls are routinely pointed over, which in theory is a bridge. But it just doesn't cause a problem in the real world.

The old slate DPCs had two overlapping layers but there was still a contiguous route through the porous mortar from the ground to the wall above, through the joints between the pieces of slate.

DPM's are more moisture resistant than waterproof. You do your best obviously, but a single hole isn't going to sink the ship.