Alternative for DPM

[Fredxxx]

I had part of a 9" wall knocked down for a doorway. Slightly below floor level are two layers of polythene membrane with a layer of floor levelling screed to make up to floor level. I would say the screed was

5-10mm thick.

As I had anticipated, the floor has cracked, presumably from voids in the polythene which never laid perfectly flat before the thin screen was put on top.

I was thinking of removing the screed and removing all but 1" or so of the polythene and painting liquid DPM above and below the membrane before applying new screed.

Is that the right approach? What liquid DPM would anyone recommend?

[Andy Burns]

I've used the Wickes one (mixture of rubber and bitumen IIRC) when building-up a porch concrete floor by 1/2 to 2 inches, the underlying concrete was a bit questionable, ants always seemed to find it easy to burrow around the perimeter, the odd seedling could appear in cracks at corners etc.

Painted the whole floor, and about three coats around the bottom edges up to about skirting height before pouring the levelling compound.

No problems since.

[Tim Watts]

Not bitumen! It's horrid to try to get anything else to reliably stick to it.

I used this one:

F Ball Stopgap F76.

The as per their instructions, primed with P131 then topped off with Stopgap 300 self levelling compound. That was my entire kitchen and it's been rock solid for years now.

The only question is whether your substrate is friable or not. If so, the F76 will just shear off in sheets with a layer of powder stuck to it.

The other problem is the F76 is only available in non tiny quantities and you need to mix 2 batches for 2 coats, so you will need to measure the 2 parts out accurately to make 2 batches.

There may be other makes which are available in smaller quantities which should be as good - but for topping it off I cannot recommend P131+Stopgap 300 highly enough!

[Fredxxx]

I presume you mean this stuff. It seems cheap enough!

This is just for a very small area just 10" x 36" or so.

Many thanks.

[Fredxxx]

The area I am talking of is around 10" x 36". The smallest quantities I can find F76 can be purchased is 5kg quantities which has far more covering power than I need and so will be very wasteful as well as being very expensive.

I exspect to find mortar under the polythene.

[Tim Watts]

Hmm - there must be another source of DPM epoxy in sensible quantities.

OTOH I recall now I had a similar bit in the hall floor after removing a wall too.

I soaked it in 1:1 diluted SBR several times over, generously until it would not absorb any more.

Have not had any problems with that bit - though it's tiled over with slate, which isn't too fussed by damp anyway.

What's going on top?

[Roger Mills]

Could you take the opening down another brick course? That would enable you to use a polythene membrane with a decent thickness of sand and cement screed (as opposed to levelling compound) over it. That shouldn't crack.

[Fredxxx]

I presume a few coats of SBR are as good as a DPM, which is what I'm looking for.

I have some floor levelling compound I can put on top. I was wondering about adhesion and if I should leave the last coat of SBR tacky when applying the levelling compound.

For time being the levelling screed would be subject to traffic.

[Fredxxx]

That would be very difficult. I'm not sure of the value of doing this as at either end of the doorway the polythene membrane would still be just below ground level? The opening is only 3ft wide.

[Charles F]

Similar problem here - 1970's bungalow where the original floor slab DPC was a painted bitumastic coating, with a 25mm-ish screed above. We removed several internal walls, which had been built on the original concrete slab, each with their own strip of bitumastic felt sheet DPC. I removed this, and painted the base of the slots where the walls had been with the Wickes liquid DPC, and then backfilled with mortar after painting all faces of the slot with PVA. So far (five years) all is well - no cracks, and no damp.

Charles F

[Tim Watts]

Not as good, but possibly sufficient for very small areas like yours if the top layer can either breath (carpet) or is water resistant (adhesive include).

But it's not really a recognised technique - I "got away" with it because there weren't really any other options.

Same as I couldn't take the epoxy DPM under the fire place in the kitchen, so I soaked that with SBR and topped with SBR screed, stopping the DPM around the hearth area.

Another poster's idea of remove one brick course and add DPM then fill - I'd use a concrete fill - but same idea is probably better.

Don't know about the others but I had Stopgap 300 exposed for a while to light traffic and it was OK. It's pretty hard.

[tabbypurr]

Bitumen. Once its painted on, sprinkle with a little sand and the new cement will bond to it nicely.

NT