DPM over concrete + stud walls + toilet = trouble?

31 Jul 2008
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United Kingdom
Hi there. I have (possibly not thoroughly enough) researched this topic, but without much success. So perhaps somebody can help me think this one through? I'm sorry about the long-winded introduction.

I have an area of around 25 square metres which currently has a very old (1930s, if not earlier) concrete slab sitting on earth. There is no DPM under this slab. The walls are solid brick and, whilst they do have what looks like some sort of DPM in them, it is 2-3 courses of brick above the floor level.

I have removed a wooden (chip over battens) floor from this area. The battens were screwed down through the old floor covering. That was (very thin) ceramic tiles over bitumen. The bitumen clearly had once had a damp-proofing effect, but the screws for the battens had pierced this in many palces. Those screws were rusty and the tile floor damp in places.

I have further removed old (possibly original) partition walls, which were formed of (very early) blockwork faced on both sides with cement and then skimmed with plaster. These walls were sat on the slab, with the bitumen butting up to them.

I have also removed a toilet (for now), which has to go back. The drain exits through the slab. Actually, more accurately, sometime (40-50 years ago) the slab has been dug up where the toilet now is (about 1 square metre), the drain installed, and fresh (poor, very sandy) cement poured in around it, and the toilet fixed to the top of that. This area is much damper than other areas, and the "new" cement is very fragile.

Damp is a problem here - it is ground moisture coming upwards. Even before the holes in the bitumen there has clearly been a problem with damp rising up the walls, chiefly, it seems, due to the internal plaster finish bridging the damp proof course, coupled with ground moisture being forced up between the slab and the walls.

This area is part of the ground floor of a larger building. It is not possible (practically or economically) at this stage to take down all of the walls in the ground floor and take up all of the bitumen / tile which runs throughout. I have to (try to) deal with the area I've got at the moment.

To add to this problem, I've got a wooden staircase rising up from this area - the bottom step is formed from a cast-in-situ concrete block which sits on the slab. The newel posts and stringers bear on this block. The staircase cannot come out.

I need to build some new stud walls in this area. These will be lightweight steel frames with fireline board on them, as there are fireproofing issues here (part of the reason the (very flammable) bitumen had to go, apart from the tiling above it being generally a bit shot...).

After a lot of reading and talking to various suppliers, this is what I'm proposing.

1. Mechanically clean the slab.
2. Patch repair where necessary.
3. Dig out the poor cement around the drain (soil pipe).
4. Remove the (clay) soil pipe completely (I'll make a new drain connection by going out through the wall and then into the ground outside - not as neat, but removes that damp problem) and dig down to about 300mm.
5. Pour new concrete into the area around the (ex) drain / toilet, but leave a "divet" about 150mm thick, 500mm x 500mm, formed by polystyrene.
6. When concrete cured, remove (possibly burn out) the polystyrene.
6. Prime the floor then apply a liquid DPM over the whole area, including into the "divet" under the toilet.
7. Apply a water-based smoothing / levelling compond over the whole area up to the stops - need between 0mm (so perhaps 3mm) and about 50mm of levelling, but not into the "divet". This compound would be stopped 10mm shy of all the walls, and also shy of the concrete block upon which the stairs are sitting (use more polystyrene as 10mm thick "stops" which can burned out later if necessary).

The idea behind the 10mm gaps all round are to allow the floor to "breath" and to get away from having moisture being forced up the walls and then "spilling" back over into the floor. The gaps can be masked with skirting later.

OK, so far so painful ;) However the next big problem is how to construct the stud walls - or, more precisely, how to secure the bottom fixing channels in place. Clearly I can't drill, plug and screw into the floor to secure them, as this will break the DPM and cause the floor to fail!

The same applied to the toilet - there would be nothing to screw it too - but that's taken care of by pouring more concrete into the "divet" I left there - this gives me a slab 150x500x500 which is solid concrete but above the DPM, so OK to screw into, yet (because the sides of the "divet" won't be perfect) will be pretty well held in place.

I can't really do the same for all of the walls - it would mean knocking seven bells out of the slab, which (being thin) would quickly start to lose all structural integrity. At that point I'm into the territory of digging out the whole ground floor of the building, which would be completely impractical and unaffordable.

My proposed solution is simply to GripFill the bottom fixing channels of the stud walls onto the top of the top coat of levelling compound. The walls would be mechanically fixed at sides and at ceiling level, but reliant upon the GripFill at the bottom.

Does anybody have any observations about this? I know it is all a bit unconventional, but this is an unconventional building with a lot of problematic constraints.

The only other option for the studwork I can think at the moment which doesn't involve just, err, "trusting to luck" is:

(a) pre-install the fixing channels before the DPM, including driling the floor and fitting the rawl plugs, then remove it;
(b) apply the DPM material and allow to cure normally;
(c) apply additional DPM material where the fixing channels will go and, whilst it is still wet, position and screw down the fixing channels;
(d) apply additional DPM material inside the fixing channels over (and around for a few inches) the fixing screws;
(e) apply the top floor levelling screed after the studwork (but before the plasterboard), again finishing the levelling 10mm or so before the metalwork to provide a gap for ventilation should any be needed;
(d) finish the plasterboard 10mm above the levelled floor, filling the resultant gap wich a skirting which (almost) touches the floor.

I'd be uneasy about this, but it does seem to make some sense. It also makes the job of levelling the floor more difficult, as it can't be done as a single activity.

Many thanks in anticipation for any insight and/or experience anyone can offer. I also have the problem of sorting out the DPM issue where the studwork meets the exterior walls, but I think I have that one sorted.
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It also occurs to me that I could cast in situ some concrete (say 65 mm thick x 70mm wide or so) on top of the DPM, having first deliberately "roughened" the underfloor in those locations before applying the DPM to help with the key. The idea being that the metal fixing channel for the stud work would then be mechanically fixed into the top of the new concrete.

I suppose the question then is whether the concrete - DPM bond will be better or worse than simply using GripFill to secure the fixing channel directly onto the top of the DPM.

The "belt and braces" approach would have been to GripFill a wooden sole plate to the DPM and then screw the fixing channel to that. However I don't want / can't have timber in these walls.
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How'd you get on? Would love to know what you did and if it worked?
I've got similar problems with similar 'unconventional' solutions :D
Especially like the idea of the air channel around the edge. Did it work?


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