Floor Inside Patio Door - Detail of Construction Needed

Hi all - bit long this one sorry (no piccys available)
I have a damp problem which appears to be coming from the strip of floor just inside my patio doors. Originally this damp was believed to be due to a leaking pipe, so a channel to the room side of this strip has been excavated to show the pipes.
To clarify - considering the view from the inside out there is:
Floor slab Channel in floor with pipes in right on edge of slab Edge of floor dpm Inner leaf of block work with concrete capping to level with existing floor Inside face of patio door
The damp proof membrane for the original main slab ties into the wall dpc at top of main slab level. This means that any dpc/membrane below the strip of floor immediately inside the patio door (above the inner block leaf) can only have a membrane below the level of the wall dpc alongside it. I should point out that this patio door is not an original feature of the property and is effectively a window extended to ground level IYSWIM, although a patio door has been there for 20 years or so.
So I guess the questions are to do with the way this area should be made up. If this door was installed from scratch, the brickwork would be cut down below the window but what then?
1. Is the exposed cavity filled with anything down to foundation level (concrete)? 2. How is the dpm for the room slab connected to dpc under this strip of added "floor"? 3. Should the floor strip be just concrete, cast over dpc? 4. How is the concrete strip supported over the cavity, if the cavity isn't filled?
Having poked about a bit, the damp appears to be coming from the area outside the main floor dpm, under this strip of floor. Under the concrete strip there seems to be loose sandy stuff. I am looking at a small area alongside the pipe channel about 600 (2') long. Although there is some dpm under the strip it is clearly damaged in parts. The insurance company put some driers in over the past fortnight, but within an hour or two of them being turned off, damp came back to the sandy area beneath the floor strip. If there is no "active" water source (i e. if it were down to ground water only) would damp reappear that quickly?
Anyone who's read this far deserves a medal - any help or advice appreciated.
TIA
Phil
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TheScullster wrote:

I think I understand.

yes.
I can't make head nor tail of any of this.

It can be but usually isn't, the cavity is normally filled with 50mm polystyrene up to about 50mm below the floor surface, then faced up with mortar / concrete

What do you mean, 'added floor'?

I assumed the floor slab was poured in one?

See above.
There's a sketch here to show how it should be:
http://tinypic.com/view.php?pic=2ztccns&s=5
click on the pic for a better view.

The sandy stuff is sand blinding, this is put underneath the membranbe so that it doesn't puncture during the concrete laying.
If this patch is an addition to the floor, IE laid long after the main floor slab and not part of it, then it will need to be dug out and replaced, so that you can insert a suitable membrane below it.
It's possible that water is getting underneath the door frame, have you checked the sealant underneath the patio door?
--
Phil L
RSRL Tipster Of The Year 2008
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"Phil L" wrote

Phil
Thanks for taking to time to read and respond.
Yes, as the patio door was not an original feature, the strip above the inner blocks was added with the patio door.
I haven't excavated serious amount of this floor strip out yet because insurance company is involved and maybe I can get them to do it. From the little I can see though, some of the dpm is damaged where it wraps over the top edge of the inner block and damp is concentrated in that area . So the only accessible bit of dpm at this point is the top broken edge emerging from between the floor slab and the inner block. My question is: how do I attach dpm to this top edge to lay over the inner block?
Also there is sand above the dpm rather than below it from what I can see.
Hope all this makes more sense.
Phil
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thescullster wrote:

The inner block should come up to floor level - if someone in the past has broken the top edge of this block off for some reason, it's almost certainly rising damp coming up from below ground via the blocks and a DPM isn't going to seal properly on a broken block.

underfloor DPM is normally overlapped by 500mm to keep damp out of the concrete. It seems like someone has taken up a strip of concrete where the floor meets the wall, possibly to make a channel for the pipes, and because the inner block is easier to remove than concrete, they've taken this out and then not joined the DPM properly to the existing underfloor DPM.
To do it properly, they obviously wouldn't take up 500mm of concrete in order to get the required overlap, so it should have been overlapped as much as possible, say about 100 - 150mm and glued, either with silicone or some other waterproof sealant.
this is what will need to be done now if it's ever to be dry again.
And if the pipes are copper, they will need to be wrapped in PVC tape prior to concreting.
--
Phil L
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"Phil L" wrote

Phil
Thanks for all your help with this. Knowing how things should be will help when the insurance contractor comes back - particularly if he starts spinning yarns/spouting the BS.
Phil
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I think this could be summed up as "bridging the cavity at a doorway". On my build, there will be 65mm screed on top of the concrete slab, and this needs to pass over the cavity to form a continuous floor across to the door which is fitted in the outer leaf. The inner blocks come up to the top of the slab, but not to top of final floor level. The DPC from the walls is cut off at the door opening. The DPM from the slab is actually long enough to cross the cavity. But I was recommended to bridge the cavity with slate, then screed on top of this across the cavity. The cavity closer will be fitted before the screed and slate, and act as a sideways damp course between the side of the brick opening and the screed. I can see there are a few "corners" where damp could leak. I may paint the area with liquid rubber just to be sure. The earlier advice to have the inner leaf up to the floor level would not work, since there would be no room for any screed on top, and any self-levelling compound over screed and onto bricks / blocks would forever cause problems. Basically, the screed must be seemless across the cavity to the actual door frame / threshold. Simon.
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Thanks Simon I think you have managed to clarify the situation somewhat here. Can you explain what this cavity closer business is about please? Is it the polystyrene cavity fill that Phil mentioned and showed in his picture? Can I do all this without removing the patio door? The above-slab screed appears to go up to the face of the door sill rather than under it IYSWIM. Also the patio door has a clearly visible black dpc under the sill. Presumably the slate that you mentioned sits on top of the inner block and across the cavity. In my case this would have to perch on the edge of the outer block (assuming there is room left before the patio door frame). Are you abandoning the dpm over the cavity completely - if not, where is it going? Are you talking about painting the slate and brick with rubber, or is this over the top of the screed as a final sealer?
TIA
Phil
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At the edge of an opening, the cavity must be closed somehow. Old method was the inner leaf turned 90 degrees and met the outer, vertical DPC put in where they meet. New method, a top hat shaped insulated strip is pushed into the cavity and the two flaps are used to fix it to the blocks/bricks either side. This is kind of similar to the polystyrene cavity fill, and will block the path of damp.

Depends. If the door is only fixed to the outer leaf, you should be able to remove the infill right up to the door. If the door/sill overlaps an infilled cavity, it might be trickier. The screed will go up to the door sill, since the sill/door is commonly set lower down in the outer leaf, so there is less height to step over to get out ! Also, screed going under and exposed to the outside would not be good idea !

Yep, there should be a small bit of outer leaf remaining to sit the slate on (assuming standard PVCu profiles). Else, the infill material in the cavity would support it. If DPC under the (plastic ?) door, the slate would sit on top of this. But can't see the point in a DPC under a plastic door really.

Well, slate would act as a DPM under the cavity by itself, but note that the DPM from under the slab should tie into the wall DPC. In this case I would lap a length of DPM across the cavity and under the slate. This maintains the waterproof barrier

I meant since the DPC / DPM detailing is a bit fiddly around the doorway, with various corners etc, I would paint the sides of the inner leaf a little way up, and the inner leaf bottom edge before lapping the DPM onto it, just as a precaution. But I bet no builder would be so fussy !
Hope that helps, Simon.
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snip................... I meant since the DPC / DPM detailing is a bit fiddly around the doorway, with various corners etc, I would paint the sides of the inner leaf a little way up, and the inner leaf bottom edge before lapping the DPM onto it, just as a precaution. But I bet no builder would be so fussy !
Hope that helps, Simon.
Brilliant - thanks for the detail Simon! Don't care how fussy I need to be, as long as the problem is fixed in one hit.
Phil
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