DPC - what is going on here?

[shabbaranks]

Hi guys,

we're having a conservatory built and its been one problem after another. We asked a second opinion as to if this builder is doing it properly and the reaction was there should be a dpc down. If said this to the builder and he has replied there is - the problem is they are using 2 different types of materials and I dont know who is correct. Could someone have a look at these pictures and tell me who is correct?

 

[xdave]

That black plastic is a DPC, and looks to be installed correctly where it has been used. The blue plastic is the DPM under the floor slab and would normally be fitted to either overlap onto the inner course DPC or laid internally and trimmed back after the slab is laid. It is hard to tell as it is just out of frame but it looks like the bottom photo shows the second approach. When the bottom wall is built up a DPC will be installed on the top of the exposed brick courses to match. I don't see a problem with the DPC there - who gave you the second opinion?

However, that cavity would normally be clear for two courses below the DPC so water getting inside the cavity has a way to drain outside and not to the inside above the DPC. Often conservatories are built to match the DPC of the existing property and from the first photo it looks like the wall is built off either a concrete slab or blocks with only one course of brick to get to DPC height. You are very unlikely to have a problem here (millions of walls have been built like yours) but it is lazy of your builder not to keep the cavity clear. (That photo might be misrepresntative of the built up cavity though - if that is clear then he may be doing it as he goes along.)

I would expect to see some sort of sleeving around the cables where they are under the slab. I can't tell from the picture if there is some.

 

[shabbaranks]

What they have done is not cut back the blue DPM so if you can imagine it - the DPM is layed across the floor concrete on top, then it goes under the first set of block work (internal dwarf wall) across the cavaty and then under the second set of block work (external dwaqrf wall) and is hanging off the outside wall ready to be cut.

The wall is built on top of the concrete base, and the second opinion was given my the father in law who used to be a builder before he got a bit too old for the job

We're just worried that the guy is taking the piddle as Im no builder but I can see when something looks right or wrong.

 

[noseall]

The dpm (blue) laps onto the internal masonry only and is then bedded onto the wall along with the (black) dpc.

The blue dpm should NOT cross over the cavity onto the extenal masonry rather it is trimmed off so that no dpm is obstructing the cavity at all.

Cavities should be kept clear.

The builder will probably trim the blue plastic when he beds the dpc.

 

[xdave]

Is the part that is layed across both skins of the wall the bit shown in the bottom photo, or has it occured where the brick skins have been built up higher? It must not bridge the cavity unless it has been closed off.

That bottom photo looks fine - it just shows it unfinished. It is normal practise not to trim the DPM back until the wall has been built, or any openings (doors and windows) are completed. If you are having a door there then it will typically sit mainly within the outer skin so the cavity will be closed off around the opening. At floor-level you could run the DPM over the closure and under the frame to keep continuity as you wouldn't have a cavity there. You would need this or the floor covering/screed would bridge the DPC as obviously the floor would run right up to the door frame.

If the DPM has been carried onto the outer skin where there is not an opening then any water in the cavity will sit on this and could soak into the internal skin leading to damp patches.

What has been the construction up to the photo point? Was a new slab laid, the walls built off of the slab and then a further floor slab laid on the DPM? Or do the walls sit on separate foundations? Is it built off of an existing concrete slab that was there before the conservatory?

 

[shabbaranks]

Sorry guys, been caught up. Ive drawn a very basic diagram of how it is the top detail is looking face at the wall and the bottom is a cut through of how they have done it.

As you can see there is a gap in the cavaty from the top of the DPM and up through the dwarf wall.

 

[noseall]

The concrete has been laid too high in relation to external ground level.

This means there is no sub floor cavity trough.

Not the end of the world on a connie but we would be requested to take it down on a regulated build.

 

[simonjay]

As you can see there is a gap in the cavaty from the top of the DPM and up through the dwarf wall.

The cavity should extend below the DPC level as has been explained previously. If the cavity is bridged right across at DPC level, this is wrong, and it seems strange the builder would do this.
However, is should be easy enough to cut this away inside the cavity before the insulation goes in the dwarf wall.

NOTE: there is a method where a cavity tray goes across at DPC level with weep holes in the outside leaf (like what is done over doors etc.)
In fact this may be required in areas with Radon gas.
This cavity tray can be formed from DPM or DPC material, but it should to slope downwards towards the outside leaf. It doesn't look like this is the method being used here though.

 

[simonjay]

The concrete has been laid too high in relation to external ground level.

This means there is no sub floor cavity trough.

Not the end of the world on a connie but we would be requested to take it down on a regulated build.

Seems like they could just about have one brick high trough, or put in a cavity tray draining to the outside.
Also, have they got 150mm from ground to DPC ?
Simon.

 

[shabbaranks]

Will try and get a better picture when I go home. As I say, we're not really happy with the work carried out and we've said we're certainly not going to pay anymore money until the problems are resolved.

Just out of interest and a rough figure - how much would it cost to make a concrete base 3 meters by 5 meters and with a depth of just over a meter - say meter and a half max. He stated that it would cost in the region of 5k is this correct?

 

[r896neo]

this is confusing

how much would it cost to make a concrete base 3 meters by 5 meters and with a depth of just over a meter - say meter and a half max

Do you mean they were strip footings 1m deep? Obviously you don't have a 1m deep raft?