laying dpc should this be on mortar or not?

[browfish]

I am ready to lay the DPC before building my walls up. I have laid the blocks work up to DPC and installed the concrete slab. The slab DPM over laps the wall. Questions are

a) should the DPM be fitted above or below the DPC roll?
b) should there be mortar, below, between, and above each layer? eg 10mm mortar, DPM, 10mm mortar, DPC, 10mm mortar, block? If not, how should it be done.

 

[tony1851]

The books say there should be mortar both under and above the dpc. I cant see how it matters - most brickies just seem to lay the strip directly on the brick and mortar on top. With mortar above and below, you could end up with a very thick joint, and the mortar underneath might start going off before you finish above it.
Don't put anything between the dpm and dpc.

 

[theoldun]

Way to go.
If top course is brickwork laid frog up, then flush frogs in.
Lay poly dpm over brickwork and cut of flush with back of brick or block.
Lay tight muck bed on top of poly.
Lay dpc,
Stick brick on end to hold dpc in place, then run a brick down and along top of dpc to squeeze dpc down into a 7mm bed. Cut of surplus muck.
Lay first course above dpc with a tight 7mm bed, then adjust bed joints above to reach the gauge you require.
Regards oldun

 

[^woody^]

If you want to lay DPC properly, then lay it on a 10mm bed which is smooth and flat with no troughs or towel marks.

Then lap the Dpm over dry, then lay the next course on another 10mm bed as normal

 

[theoldun]

Not correct woody. .Suggest you read NHBC Standards, Section 6.1 page 11 and 21 and Section 6.2 page 21. Clearly states dpc on top of dpm. 99% of new builds are built like this.
By the time you have laid 2 or 3 million brick you have learnt how to lay a flat bed under dpc..
Regards oldun

 

[^woody^]

The NHBC show an illustrative diagram with no supporting information to back it up

 

[stuart45]

Could be a regional thing. In the South East the DPC on the facework was always bedded, but in the South West is often laid dry on the brickwork which does usually look neater than some of the Sunday joints you see at the DPC.
Saying that I have always done it like Oldun except back in the old days when the DPM was often cut flush with the oversite.

 

[theoldun]

The NHBC show an illustrative diagram with no supporting information to back it up

Wrong again woody, supporting information is in black and white.
Why not try reading supporting information?
Should a knowledgeable person require supporting information to understand a simple illustrative diagram or drawing?
Regards oldun

 

[noseall]

DPM first then dpc.

Why would you lay it any other way?

We tend to cut the dpm so that it is only 50mm or so lapped onto the brickwork allowing a large margin of mortar - masonry contact from the dpc bedding whilst not detracting from the performance of the dpm lap.

 

[^woody^]

Err no, it does not detail the chances of the dpm tearing on the sharp arris, or the route for moisture. It's a poor detail

 

[noseall]

Err no, it does not detail the chances of the dpm tearing on the sharp arris, or the route for moisture. It's a poor detail

Baffled?

 

[^woody^]

It was a reply to the post before yours - ie the nhbc diagram is not definitive

 

[browfish]

Right, now i am confused.
I like the idea of only lapping the DPM by 50mm. It has concerned me that the only thing holding the wall in place is its own weight as the mortar surely can not get a good key on a smooth peice of plastic.
It looks like i fold over the DPM onto the breezeblock, lay a thin mortar bed, lay DPC and then lay bricks as normal with a 10mm mortar joint.

Any idea at what level i should start the wall ties. On top of DPC? Should the celotex insulation start at DPC or go below?

 

[jeds]

It's not simple because the proper answer to your question is you should follow the manufacturer's installation instructions for whichever product you are using. What makes it not simple is that different manufacturer's give slightly differing instructions. Most of them base their guides on the relevant British Standard - but unfortunately that isn't clear. It just says the membrane should be lapped and sealed with the dpc. What it does say though it that the dpc should be bedded on wet mortar. And Stuart45 is right. It is very common practice to lay the dpc dry in my neck of the woods - in the South West that is. But I don't personally recommend it.

If you take Visqueen as an example; they do not recommend lapping polythene membranes with dpc products at all. Their advice is to use a dpc cloak which should be sealed with the dpc using sealant tape and then similarly sealed with the membrane. Of course they might just be trying to sell more products. But that's their advice so if you use their products you should really follow it.

My advice is bed the dpc on a tight bed and lap the dpm over by about 75%. That way is easy to do, satisfies the requirement of BS5628 and will do the job perfectly well.

 

[^woody^]

I've had a look at quite a few tech documents about this simple thing, and there is no definitive answer