Question on Damp Proof Membranes

awishawash

Hows it goin everyone, I have a question on damp proofing when building walls. When a 4 inch dpc is rooled out over the first courses, it surely now stops any moisture from going above or below that point. What I'm getting at is when the bricklayer beds his next course down over it, how is it possible for that course to be stuck to anything? The mortar doesnt reach the joints below so theres no continuity.Surely this weakens the whole structure. Also the blocks below the dpc are almost always damp/soaked so how do they not crumble over a long period. I must be missing something very obvious here.

Mellor

The blocks below the DPC would often be damp, but they going to be weakened by this to the extent that they would fall apart.

As for there being no bond between the first courses, you're right. But this isnt an issue. The mortar is used to hold the blocks together, not to hold them to the course below. This is a very slight difference, and as they are held together as a whole structure they are very secure. One section cant slide out of place without the rest moving too. And the sheer weight of all the courses above the DPC is enough to hold the bottom course in place.

ircoha

Mellor wrote:

The blocks below the DPC would often be damp, but they are not going to be weakened by this to the extent that they would fall apart.

As for there being no bond between the first courses, you're right. But this isnt an issue. The mortar is used to hold the blocks together, not to hold them to the course below. This is a very slight difference, and as they are held together as a whole structure they are very secure. One section cant slide out of place without the rest moving too. And the sheer weight of all the courses above the DPC is enough to hold the bottom course in place.

I preseume the are not was left out by mistake.

just one comment on the dpc, it needs to be wide enough to stop damp rising via the finishes

Viking House

Hi awishawash

I also don't like that plastic DPC so we have started to use a Bitumen based sticky one from Germany lately. When you kick a low wall built on that plastic DPC it always breaks at that point.

www.viking-house.net

awishawash

wow thats funny. So the structure is completely dependent on its weight?I'd love to know what the procedure is for countries with high risks of natural disasters, they wouldnt stand a chance.
Another thing - when the render finish is applied 18mm thick or whatever right down to the ground, is that not bridging the damp up over the dpc? On some houses there is a lip at the bottom, around 150mm up, which I presume is where the dpc is, but there is still the 'splashback' area below that which has a thin coat of sand cement, (I think thats carried the whole way to the top as a scratchcoat, but just not scratched down there), surely thats gonna bridge moisture up the wall? What about finishes that run right the way below the soil? Are the supposed to stop an inch or so up via a stop bead?

awishawash

awishawash wrote:

wow thats funny. So the structure is completely dependent on its weight?I'd love to know what the procedure is for countries with high risks of natural disasters, they wouldnt stand a chance.

In earthquake zones the structure is tied to the foundations, sometimes via resilient mountings that flex, it's the sideways movement that really does the damage!

Mellor

Deleted User wrote:

In earthquake zones the structure is tied to the foundations, sometimes via resilient mountings that flex, it's the sideways movement that really does the damage!

Also, countries that suffer from earthquakes don't suffer from as much rain as use. So the DPC is not always used.

On some houses there is a lip at the bottom, around 150mm up, which I presume is where the dpc is, but there is still the 'splashback' area below that which has a thin coat of sand cement

Yeah the DPC is here, above the plinth. It has to be atleast 150mm up from the ground, but depend on the way the courses run it might be slightly more.
If done right the DPC should come through the render plinth and be hidden by a bellcast,

squire1

Mellor wrote:

Yeah the DPC is here, above the plinth. It has to be atleast 150mm up from the ground, but depend on the way the courses run it might be slightly more.
If done right the DPC should come through the render plinth and be hidden by a bellcast,

So, if I am putting down a patio or footpath (I believe should be 150mm below DPC, which I cant see) then if I put them 150mm below plinth level I should be OK, right?

awishawash

That sounds right squire1, according to Mellors post. Maybe make it 160mm to be on the safe side. Thanks for the replies lads, much clearer now thanks.

Mellor

yeah, thats fine.
If you get down and look up under the plinth you could possible see the very edge of the DPC,

Once heard a story of a self builder, who didn't want the edge of his DPC tray sticking out from under his window cill, so he tried a quick fix and burnt the protruding edge off his DPC with a lighter. It burnt and disappeared right back to the brick. He went in for a cup of tea and when he came out there was smoke coming out from the top of the cavity. Seams the insulation caught on fire too. Quick fixes, they rarely fix the problem, and in the long run aren't really that quick.