Building above DPC

[blindside]

Hi all,

This isnt for particular job I need to do, Just something I have been wondering about.

Why cant we build above DPC? if there is a clean cavity between internal and external skin in theory there should be no issues with damp?

Also, why have a DPC on the outside skin anyway?

Any thoughts?!

Thanks

 

[noseall]

Hi all,

This isnt for particular job I need to do, Just something I have been wondering about.

Why cant we build above DPC? if there is a clean cavity between internal and external skin in theory there should be no issues with damp?

Also, why have a DPC on the outside skin anyway?

Any thoughts?!

Thanks

The masonry below DPC is built to a standard that will accept moisture and not be affected by frost etc. Bricks above DPC are often not of similar standard therefore need to be protected from being permanently damp.
Also,(as with extensions additions etc) external walls can become internal walls.

 

[John D v2.0]

When you say build I assume you mean the ground levels. And yes, you could but as nose all said and you also have to fill below ground level with weak concrete to resist the ground pressure which would bridge your cavity. Also nor all cavities are as clean as you hope especially towards the bottom, plus wall ties can allow damp across if not levelled correctly.

 

[^woody^]

Why cant we build above DPC? if there is a clean cavity between internal and external skin in theory there should be no issues with damp?

Also, why have a DPC on the outside skin anyway?

If you are refering to ground levels, then you are correct, and the cavity should stop damp passing across the wall from built up ground

You are also correct that a DPC is not necessary on an external leaf.

Current thinking is still stuck back in the days of solid walls, and you will find it hard/impossible to convince anyone otherwise.

But there is a slight proviso here, in that too much, or constant dampness of the outside leaf can cause elevated moisture levels (humidity) in the cavity, which could be detrimental to the insualtion, thermal qualities of the wall, and create possible condensation issues.

 

[blindside]

Hi,

Thanks for the responses, Ill try and address all the points made.

Noseall - Point taken about external walls becoming internal. If the house was built out of suitably frost rated bricks or any that could be used under DPC would this in theory be acceptable?

John D v2.0 - When I was talking about building above DPC i was thinking in my head about paving, but like i say this isnt for a particular job, just out of curiosity. Agreed about cavities not being as clean as they should be and walls ties. I guess I'm just looking for reason why if everything is done correctly the cavity wouldn't keep penetrating damp at bay

Woody - Yes i think some people are of the opinion that we could do away with DPC's altogether. Do you think you could say that the internal DPC stops gravity taking water out of a wall aswell as stopping it raising up the wall or is that a step to far? Condensation inside the cavity is something i hadn't thought about though!

 

[John D v2.0]

Also if you build the outside above inside dpc level then the water has no where to drain to the outside. Then water could build up in the cavity. So then if there is water sitting inside the cavity it would just soak through.

 

[blindside]

Good point! I hadnt thought of that!

 

[John D v2.0]

Me neither - until just then! It's amazing how houses have improved as people found problems over hundreds of years and refined the building techniques. But it's all basic principles about balance and using gravity in your favour. If you're interested in the topic, one place to start would be this site: https://fet.uwe.ac.uk/conweb/house_ages/period/print.htm

 

[blindside]

Interesting site, thanks. Yeah i agree, i think advancing methods with technology and newer products can be a good thing, although i bet if you go to a newbuild estate in 200 years time, the houses there wont be in anything like the condition of some current 200 year old house.

I wonder sometimes about the damp course though. I may be wrong but i think were one of the only countries to use them so it makes me think maybe we could do without them, i mean as far as i know they dont have DPC's in venice!

 

[noseall]

We (cheaply) annexed a wee side porch thingy between a garage and the main dwelling whereby we stuck a flat roof over the gap between them and a door at each end. The garage was much lower than the house so the house was showing a lot more masonry below DPC than the garage. We just dabbed the walls and skimmed them. All of the plastered masonry below DPC turned brown with damp. There is a very distinct line where the damp stops. No guessing where.....?

 

[stuart45]

Interesting site, thanks. Yeah i agree, i think advancing methods with technology and newer products can be a good thing, although i bet if you go to a newbuild estate in 200 years time, the houses there wont be in anything like the condition of some current 200 year old house.

A lot of houses built 200 years ago have long been demolished. If a house is well maintained it's lifespan can be greatly increased.
I've worked abroad many years ago and have used a DPC in places like Belgium and Germany. Contary to popular belief rising damp is recognised in the USA as well.

 

[blindside]

OK, fair enough. I dont have enough knowledge to have a strong opinion on this, but for the record I'm not anti DPC, just like to know the reasons and science behind these things!

 

[John D v2.0]

Only the lucky few old houses have survived!

There's a good reason we have dpc and cavity walls here, it just rains all the time round here! And it's very humid the rest of the time.
Whereas in Scandinavia they have better insulation, etc.