Dpm /Dpc

[paulrees1]

Hi all,

Life and cash are getting in the way of a workshop build at present, but in the meantime I want to get the design work done.

There's a couple of questions on Dpm and doc which I can't get my head around, so I thought a picture might help.

So starting off with an easy one, have I got this picture right?

Thanks


Paul

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[9fingers]

Welcome Paul!

I'd give it 9/10. Usually the edge of the DPM is tucked into the DPC

Your bricks will of course rest on the slab rather than float in mid air :lol: and the slab edges are often deeper into the ground.

Keep the questions coming - there are no daft questions only folk daft enough not to ask first :lol:

HTH

Bob

 

[sjalloq]

Can I ask some dumb questions too please.

So why would you have a DPC _and_ DPM if you tuck the DPM under the DPC?

 

[Mike G]

Firstly because you have to (in a controllable building). Them's the rules. Secondly, because DPCs are higher grade, thicker stuff than DPMs, which are very hard to get to sit flat. Thirdly, DPMs never show on the outside of the brickwork because they're ugly, whereas the DPC does, and thus the point of the DPC is to make certain that the damp protection extends to the furthest out face of the masonry.

 

[paulrees1]

Ok so I'm stepping it up a bit, does the attached explain cavity walls?

I'm aiming to build a largely timber frame, so this is no use for that, but we're also contemplating adding a porch to the house. Hoping that this picture is right for both situations?

Thanks again, paul

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[TrimTheKing]

That's how I've just done mine, the pic on the right. Well, mine has insulation on top of the DOM under the slab, then sand, then slab but same DPM arrangement.

Cheers
Mark


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[paulrees1]

Excellent, thanks Mark, I've never understood it before.

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[TrimTheKing]

Can't remember if the DPC spanned both skins, think it was 1 run per skin but same layout in terms of tucking in the DPM.

Cheers
Mark


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[9fingers]

DPC must not bridge the cavity.
Bob

 

[Robert]

DPC must not bridge the cavity.
Bob

You sure bob?

On our extension I remember the builder using a very wide damp course one side to change levels. he called it a 'tray' damp course. It certainly bridged the cavity. hmm maybe the downward angle from inside to out makes it ok.

 

[9fingers]

DPC must not bridge the cavity.
Bob

You sure bob?

On our extension I remember the builder using a very wide damp course one side to change levels. he called it a 'tray' damp course. It certainly bridged the cavity. hmm maybe the downward angle from inside to out makes it ok.

Certain not as drawn above. My feeling is that IF something has to bridge the cavity, such as a lintel or window/door opening or other reason, there needs to be a cavity tray stepped down toward the outer leaf to guide any water away from the inside of the building and weep holes provided in some cases.
Best damp roofing has got to be where there is no route for moisture to pass from outside to inside.

Bob

 

[Mike G]

DPC must not bridge the cavity.
Bob

You sure bob?

On our extension I remember the builder using a very wide damp course one side to change levels. he called it a 'tray' damp course. It certainly bridged the cavity. hmm maybe the downward angle from inside to out makes it ok.

Certain not as drawn above. My feeling is that IF something has to bridge the cavity, such as a lintel or window/door opening or other reason, there needs to be a cavity tray stepped down toward the outer leaf to guide any water away from the inside of the building and weep holes provided in some cases.
Best damp roofing has got to be where there is no route for moisture to pass from outside to inside.

Bob

Bob has this covered.

The DPM meets the DPC on the inner leaf. Sometimes, such as with a building set deeper in the ground than normal, there is a vertical DPC to the inner skin, which amounts to a wide piece of DPC being set in one course, folded up the cavity face of the inner skin of masonry, and then folded into the blockwork higher up.

If a DPC spans the cavity I can't think of any circumstance in which it would do so horizontally. It would always be set in higher up the inner skin than in the outer, so that any moisture which landed on top of it would fall to the inside of the outer skin of the wall. There would generally be weepholes built into the outer skin at that livel to allow any such moisture out.

 

[RogerS]

Paul, slightly OT but what program are you using (on your phone I guess?) to make the drawings?

 

[paulrees1]

Hi Roger, i do them in ms paint, email that to myself, then post from my phone. Sounds bonkers but i find it less of a faff than posting photos to a web page or editing on a phone

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[RogerS]

Hi Roger, i do them in ms paint, email that to myself, then post from my phone. Sounds bonkers but i find it less of a faff than posting photos to a web page or editing on a phone

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That makes a lot of sense as I couldn't really see how one could easily edit on a phone.

 

[paulrees1]

Thanks guys, point taken about the bridging so I've changed that, so learning a lot as I go. If you don't mind now a follow up.

For the porch we need a ground level door (to match existing interior porch), so have I got the dpm/dpc right for that?

Thanks
Paul

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[9fingers]

Bit of a problem with the right hand diagram. The external dpc must be 150mm or more above ground level which means you should not have a ground level door. This would give no threshold for rainwater/flooding etc

Bob

 

[RogerS]

Bit of a problem with the right hand diagram. The external dpc must be 150mm or more above ground level which means you should not have a ground level door. This would give no threshold for rainwater/flooding etc

Bob

That doesn't make sense to me, Bob. If what you say is true then all the houses round here are wrong!

 

[paulrees1]

Hi Bob,

The current door is ground level, and had one on previous house too which were bifolds (that one had an "aco" I think drain fitted in front of it)?

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[Mike G]

DPCs step down at door thresholds. But Bob is right........the DPC has to be a min of 150 above FGL.

 

[paulrees1]

Hi Mike, but if the doors is at ground level isn't the dpc underneath it?

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[Mike G]

The DPC runs directly below the threshold, then steps up the returns back to 150 (min) above FGL.