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Dpm /Dpc

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Dpm /Dpc

Postby paulrees1 » 19 Apr 2017, 08:40

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


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Re: Dpm /Dpc

Postby 9fingers » 19 Apr 2017, 11:26

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

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Re: Dpm /Dpc

Postby sjalloq » 19 Apr 2017, 12:43

Can I ask some dumb questions too please. :D

So why would you have a DPC _and_ DPM if you tuck the DPM under the DPC?
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Re: Dpm /Dpc

Postby Mike G » 19 Apr 2017, 15:20

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.
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Re: Dpm /Dpc

Postby paulrees1 » 19 Apr 2017, 23:31

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, paulImage

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Dpm /Dpc

Postby TrimTheKing » 19 Apr 2017, 23:37

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.

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Re: Dpm /Dpc

Postby paulrees1 » 19 Apr 2017, 23:39

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

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Re: Dpm /Dpc

Postby TrimTheKing » 19 Apr 2017, 23:54

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.

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Re: Dpm /Dpc

Postby 9fingers » 20 Apr 2017, 07:47

DPC must not bridge the cavity.
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Re: Dpm /Dpc

Postby Robert » 20 Apr 2017, 09:34

9fingers wrote: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.
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Re: Dpm /Dpc

Postby 9fingers » 20 Apr 2017, 09:52

Robert wrote:
9fingers wrote: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.

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Re: Dpm /Dpc

Postby Mike G » 20 Apr 2017, 12:24

9fingers wrote:
Robert wrote:
9fingers wrote: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.
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Re: Dpm /Dpc

Postby RogerS » 20 Apr 2017, 19:28

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

Postby paulrees1 » 21 Apr 2017, 16:11

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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Re: Dpm /Dpc

Postby RogerS » 21 Apr 2017, 17:20

paulrees1 wrote: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.
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Re: Dpm /Dpc

Postby paulrees1 » 22 Apr 2017, 23:10

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
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Re: Dpm /Dpc

Postby 9fingers » 23 Apr 2017, 00:19

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

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Re: Dpm /Dpc

Postby RogerS » 23 Apr 2017, 05:40

9fingers wrote: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!
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Re: Dpm /Dpc

Postby paulrees1 » 23 Apr 2017, 06:51

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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Re: Dpm /Dpc

Postby Mike G » 23 Apr 2017, 08:36

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

Postby paulrees1 » 23 Apr 2017, 09:11

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

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Re: Dpm /Dpc

Postby Mike G » 24 Apr 2017, 08:11

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