# Parapet & coping stones



## Irish Rover (8 Jan 2020)

Hello

First post with a question.
We have just had a single story extension built with a glass lantern and a parapet wall topped by coping stones.

After heavy rain, damp patches started appearing on the internal plaster. The builder gave lots of excuses. I asked him if a cavity tray had been fitted and he initially said yes, changing it later to 'we don't fit cavity trays anymore we put .... in instead' (I can't remember what he said they put in instead)
Also there are no weep holes, which he said was ok.

They covered the coping with plastic over Christmas and the damp almost dried up.

He has come back to us now, claiming he has sacked one of his workers because it turned out he skipped putting a cavity tray in because he forgot to bring it with him and going to collect it would have made him late getting home.

I know. (homer) 

So anyway, he says he will return next week and put a cavity tray and weep holes in.
My question is, can anyone tell me the correct way to construct a parapet with coping stones?
I would like to have an idea of whether or not he is doing it correctly the second time.
Is there an optimum layer to put the cavity tray between? Should he replace the insulation material?
The coping stones he put on overhang the outside and have a drip groove, but on the inner side they are flush to the inner wall. The upstands from the GRP roof come up to them.

I would post some photos but this is my first post.

Diagrams, if possible, with any answer would be extremely helpful to me.

Thanks in advance
John


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## MikeG. (8 Jan 2020)

Welcome to the forum.

I would suggest calling the building inspector and ask him to visit at the time the redemptive works start.

The problem with asking for details is that there are multiple ways of doing this properly, and which one is more suitable for you will depend on all sorts of variables. A parapet is essentially a box gutter/ back gutter, which I would specify in Code 5 lead or, preferably, EPDM. The source of the dampness intrigues me, because if it is from the roof side of the cavity wall it suggests that the box gutter isn't working properly. If it is from the coping or facing wall, I'm amazed that damp would show through so quickly. The cavity tray's positioning is in relation to the lead/ EPDM flashing level (ie just below where it tucks into the masonry). Retrospective fitting of a cavity tray is almost certainly going to mean removing multiple courses of brick or stone (you don't mention the construction). Other than that, without photos I wouldn't hazard a guess as to what is going on.


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## Irish Rover (8 Jan 2020)

Thanks for the reply.

The building "inspector" is another bone of contention.
Our architect signed us up to a company called JHAI. The inspector allocated to us is harder to get hold of than Lord Lucan.

He just asked the builders to send him photographs of each stage.

I realise a photo would explain it better than I can.


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## Irish Rover (8 Jan 2020)

Doing some research I can see that there is more than one way to build a parapet correctly.


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## Irish Rover (8 Jan 2020)

It seems there are plenty of ways to do it incorrectly too :?


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## Irish Rover (8 Jan 2020)

Photos


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## Irish Rover (8 Jan 2020)

More


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## MikeG. (8 Jan 2020)

You've a wet ceiling, which means that even if there is an issue with a cavity tray it isn't your only issue. Water is getting under your roof sheet.

_Edited_


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## MikeG. (8 Jan 2020)

Irish Rover":3ajc0rq6 said:


> ......Our architect signed us up to a company called JHAI. The inspector allocated to us is harder to get hold of than Lord Lucan...........



You've an architect involved?

OK, then you should be talking to him, not us, and I'm outta here (professional protocol.....we don't step on each others' toes). Was there a contract? If so, there is a dispute procedure. Don't forget, if the architect was administering a contract then he works for you, not the builder, and it is him, not you, who should be seeking redress from the builder. He should be overseeing the works, not you, and checking that the detail as built is the same as specified in his drawings.


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## The Bear (8 Jan 2020)

Looks almost identical to the extension I built in my last house.

Whats the black stripe on the vertical bit of wall between your cap stones and the aris rail thats been fibreglassed? When we did mine the fibreglass went all the way up to and under the caps.

Mark


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## Irish Rover (8 Jan 2020)

Hello

I'm not sure what it is but it goes all the way round.

I don't know how far, if at all, they have taken the GRP under the copings.


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## Woody2Shoes (9 Jan 2020)

Just to add some more thoughts to the good stuff you've already read:

I think that there are at least three ways that water can be getting into your new ceiling and walls:

1) No cavity tray would appear to have been installed in the two existing walls of the main house. The outer skin of a cavity wall can and does become surated with windblown rain and moisture can run down the inside face of the outer skin - effectively the outer skin of these existing walls become internal walls under the new roof and need to be protected by an inserted DPC 'tray' e.g. https://www.youtube.com/watch?v=zkuMGy0sm14

2) a) The "flat" roof needs to fall continuously towards the gutter(s) with no 'dead' zones where water can accumulate. This is normally done by installing 'firring' pieces under the OSB (in this case) boards supporting the roofing membrane - there appears to have been a bit of interesting 'terracing' in the outer corner of the new roof to try and keep water from pooling in that corner. I would question whether there is a proper fall on your roof - if I'm right, this would increase the chance of overtopping the upstand.
2) b) Any water that pools - or indeed which is simply blown by wind in the wrong direction in a heavy downpour - can breach the upstand on the membrane (which should be a good 150mm high minimum everywhere). The vertical upstand needs also to be protected by a 'flashing' coming down over it - this seems to be missing on the existing wall, where there simply seems to be some attempt to 'dress' the upturned membrane into the wall and seal it with a bit of gunk (not great workmanship). I would also check the integrity of the upstand at internal and external corners (150mm minimum needs to be maintained everywhere).

3) Since both skins of the parapet wall are potentially exposed to windblown rain, they both need to be considered to allow water into the cavity, and a stepped DPC (which could simply be a sheet of lead or, much more likely, black plastic DPM sheet) needs to be built into the parapet wall (with proper detailing at joints etc.) - shedding water to the outside not the inside. It's not obvious that such a thing has been done.

I wonder if the parapet wall is the primary problem in this instance it could well be 2a/b - the brickwork looks like the work of someone who knows what he's doing, the roof structure (chippy?) and covering (apprentice roofer?) less so...

Interesting point made about building control here (an interesting channel too): https://www.youtube.com/watch?v=p1ad9QA-jKw

Interesting article: https://specifierreview.com/2017/11/24/ ... ity-walls/

Cheers, W2S


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## Irish Rover (9 Jan 2020)

Thanks for the reply and the links.
There has been no water ingress on the two walls of the existing house. Only on the two new walls and most of that on the end wall facing the weather direction.

The water ingress problem improved a lot after the whole run of coping stones was covered with plastic sheeting over Christmas, which seems to point to the coping/parapet being at fault?


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## The Bear (9 Jan 2020)

W2S makes several valid points.
I'd be looking closely also at that black strip, what is it, how does it join the fibreglass and is it dressed under the copings/caps.

Mark


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## toolsntat (9 Jan 2020)

No expert but looking at it logically my concern is the copings finishing flush with the GRP.
Assuming their fall is onto the roof perhaps the grp is letting in.
Wider copings may have been required or at least the grp going under them.
Cheers Andy


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