New Flat Roof Extension - Parapet Copings

[commsbiff]

Hi all,

We are having a single story extension built. The house was originally built as one of a pair of similar properties in the late 70s. Our neighbours already have a single story extension with a flat roof and parapets. Due to this, we opted to go for the same style of extension to keep things looking consistent.

I have been away from home with work and when I returned yesterday, I found that the builder had 'finished' the parapets by coping them with 1 inch thick concrete paving slabs. Not only do these slabs look poor (in my opinion) as a capping, but they do not have a drip edge or slope back to the flat roof for run off.

I have spent a bit of time searching this forum and 'think' I've learned a couple of things. After I asked my builder, he told me there was no dpc under the slabs because it didn't need it. He said this is due to the fact the wall has a cavity tray and if water gets in it will just come out again.

I was under the impression that the first line of defence of a modern cavity paraper wall should be a coping stone with drip edges and a slope back to the flat roof to prevent water staining the brickwork (does anyone know the required overhang?). The second line of defence should be a dpc that is supported over the cavity (by eg 3mm slate) and extends 10mm out from the mortar/ brickwork on each side. The third line of defence, if everything else fails or if water gets in from somewhere else, would be the cavity tray.

Am I right or am I wrong? Has the builder 'finished' the walls in an acceptable manner?

If he won't put this right, I may need to DIY the remedy, how should I proceed?

I would really like to know your opinion wether it's anecdotal or evidence backed. I could really use some evidence to take to the builder but I can't find anything on building regs.

More info, my mate who is an architctural technician did the plans and the extension has gone down the building consent notice rather than planning permission.

Please help! TIA

 

[^woody^]

It's a nice job but partly wrong.

There should be a slope and a drip on any wall copings.

I can see where the builder is coming from regarding a DPC in context of the parapet not being very high. Not quite right, but not the end of the world.

For some of your evidence, just put " parapet wall NHBC " into google.

Other evidence will be in various BS or EN standards which will deal with the relationship of roofs, flashings, copings, parapets and cavity walls.

If your mate is an AT, then he must surely know how to design a parapet. But if you look over at the neighbours, if that is the same and there are no issues, then you could expect the same performance.

BTW, that bottom row of tiles rubbing on the flat roof membrane would be a bigger concern and potential cause of problems.

 

[freddiemercurystwin]

So did the details of the parapet specify a proper coping stone with drips and adequate overhang? Oh wait that's right another Notice! And a repeat of Woody's concern.

 

[commsbiff]

Woody,

Thanks for your helpful response. I agree that the work that's been done topping the parapet is to a good standard and a nice job in that respect. I guess I was just surprised to see paving slabs topping the parapet.

Next door's that is shown in the pics is topped with the appropriate coping stones (75mm / 50mm once weathered sloping back to the roof with drip edge). I had specified to my builder to make it like next doors (I was coming from an aesthetic point of view as I knew nothing about copings), and he responded that it was an ugly concrete and he would get better colour match. There was no mention of 25mm buff concrete paving slabs. It was only when I started to search on whether this was an appropriate topping that I discovered it probably isn't - primarily as you have stated that it should have a drip edge and a slope.

I have been in touch with my AT friend who does indeed know how to design a parapet. The parapet construction was not specified in the drawings, but would have been had the builder requested them or not informed me that he was happy to build from the plans that he was given. I don't have much experience dealing with builders, but if one tells me he can build something with the information he has, I assume he's correct.

Thanks for your info about NHBC etc and I have found a 'best practice' drawing to give my builder.

A really big thanks for pointing out that bottom row of tiles. I'll see if a row can be removed. If the flashing doesn't run high enough for this, perhaps I can add small rubber blocks under the front edge of the tiles to prevent rubbing the epdm.

 

[commsbiff]

freddiemercurystwin,

I made a mistake - my plans actually did go through planning permission. This was done by my AT mate while I was serving in the Middle East for a long period and I forgot as this was a change to our original approach.

The plans did not specify construction details of the parapet. However, if my builder tells me he can work with what he's got, him being the building professional (and me the novice), then I'll take him at his word. If he'd asked for details, I'd have got my mate to make them. He didn't ask me for details of how I'd like the walls tied together, or which floor screed he should use or what mortar mix ratio I'd like either and so he didn't get that info from me. This may sound petulant but my point is in response to your unhelpful sarcasm and is merely to point out that I am a layman, unfamiliar with building processes. When dealing with a building professional, I expect them to ask for detail on anything they need. Essentially, if my builder didn't know how to build a parapet properly, he shouldn't have said he did.

Thank you for echoing Woody's concern. I will look into it.

 

[Notch7]

freddiemercurystwin,

I made a mistake - my plans actually did go through planning permission. This was done by my AT mate while I was serving in the Middle East for a long period and I forgot as this was a change to our original approach.

The plans did not specify construction details of the parapet. However, if my builder tells me he can work with what he's got, him being the building professional (and me the novice), then I'll take him at his word. If he'd asked for details, I'd have got my mate to make them. He didn't ask me for details of how I'd like the walls tied together, or which floor screed he should use or what mortar mix ratio I'd like either and so he didn't get that info from me. This may sound petulant but my point is in response to your unhelpful sarcasm and is merely to point out that I am a layman, unfamiliar with building processes. When dealing with a building professional, I expect them to ask for detail on anything they need. Essentially, if my builder didn't know how to build a parapet properly, he shouldn't have said he did.

Thank you for echoing Woody's concern. I will look into it.

I think you may be getting confused with planning permission and building regulations.

We are only talking building regulations here, you can either do a full plans submission which gets plan checked -mostly for building regulation compliance. Or you can apply for a notice and just build -the danger is that the inspecton only checks what has been built (apart from footings) If its wrong it has to be re done.

I think copings are usually 400mm for a 300mm-350mm wall, you can have once weathered or twice weathered.

Parapet walls needs careful designing, they are prone to leak!

Your builder could've fitted parapet cavity closer, they are only £20 or so /2.4M

https://www.roofingsuperstore.co.uk...rFY5cybbMvsFO3Cbvc_tEB4qQWiZer3UaAvDREALw_wcB

The work looks nice and neat its shame the builder didnt go that extra distance with the correct detailng

 

[commsbiff]

Notch7,

Ah, that rings a bell. Yes, I think we only applied for the notice. The same drawings were used for building control and planning.

I think you're sizings are correct, 50mm appears to be the required overhang from NHBC documentation.

I was under the impression that the cavity closer was to prevent the DPC troughing into the cavity. As there is no DPC, I don't think there's a closer fitted but I shall ask him. Does it fill another purpose?

Yes, it is nice and neat. I like the guy, but want it done right for peace of mind, especially when people keep saying they are prone to leak!

Thanks for your input.