parapet wall with coping stones leak

[rado16v]

Hi,

We have a single storey extension which has a flat roof and parapet wall finished with coping stones.

Rainwater is leaking into the room below from the coping stones. A builder lifted one stone and it seems the mortar between the damp proof and stone has weighed down the membrane creating a channel that water is seeping through and getting down the cavity.

What's the best remedy? He's advised of lifting the lot replacing the wet cavity insulation and replace the membrane with lead, pop the stones back on and coat the stones with a protective coating.

Is this the best solution? I want it done right this time.

Appreciate any comments.

Thanks in advance.

 

[dann09]

You will get better advice if you post general pics, and lift the loose coping stone and pic that area as well.

Also, perhaps scan a section detail of what you have at the parapet - including the stones.

eg. the membrane you mention could be a bridging DPC, or some other flashing detail, as well as what it sounds like.

 

[pinenot]

"Rainwater is leaking into the room below from the coping stones"

Doubt it?? the coping stones in themselves are a water shedding in profile, unless they have been specified by someone out of the know.
Either double pitched or singe pitched cope stones, shed water either to the external/internal wall sides simultaneously, say on a party roof all head, garden wall etc., or mono pitch stones which regularly shed the water into a gutter, Also proper cope stones should provide a water drip arrangement, so that the diverted water does not return to the wall head being covered/protected.

Untitled

• pinenot
• 4 May 2013
• 1

All very much like this.

What you might try to do is inspect yours, and like the previous request post some pics, that way we will be able to help...pinenot

 

[Joe Malone]

Construction detailing is everything when it comes to dealing with Parapet walls. I'm a building surveyor and I have very rarely seen them done right.
It's difficult to understand what's gone wrong because I can't see the detail but one thing that immediately strikes me is that you made no mention of a damp proof course being installed under the coping stones? This is an essential part of parapet wall construction and the coping stones alone do not stand a chance of keeping the water out. I recently finalised a 12 month battle with a construction company in Birmingham who were paid an awful lot of money to renew a flat roof on a large medium rise block. From day one the roof leaked and I had every excuse under the sun as to why this was but it was obvious to me that the construction detailing never stood a chance of working. The cavity trays were stepped down a course in the brickwork so these also failed to do their job as they were leaking at the stepped overlap. I eventually got the DPC installed under the copings and the building has been leak free for 12 months now.
Have you got a cavity tray installed at the top of the parapet with weeps terminating above the flashing detail? Another potential issue is the EPDM. Is it fully bonded because often it isn't so when I does get a leak it is difficult to trace the source because the water can track underneath an EPDM system that is not fully bonded. How is the EPDM turned up into the mortar bed on the parapet wall and how has this been sealed? What is the quality of the leadwork and has this been correctly sealed and overlapped? A number of issues to consider here but my money is on you not having a DPC installed under the coping stones and some might argue that the copings are good enough but they are not. The mortar joints alone often leak because they crack through differential expansion so even if the copings are fully bonded to the wall the joints will often leak.

 

[rado16v]

Thanks for your replies. Pics can say a thousand words, I got up on the roof today.

You can a plastic sheet we've put over the coping stopes weighed down with bricks to temporarily fix the leak during the winter. I believe the leak to be coming through the coping stones because placing the sheet over them resolved the leak.













I put my hand under the black membrane you see here and found wet insualation. Below this I could feel another plastic membrane, I'm guessing this is the cavity tray?

No leak here, but you can see a crack in the mortar, nearly all the stones have this



I think the job has been done to standard but somethig has obviously failed, cracks in the mortar then?

Thanks

 

[Joe Malone]

Hi, yes, the pics are much better. Ok, the first and very obvious construction detail fault is that the DPC is not supported across the cavity so it is sagging into the cavity and will therefore shed water into the cavity if it bypasses the copings. Ideally you need to bridge the cavity with slate or use a material that will not sag into the cavity. Furthermore, the DPC has not been bedded onto mortar. Effectively there is no bond between the brickwork and the DPC.
Also, I'd like to see more of those copings. Do they have a drip on the underside and what overlap have you got on the parapet?
Check the overlap on the DPC under the copings, I'd be looking for a minimum 300mm overlap but as I said if its draping into the cavity, as yours is, then the overlap won't help a great deal.
Do you have a cavity tray installed under the DPC?

The EPDM roofing detail looks very good and you look to have a fully bonded system. The photo's confirm my earlier point that all the mortar joints are cracked between the copings and therefore will be a major moisture pathway for penetrating damp.
The more I look at those copings, the more they don't look right. The last job I looked at where there was a problem, the contractor had used concrete paving slabs rather than coping stones. You're problem isn't that bad but those copings do look a bit rough.

I've zoomed in on the first picture of the wall and I think I can see the drips installed at the mortar perps so I'm almost certain there's a cavity tray installed in the wall. Personally I'd want to inspect that before dealing with the parapet wall detail but any faults found there can only be corrected by taking the wall down enough courses to replace or repair it. Pragmatically, the correct detailing on the head of the parapet wall should solve your problems.
If you can't get slate to bridge the cavity to stop the DPC sagging then I'd recommend changing the polyethylene DPC currently installed to a thick bituminous roofing felt. It won't sag and it will provide a better mortar bond.
Your mortar is also important and I'd recommend adding lime to the mix to help prevent cracking through differential expansion.

One last point of concern I have... The EPDM flashing detail appears to have been pointed in with clear silicone? I can't confirm this but finding a clear sealant for roofing work is quite unusual. I have seen this done many many times and if they have used a cheap silicone sealant then it will fail both adhesively and cohesively very early on. Best practice would have been to first point in the flashing detail then once dry finish with a good polysulphide roofing sealant.

Lead capping under the copings will be far superior to the plastic dpc but why? It is a very expensive and unnecessary measure to take in my opinion. Also, putting something like Thompsons water seal on the copings is also completely unnecessary and will do no good whatsoever.

Can you also show me the outlets for roof drainage please? Be interested to see this particular detail, size of outlets, how many are there?

 

[tony1851]

OP suggests there is a cavity tray lower down. This is unlikely to be in one length so will be overlapped at some point. If water is getting below the top dpc (which it will if that is also not in one piece or not physically sealed together) then it is inevitable that water will get down into the cavity.

The architect may well have done a correct, standard detail for this but drawings only show it in two dimensions; how it's detailed at the end is crucial.

Parapet walls look nice and modern but require great care in detailing and construction.

 

[freddiemercurystwin]

There should be a stepped cavity tray dressed above a lead flashing. Neither is evident from the pics.

eg https://environment7.uwe.ac.uk/resources/constructionsample/Conweb/walls/parapet1.jpg

This is what happens when people avoid belt and braces construction and don't bother with cavity trays above abutments. They know who they are. Bozos!

 

[dann09]

Thanks for the pics. FWIW, i agree with lots of the above and FM'sT excellent details.

radio,
i doubt that you would want to re-build the whole parapet and insert the necessary best practice detail?
However, i think that you are going to have to remove all the copings and DPC and mortar to bare brick, and do as suggested above.

Smear on ( lightly butter ) the stone edges before loading with mortar and setting them. This will give a tack for the mortar to grip.
There's something not quite right with the in-place mortar.

With the copings off, you could grind out throatings on either underside of each stone.

What state is the cavity insulation in?

Stones perched on sheeting above doorways and paths is potentially dangerous.

 

[catlad]

I can't see the point of finishing the wall like that especially with only two coarse of brick showing it looks destined to fail in more ways than one, why not get the roofers back and cover the wall with rubber and your water ingress will disappear.

 

[noseall]

Parapet walls are a useless waste of time and frequently leak. This forum is testament to that.

No amount of cavity trays are gonna stop 'em leaking, rather they need designing out of buildings.

To suggest fitting c.t.'s will stop parapet walls leaking is a bit like shutting the stable door after the horse has bolted. You may as well give parapet wall homeowners a brolly!

 

[tony1851]

No amount of cavity trays are gonna stop 'em leaking, rather they need designing out of buildings.

There are many older buildings with parapet walls that don't leak.
BUT, I'm talking about larger, architect-designed city-centre commercial buildings built from the late 19th century up to 1939.

Parapet walls in those were often well-constructed in solid masonry.
The problem nowadays is that building technology has become complex, with cavity walls, insulation, lightweight blocks, cavity trays and so on. The skills to design and execute these correctly have, however, fallen.

I agree that for small domestic extensions, parapet walls should be avoided.

 

[Joe Malone]

No amount of cavity trays are gonna stop 'em leaking, rather they need designing out of buildings

Don't agree with this at all; the problem is that we've lost the knowledge to adequately construct a parapet wall. The vast majority of builders just don't know how to build them anymore. 100 years ago when they were common place everyone knew how to build them. So it's not a matter of designing them out, it's a matter of properly designing them in.
The two modern apartment blocks I recently dealt with in Birmingham both had leaking parapets and a builder that denied it was anything to do with the parapet walls. After almost ending up in court the builder put right the work at his expense but I had to virtually stand over the builders to guide them through the detailing. On one building I even had the parapet taken down and rebuilt because the CT's were so bad. Agree that of course a cavity tray won't in itself stop water ingress but it's a third line of defence, and an important one. I often get an awful lot of 'recommendations' from roofers these days to run the flat roof covering up the side of the parapet and under the copings, it's a recommendation borne from the fact that they don't know how to detail a parapet and its easier for them; I always refuse.
The two modern apartment blocks in Birmingham have been leak free for the last 12 months for the first time ever and I've no doubt that they'll be leak free for many years to come because they have now been properly designed and constructed.

 

[noseall]

So what your saying is basically all parapet walls are expected to leak and that the c.t. is there to deal with this?

Sounds like brilliant design.

 

[Joe Malone]

So what your saying is basically all parapet walls are expected to leak and that the c.t. is there to deal with this?

Sounds like brilliant design.

No, I've been very clear on the point. The CT is a third line of defence in dealing with water ingress at the parapet head but let's not forget that moisture can get into the parapet cavity by penetrating through the masonry or by interstitial condensation and on those occasions the CT is the first line of defence. It is the detailing on the parapet head that is critical, properly specified copings, preferably with a cavity lock system or a saddle design to lock them in place on the parapet, properly specified mortar that doesn't crack at the joints, a hard support across the cavity to stop the DPC sagging into the cavity and a mortar bed either side of the DPC.
I don't expect any parapet walls to leak when they are built to this standard, I expect them to leak when they aren't.