Leak in internal wall under downpipe carrying roof rainwater

We built an extension on ground floor of a Victorian house. The extension is L shaped with a thin part connecting it to the existing structure. The roof is a green sedum roof, with a waterproofing membrane collecting all the rain water and channeling it to a pipe that goes through the ceiling and out in the area just by the connection between main house and extension. There were two professionals involved, a main builder who did everything except the roof and another one who did all the roof, and pipe. The roof guy left the pipe extending about 25cm from the house and when the builder finished the wall and render, he cut it close to the wall and created an angle connection to a vertical pipe taking the water down to the sewer.

Within a year after the construction we noticed damp patches on the wall very close to (just under) the pipe mentioned above. The builder came to see it and said that water was escaping from the corner connection so he glued it to create a seal. Soon after though the damp came back. We asked the roof guy, who said that the pipe should not have been cut short because water can travel back from the end of the pipe back into the wall, the safe distance being 5cm from the wall at a minimum, pointing the blame to the builder who should know better. The builder on the other hand said that he cut it 2cm outside the wall (although my wife remembers it being cut flush, not 2cm) and especially with the additional seal from the glue there shouldn't be any water coming in, and suggested that most likely there's a leak further upstream perhaps where the membrane leads to the pipe. The roof guy said that's impossible because there's a huge area of a complete seal of the membrane on both surfaces of the pipe. He quoted about £800 to remove the pipe and install a new one whose outlet sticks out the recommended amount from the wall(and of course recreating the seal with the membrane). In the absence of an agreement between the two on what's the fault and what needs done, and it seems they don't get along very well which makes communication difficult, and given that we can't have a continued leak, we feel trapped into paying that.

What I am not convinced of though is that even if best practice is indeed having the pipe extending by a lot from the wall, there must be a way to waterproof the wall if it stands out even a tiny bit. Either by having a fully sealed extension of the pipe, or sealing all the surrounding area, or something else. Surely it can't be that the solution is replacing the entire pipe?

I am trying to get both of them on site and discuss it and agree on something but I would like to gather some views and ideas on it beforehand.

Pictures are here: https://imgur.com/a/OTZs2xV

There are photos of the area in question, before, during, and after the works. You can see the pipe sticking out a lot before the wall render was applied, and now neatly (but potentially dangerously) tucked away in the L corner leading the water down. You can see the damp wall inside and some stains on the render outside. There's also a picture I took of the inside of the pipe from the front, as well as two photos from the other side (on the roof).

Any suggestions/thoughts welcome.

 

First i’d test it with a hose inside the pipe, past where it meets the membrane and before the point he cut the pipe. Simulate some heavy rain there, making sure non flows back to the membrane joint, and see if it shows inside.

Then you know which joint is leaking.

 

Thanks but the problem is that there's no immediate feedback because the water travels slowly inside the wall and eventually shows on the inside much later. The leak is likely small because even during heavy rain there's no noticeable immediate change in the wall.

 

I do not understand, if the roofer is saying the leak is caused because the outlet pipe has been cut too short so the elbow connector is not clear of the wall, why is the damp showing on the internal face of the wall?

Surely if that was the case you would just get some localised staining of the external render around the elbow connector. The fact that the dampness is showing down the internal reveal suggests to me that the leak is around the roof outlet end or is somehow tracking back along the outlet pipe to reach the inner leaf of the wall. I would be clearing the gravel away from the roof outlet and having a very close look at the roof membrane sealed around the outlet and checking that the square outlet pipe actually falls towards the elbow connector and downpipe. Looking at photo 8 I think it is, I assume that is looking inside the outlet pipe towards the roof, the water appears to be standing at the roof end which might suggest it is falling the wrong way.

If it really is just a matter that the outlet pipe has been cut too short (which I doubt), one manufacturer I looked at supply extension pieces if the outlet pipe has to pass through an especially thick parapet wall so I do not see why you couldn't just add an extension piece rather than replacing the entire roof outlet.

 

If the pipe is male and the outlet female it could track back and down the cavity.

Could you block the downpipe and fill it sonit backs up past that joint?

 

The roof is a tidy job so my initial thoughts would be to blame the builder, but that does not help

What the roofer is saying deffindefi makes sense.
The brick wall is wet also so it looks like the water is getting into the cavity.

You need to pull back the stones on the roof around the drain, make sure all joints on there are water tight and stuck down well. They should have had mastic on the seams.

If you can try and pull the box off the down pipe see what he's glued it with.

 

Divergence of opinions here
There isn't any localised stain on the outside at the point of the connection (end of the pipe), but there are signs of water travel in all directions downwards. It could be that the leak is further upstream. A crack in the pipe perhaps? although from the photo I took it doesn't seem there is one. Or at the point that the membrane meets the pipe? But the roofer is adamant that this is not the case because there is a big overlap and a complete seal with membrane both inside and outside the pipe. Another possibility could be from outside the whole membrane-covered roof surface? That would explain more easily the wet brick wall that tomfe mentions. By the way there's no cavity on the wall, it's a solid brick wall (very old Victorian construction, from before they started using cavities).

The pipe is level with the ground I think, so there is some water held in the roof membrane almost at all times. The excess travels down eventually as the pipe is at the bottom of the roof structure and there is some slope in the roof towards the pipe (you can see it in one of the photos from the construction phase where the roof is partially wet). In any case there should be no water escape from the roof end of the pipe even if there is some stagnating there. The roofer's point is that the water escapes from the front end of the pipe and because it's not far enough from the wall, it travels back towards and into the wall along the outside part of the pipe, and that there's nothing you can do about it, short of replacing the pipe and not cutting it too short this time.

The builder is telling me that he used CT1 glue. He said it's very strong and you can't open it without cutting it (he had said that before he put it on and I agreed after I was assured that there wouldn't be any more leak).

I could block the very bottom of the vertical pipe, then the water would build up in the pipe (and then go back out the roof end but also presumably leak into the wall as well). The problem is that it takes time to see the effect (damp inside), so there won't be any useful conclusions, certainly not being able to identify where the leak is.

 

Turn the hose off once the water passes the builders joint and top it up if it subsides. It will come through eventually.

 

Maybe I don't fully understand what you propose. The water will come through, yes, the same way as it comes now presumably, but how will we be able to tell what's at fault. Filling the pipe with a hose is similar to water coming down during/after heavy rainfall (there's about 45sqm of roof surface water all coming down through that 10x10cm pipe). We don't see the wall immediately wet, but possibly not until the next day. And there's no way to tell where it's coming from. Wherever it is, it certainly travels along/through a lot of wall structure and the path is not clear at all.

 

So you new extension does not have a cavity, what is the wall construction?

 

No the new extension has a cavity, the existing structure (brick wall) doesn't. I meant the brick wall previously, sorry if it was confusing. The extension (that starts at the point of the render as seen in the photo) is a wall made of two leafs of blockwork with a cavity, and K-rend render outside that.

 

Yes, there is water getting in the cavity of the extension.

I am not sure is there would be staining around the pipe box as it looks long enough to penitrate into the cavity.
The cavity wall insulation will be directing the water all over the place, did they use full fill or PIR?

But anyway you need to pull back the stones on the roof around the drain, make sure all joints on there are water tight and stuck down well. They should have had mastic on the seams.

 

Are you 100% sure it is the shoot / downpipe?

I cant tell from the pictures but there is quite a bit going on in that area.

You have a parapet with coping stones -cant tell if thats a cavity or single skin parapet. The parapet abuts the existing wall. There is a door or fixed light with a lintel and I guess a cavity closure or vertical dpc. There is also a rain cover with brackets over the door. All unlikely perhaps, but its part of the diagnosis.

The damp internally is on the reveal and the wall face just an inch or so from the corner of the reveal. So the water must be getting in at high level (as it starts high up) and getting across to the inner blockwork leaf.

If the thought is that the water is coming out of the elbow externally and running back outside, ie where the staining is showing, then sealing between that and the render would stop it.

If the builder cut back the square pipe outlet, does that mean the joint is now within the wall / render?

I have to say, the exit shoot / pipe seems a poor detail for 45sq metres of roof.

 

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