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My terrace (which I bought three years ago) has a single-storey rear extension which forms a butterfly type roof with next door's single-storey rear extension.
This butterfly roof is served by a valley gutter in the middle which runs more or less down the boundary between both properties.
My extension was built in something like the 80's with next door's extension being built by the current owner in mid-2000's or so.
Originally my extension roof was served by a normal gutter arrangement. There is a bit of a question mark about where the boundary is exactly and it looks like my extension may have encroached a few inches when built. It seems that what is now the party wall (on the extension) is a cavity wall on my side, and on next door's side, they built a half-brick thick solid wall butted right up to my outer leaf as their internal wall. I'd wager that the cavity wall was built to be a future party wall if next door built an extension. In the event, they just built their own wall against it though.
When next-door built the extension the valley gutter was obviously put in at the same time.
Next door has rainwater ingress, particularly during heavy rain. Out of courtesy, I improved the outflow arrangement with a better hopper to stop a lot of 'splash' that I was seeing that was wetting the outer wall.
A couple of cracked tiles were also replaced their side but apparently the problem has not gone away. Next door has been told by several roofers, and also a builder friend of theirs (who I more or less trust), that the valley should be torn up and replaced/refurnished. The valley doesn't look great I have to say and it's possible that normal wear and tear over the years has resulted in tears in the lead chute, movement of joints, etc., resulting in leaks. This would cost me about £500 as I accept joint responsibility for the upkeep of the valley gutter.
I've been paying attention to the state of my roof the last few days considering this. The covering is those old Stonewold concrete tiles (single-lap type) dressed to a wet verge with some mortar (in a poor state) sitting on undercloaking boards. It's a fairly low pitch. At some point, a short tile of a different brand was put it resulting in the tiles on that course being about 1" short of being flush with the undercloaking at the verge. Another course ending at a half-tile is also well short and so was the bottom course where a full tile was cut too short. I've replaced these tiles to make them flush but not the course with the half-tile yet.
I mentioned this because being short of the undercloaking/verge, and with the poor state of the mortar there, I noticed during rain that water was dripping down the top of the undercloaking towards the valley, where to my horror I noticed that the lead chute is dressed over the undercloaking. This to my mind means that it was never detailed properly, and when I lifted the tiles I saw that the lead is dressed over the felt/membrane too. This means that any water getting on the felt, getting behind the cracked verge and falling onto the proud undercloaking just runs down and wets the structure/wall under the lead. I don't think I see any damp in my extension because I have a cavity wall, unlike next door.
I've not lifted tiles on my neighbour's side but it looks like it was detailed the same; I can see the lead dressed over their undercloaking on the verge.
What I'm thinking:
1) Valley was built with a latent defect and this legally is the responsibility of next door given that they were the client who commissioned it.
2) Until valley is torn out and tiles lifted can't really rule out normal wear and tear causing a leak.
I have brought up 1) with the owner and they weren't very receptive although they did try and change the subject which tells me my argument worried them. I don't like the owner but in some minor dealings in the past she was reasonable and she let me replace the existing fence with a taller one.
If I play the lawyer here and refuse to pay half the costs of the valley works I'll no doubt destroy any relationship we currently have. I think I'm legally right but I could also be wrong. This could cost me £££ if she went to court and won. However, I don't like to feel like a pushover which is a tone she seems to take. I'm sure she's divorced 😀 or driven her husband mad.
There is also a question mark about the state of my verge and the tiles not being flush. I really think that water running down my undercloaking has contributed to the damp, and this is consistent with her saying that the problem started when the previous owner of my property replaced a cracked tile. I believe this tile is the 'short' tile of a different brand that I just changed out and which left that row about 1" short of being flush with the undercloaking. There may be some liability on my part here if a surveyor started pocking around, but it shouldn't have been an issue if the valley was properly detailed originally.
I think having written this that I'll just eat the £500 given the issues at play, especially since I don't want to burn the relationship. She's also getting her roof covering changed out at the same time, new battens, felt, etc. I've checked some of my battens and they look ok. I'll ask the roofer to instal a dry verge on my side too and I need to discuss with him how he plans to detail the valley. I am a bit concerned that some more significant work is needed to lower the valley to allow the felt to properly lap over and my tiles to sit on a fillet (which they don't currently do).
It sounds very complicated. I realise you may not want to to, but if I was in your position I'd at least look at the cost of recovering both roofs and reversing the fall on the central half of each to form a shared ridge, rather than a valley. It's more disruptive and a bigger job, but less complex (and you can sell the lead). I can see a shared valley being an ongoing pain.
I was thinking about that, or a shared flat roof, but there are pitched roofs again either side of our properties. I have a shared valley and a shared ridge. It's a row of 4 terraces and each has similar single story extensions forming an M profile when viewed from the rear.
I think what you need there is an independent roofer / builder as what you've just written reads like several paragraphs of educated guesswork.
It may be that the previous owners of the two houses agreed that "your" cavity wall could straddle the boundary line to enable use of it by your neighbours in the future. What sort of gutter was there before your neighbours built their extension? A good Building Surveyor with party wall experience would be able to advise on liabilities and the best solution, but that advice would probably cost more than the remedial work! Valley gutters usually cause problems. As said above a ridge would be best from a weatherproofing point of view or a flat roof between the two existing ridges, subject to Town Planning etc.
I was thinking about that, or a shared flat roof, but there are pitched roofs again either side of our properties
Ah, OK. When I had a terrace with an extension there was a narrow yard down the side, and extensions were back to back in pairs. Although pitched roof with a shared valley sounds like a problem, it's still better than a flat roof - which will leak eventually, and you still need to take the water away somehow.
I'm deeply prejudiced against flat-roofs too, although I hear they can be much better now than the horrible torch-on-felt jobs I used to see on the council estate I grew up on lol. Would look like crap I think too sandwiched between back-to-back pitched roofs. The other four terraces the other side of the ginnel all have flat roofs on their extensions and it looks like a slum :D.