width of dormer cheek?

18 Jul 2011
Reaction score
United Kingdom
Hi all,
we are about to get a dormer loft conversion done and have a Party Wall Award and drawings.
We are mid terrace and the loft company have designed it so the 100mm x 50mm soft wood frame of the dormer cheek will be flush to the inside of the party wall.
To the exterior of this will be 9mm Supalux then 18mm weatherboard I think. Then felt, battens and tiles and also fascia boards and the junctions will have zinc soakers and lead flashings.
As our half of the party wall is only approx 100mm thick, I'm concerned that the dormer will touch or cross the centre line of the party wall.
1 neighbour has reluctantly agreed to us stepping on their roof to weatherproof the cheeks of the dormer. She insisted on her own party wall surveyor, forbid him from acting for us, and has objected from the start to any kind of a dormer, so verbal permission to step on her roof could easily be withdrawn.
I haven't asked the other side yet about stepping on their roof, but they plan to do a loft conversion soon, we signed each other's informal party wall agreement, and we will agree to their dormer adjoining ours.
Does anybody know the full width of the dormer cheek including weatherproofing? I have read that the flashing alone needs to be 200mm wide.
Would we be advised to step the dormer cheek in a little bit?
We want to be absolutely certain that the flashing, or fascia will not cross the centre line and we want to be certain it will not impede either neighbour from doing the same as us in the future.
If we step far enough in on both sides to allow a workman to weatherproof the dormer without stepping on the neighbour's roof, the conversion will be too narrow to fit a bathroom in as it is a narrow house.
I've attached drawings.
thanks in advance for any pearls of wisdom


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At no time should any of your construction materials overhang onto the neighbours side, including gutters etc.

Would we be advised to step the dormer cheek in a little bit?
Yes, of course you should. Is your designer stupid?

Dormer cladding materials would equate to about 100mm on top of the framing, i.e. 18mm ply, battens, tile hanging etc. There could be an easement for the flashing but I think you would have a fight on your hands about any solid, intrusive materials crossing the boundary, by the sounds of it.
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I can't see how you have had someone draw this and not consider the dormer. Nor how you can have a PW agreement in place without this detail resolved in the Award.

Anyway, that design of dormer check will be 170mm from internal face to external. You would be permitted to flash across the boundary, but not build over it.

BTW, a party wall surveyor can not object to any dormer, but only ensure that the building of it is done in a way that is not detrimental to his client.
@op; as a matter of interest - and if one may be so bold - how much did those drawings cost?
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I've no idea how much the drawings cost, as they were done as part of the deposit to the loft company. They told me they pay their architect £1200 but I don't believe it as he didn't seem very experienced. I asked him if one of our walls was load bearing and he said "I've no idea, its not my job to know. Ask the project manager" :-0
I think it is possible to get them done round here with structural calculations for around £700. If I did this again, I wouldn't get the plans done via the loft company.

The Party Wall surveyors for months assured us that this design would be absolutely no problem as I raised the question about being too close to boundary many times, before and after sending them the drawings. Then just before we got the award we were told if you go on to neighbour's roof to weatherproof your dormer, it is a trespass. Apparently, if we decide to break open the roof on both sides and build on top of the party wall, then we have a statutory right to access the neighbour's roof do so according to the Party Wall Act, but as it is not a parapet roof, this would mean additional mess and disruption.
As the party wall is approx 200mm thick and the cladding on the dormer cheek is approximately 100mm then we would be right on the centre line not including flashing, providing we can step on neighbour's roof to do this.
If we can bring it in just 50mm then that might be better, but if our neighbour builds to the same size later on, we need a good way of weatherproofing the gap between the two dormers especially as our neighbour wont be able to tile their cheek due to ours being in the way.
I'll raise this with the building inspector on their first visit.
Interestingly, some of the dormers round here don't appear to have any flashing at the junction of dormer cheek and roof, though they do have flashing at the junction of dormer face and roof.
Thanks for the valuable info everybody, very much appreciated. priceless in fact.
Funny isn't it. you can pay party wall surveyors a few grand and they tell you sod all, whereas the helpful folk on here share their widsom for free
we were told if you go on to neighbour's roof to weatherproof your dormer, it is a trespass

Get another award - and a a refund from those surveyors. The right to access the neighbours property to do the work is one of the fundamental points of the PW Act.
No way those drawings are worth £1200.

Rather than employ 'loft conversion specialists', lofts are best done by builders or joiners off proper independant plans.
we were told if you go on to neighbour's roof to weatherproof your dormer, it is a trespass

Get another award - and a a refund from those surveyors. The right to access the neighbours property to do the work is one of the fundamental points of the PW Act.
Your surveyor should be demanding access and to protect their client your neighbours surveyor should be agreeing, maybe with a condition that he inspects the roof and flashing etc on completion of the works.
Complete refusal on your neighbours part is rather short sighted, who knows when they might want access to your property for maintenance.
Some neighbours just like to exercise their rights and power because they can and not for any real concerns.
I've got a landlord neighbour just like that next door. I phoned him to let him know I would be starting a loft conversion and he started going on about a PWA, well reluctantly I agreed to keep him sweet. I included information about access to his roof and what was going to happen at the abutment etc. He came back and said he wanted dates and times that I would be on his roof, when I said I couldn't give those he came back and said he wanted 24hrs notice of when I would be working on his side.
When I pointed out to him that last year he hadn't asked for permission when he dropped scaffolding into my garden, when he re-pointed his house and had covered my patio with rubbish and had filled my spare bedroom with dust and still hadn't given me the money to cover my cleaning costs he went all sheepish and started wittering on about covering himself as he had had a problem with a loft conversion at another house he owned.
Anyway I sent the PWA back to him again and 3 weeks later he hadn't sent it back, so I phoned to ask if he was putting it into dispute, he replied "oh no I just wanted you to know I would be watching what you we're doing".
I agree, getting a trusted local builder and independent plans would be better. the builder I wanted to us is not well and taking time off. The others I spoke to are booked up for most of the year. We've been putting it off for a year and can't wait any longer. Apparently the surveyors can only agree access if the works relate to the party wall itself which in this case is the steels. As the dormer cheek is flush to the wall and not on top of it, they say they have no authority to agree access. Shame they only told me that after the award was agreed as I had asked them this question for months. They had given me many verbal assurance and one email assurance that building to plan would be no problem whatsoever.
It sounds like stepping the dormer in would be more sensible. What is the minimum gap we should leave between external face of the dormer cheek and the centre line of pary wall? I was thinking 25cm and if our neighbour did the same in future, a 50cm gap would still allow for maintenance . Or does a roofer need more space than that to maneuver and swing his hammer?

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