So I'm planning to do this build myself due to unavailability of local builders.
2 extra rooms to be created either end of the house. House currently is 2 story at the rear, single story at the front. New rooms will be approx. 2.6m deep and 2.9m wide.
Rough cross section. Walls A, B and C are 9inch brick and load bearing. In the long distant past the roof line of the lean-to front was lower, but raised to match the main roof in the 70s.
Rafters are currently 50x75, as are the ground floor ceiling joists. Purlin on slope approx. halfway between A and B.
Building inspector has agreed new floor joists spec to replace the ceiling, 45x145 @400 centres. It might have to be slightly lower its height depends on the number of steps up into the new room. Front rooms have higher ceilings than the rear.
Bloke who did the plans for PP and the calcs, originally specced triple 50x200 rafters from A to the ridge. I pointed out that this was impossible as the one of them passes over the existing stairs which already had restricted head room and I cannot make it worse. He re specced them as triple 75x125 C24, which at the time I did not query further as I was not going to be building it. He also said that extra height of rafters could be upwards into the cheeks so as to not reduce headroom over stairs.
Now obviously (to me now I've been forced to look at it) you cannot have deeper rafters running up to the ridge without raising the roof which is not possible.
I did speak to an SE who was advising me on another project, who stated there was no need for the new supporting rafters to go right up to the ridge as they could stop at Wall B. I'm now wondering if they could also be smaller section as the span might well be much less than the initial chap calculated. I have no desire to contact him again. I have asked the Building Inspector for his opinion and await an answer. The second issue with the sizes is obviously the second option although lower would need a 9inch thick cheek to hide it! As I say I don't have much faith in his numbers. I feel he just fed stuff into a calculator without thinking around the practicalities of it. Lesson learned.
The front dormer wall is over the existing Wall A and a gabled roof will join into the existing roof. The scale is off, but there is about 6ft of roof behind wall B.
I was thinking that with a ridge beam From A to B and decent wall plates (or flitch beam) on the cheeks from a corner post at the front back to Wall B, the roof load would be mostly taken off the cheeks and the rafters would only have to support the cheeks. They will be 100mm thick, rendered finish. The remaining roof section could be built up on layboards, with diminishing rafters, on the existing roof I believe. Roof is composite slate BTW.
The plan man included spec for trimmers at the rear of the dormer, but I have no idea why given the existing rafters will only be cut back to Wall B and be supported by them.
Someone else who offered advice seemed to think I would need steels put in running parallel to Wall B, above and slightly behind it ??? No idea what it would be supporting,
I believe that the solution should be quite simple as the spans are not huge, and there are load bearing walls to make use of. I suspect a builder would just build it and have the knowledge to know what would work and what would be acceptable to BC. I don't have that knowledge, and even though my Father is a retired carpenter I don't think they had regs in his day 😉.
I was brought here by this thread I came across when searching Doubling up rafters - Page 3 - Lofts, Dormers & Loft Conversions - BuildHub.org.uk it is the only example I have come across that is similar to mine (i.e. the extra wall at the rear of the new dormer room). Such a shame the thread just stopped.
Pic of current space, taken from the other end just to be awkward. Wall is wall B. End wall is a party wall, we are an end of terrace.