Marrying new roof into old terraced roof

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Hi,

I'm a general builder and I've been thrown into a job where there was a house fire, 3 roofs have been replaced, first 2 roofs by another builder, I'm the last to start as we have been waiting for structural drawings, having looked at the roof, on my right I'm joining into a new roof, built with 6"x2" rafters, sitting on a 4"x3" wall plate and a 8"x3" perlin in the middle, my problem is on my left the terraced house I'm joining into has a small 3"x2" wall plate, with 4"x2" rafters, I can't just have a small birds mouth across the roof as my new joists are huge compared to the old ones, my guess is I'm going to have to cut out a huge birds mouth to start with to join into the shape and angle of the old roof to the left then as I go across the roof the birds mouth will become smaller to match the right roof?

Any actual roofers on here that can point me in the correct direction would be appreciated!

Rich

 
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Wall plates need to be the same height if they are not then you will have problems, if you are using different rafters sizes from the other two houses then you will have problems with h.a.p. which is height above plate which can be manipulated with the birds mouth cut. If you are trying to get the slates to stay in the same plane as the other two houses then it sounds tricky, it might be better to concede that one side has to be a different level and use some lead to flash between the slates.
 
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At the rafter/wall-plate detail, you can cut a relatively large birdsmouth (so that more of the rafter-foot is bearing flat on the wall plate). It isn't so critical at the bearing. Clearly you will have to cut each one individually.
 
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The problem is as it is a new roof, has to be passed by building control, can't change the wall plate size, can't change the rafter size so the option of finishing at the taller height seems plausible, but won't that look odd? There will be a 3" height difference between the two roofs? Was going to use a tray for under the two roofs and job done but I suppose lead will do the job, just concerns me on the height! Has anyone else ever don't this?
 
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The problem is as it is a new roof, has to be passed by building control, can't change the wall plate size, can't change the rafter size so the option of finishing at the taller height seems plausible, but won't that look odd? There will be a 3" height difference between the two roofs? Was going to use a tray for under the two roofs and job done but I suppose lead will do the job, just concerns me on the height! Has anyone else ever don't this?

You can change the wall-plate size if it helps - it's not a structural element.
You can cut the rafter down at the wall-plate to give a flat bearing, and cut the tails to match
 
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Thanks for the diagram, I understand what you are saying, but again, a issue I'm going to have is that obviously as I come across the cuts, I'll be cutting a lot from the first birds mouth and not a lot when I meet the 6"2" roof to my right, also that will mean the ceilings inside will slope 2" out in the bedroom?
 
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Can you use deeper ceiling joists (say 6") and notch them progressively deeper as you go to the higher end?
 
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