Question around rafter replacement as part of new roof & dormer setup

[d1str4ct3d]

Hi,

 

I'm doing a self build new roof and dormer (slow progress as it has been in the works for 2 years but really have to do it next year as the roof does need replacing). I've had a structural engineer  at the beginning of the year provide the calculations for steels and new rafters (to take the load of solar panels in the future too) and looking to submit party wall notice in January along with building control.

 

My question is around the birdmouth for the new rafters.

 

The house was built around 1941 to house airmen/support crews and their families for the 3 local air fields during WW2, the rafters are 70mm x 50mm (or 2 3/4 by 2 inch if you prefer) which span 4.6m from ridge to wall plate (supported by 2 purlins at 140mm x 50mm, though that is not relevant as they won't be staying).

The new rafters will be 150mm, and we have no plans to change anything related to the ceiling for the floor below, so the ceiling joists (also 70mm x 50mm) will be remaining - which means the wall plate will also be remaining.

As it's permitted development we can't change the hight of the roof (it is also a semi so another reason for not being able to change the hight).

 

If you place 150mm rafters to match the existing rafters, the birdsmouth will reduce the thickness to 35mm (which is no different to what is happening now, except the original thickness of the rafters from old to new is vastly different). I've thrown in a drawing to show this, old on the left, new on the right.

 

How would you go about this as I'm fully aware the birdsmouth shouldn't be more than a third of the thickness?

 

 

Thanks in advance.

 

[Redbeard]

And the gutters are on the end of the oversailing spars? A lot of stress on that joint, I'd guess. How many rafters? 1st thought was steel plates, but that's a lot of fabricating, particularly if you do them both sides.

Edited yesterday at 13:36 by Redbeard

[d1str4ct3d]

Correct, the gutters are at the end. I had thought of ending the new rafters at the wall plate, so no birds mouth at all, and then connect a cut down segment of the old rafters to the new ones - then it is technically no different to its current state but it also seemed a bit pointless when there are new rafters.

 

14 new rafters at 450 centres - 2 of those will actually be triples to support a 2 in 1 Velux window - so 12 singles, 2 triples  - still working on this drawing so it isn't 100%

 

Edited yesterday at 14:09 by d1str4ct3d