Doubled up rafters with centre support

[akist]

My roof's rafters are about 7m long with a support structure in the middle. Each "rafter" is made of two shorter pieces, resting and overlapping over the support in the middle. This is for practical reasons as 7m rafters would be rather difficult to source and to handle.

Dormers require doubled up rafters. The carpenters have created the double thickness rafter by bolting on two "filler" pieces onto the overlapped rafter, so that over all you see a double thickness long rafter. However I think this is not right, because it is only single rafters overlapping over the centre support section.

I am attaching a sketch I just made. Could you please tell me how would it be done another way?

 

[blup]

I'm not an expert but doubling up is so the roof can take the load of the new dormer structure, plus the load transferred from the removed rafter. "Doubling up" in the way your chippy has done it simply transfers the load to the single rafter.

Did your SE inspect the roof and note the set up of the existing rafters?

Blup

 

[Dereekoo]

Why are you doubling up both above and below the central support section, surely your dormer will be constructed in the top half or the bottom half of the rafters so strengthening the unused section will not be necessary?

 

[blup]

Maybe when the roof is stripped for the dormer take off a few more tiles and add new full length rafter.

Blup