Solid wall / cavity wall plate and roof issues

[Dillz83]

Hi
im new the the forum.
im currently in the process of putting a 6m rear extension on my home.
part of the work is to replace the whole of the hipped roof and form a new roof with gable ends to allow the more space in the rooms within the roof.

im at the stage where im planning on replacing the the floor/ceiling joist and the roof.
the orignial walls are solid 9" walls and the extension has been build with 12" (300mm) cavity walls.

Because the walls are different thickness this is posing a few issues regarding the roof.
the roof is to be cut and pitched.
as the wall thickness differ in thickness, i can't get a continuous straight wall plate as part of the wall plate would be floating in the cavity space. this in turn makes it impossible to cut the roof rafters at the same size etc.
i suggested to my BC, to put a wall plate on top of the brickwork - put ceiling joists on 1st wall plate - attached 2nd wall plate to top of ceiling joists - cut rafters to new fit 2nd wall plate.
they said its okay to do this but would like a 2nd opinion?? is there a different way to do this??

if i chose this route, would i still need to bolt the ceiling joist to the rafters?
if so, I'd have to extend the joists beyond the exterior wall to meet up with the rafters and hide them above the sofit which would be quite large!! which i didnt really want!
could i attach a plate fixing the rafters to the joist??

attached is a basic detail of what im tring to describe.

any advice is welcome

 

[^woody^]

It makes no sense - the diagram or the need to do that.

Are you framing a new roof over the existing wall or is anew roof joining to an existing roof?

 

[Dillz83]

sorry, posted the wrong diagram regarding wall plate detail. but its still the same principle? if wall plate is over solid wall, part will be over the cavity on the extension??

its a whole new roof over the original building and extension.

 

[^woody^]

You need to set the plate height and then adjust the seat cuts of the rafters.

If you look at the seat cut in the first rafter on the existing wall, you will see the difference in the cut over the new (wider) wall. Now if the rafters aren't deep enough for a larger seat cut, you would split the difference and reduce the seat cut on the new wall which will then lessen the seat cut over the existing wall.

In all cases, the ceiling joists must be fixed to the rafters to act as restraints to prevent the roof spreading

 

[^woody^]

Or, have all the rafter seat cuts the same, and set the plate on the existing wall lower, then add a packing piece between the rafters over the existing wall to raise the ceiling joists to keep them all level

 

[Dillz83]

thanks for your reply.
i did think about the 2nd option but was put off by the amount of extra work that is required with the brickwork and joists.

 

[^woody^]

The second option is potentially the easiest, as all the rafters are cut the same.