DIY flat roof dormer - doubling up rafters, removing purlin

[casperslo]

Hi! 

We are constructing flat roof dormer in our existing attic bedroom (it was there since it was built - 1900 - not a loft conversion). The area underneath the sloped ceiling was used as a storage, but we decided that it would be better utilised as an ensuite bathroom. We did pay for building notice, and about to book in first inspection before we proceed with further works, however I though it would be useful to get your opinion as I bet there are many experienced roofers and engineers here who could advise if our plans seem reasonable. 

 

The area is 3m long by 3.8m wide.

 

Please check the description below against the picture attached.

 

Floor joists will be doubled (5" x 2" C24 3m). In order to remove purlin, green marked old rafters will be replaced with doubled rafters (9" x 2" C24 5.6m). Short sides of the purlin remaining would be screwed into doubled rafters to still provide support for the endmost old rafters. Trimmer boards will be installed (red and yellow line) to take load from cut rafters from the top and bottom (any advise what size timber should we use for them?). 8 x 2 C24 4m dormer ceiling joists will also sit on the trimmer board. Doubled up rafters (green marked) will be supporting cheeks of the dormer. 4" x 2" board will be screwed to the floor joists (blue line) to create a base for the dormer front wall. 

 

Thank you in advance!

 

Edited July 8 by casperslo