advice on sistering rafters and birds mouth cut

23 Mar 2016
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United Kingdom
Hi All
Can anyone help.
The wife wants roof windows in the bedroom.
The pitch on one side of the of the roof is 15 degrees. the other side a lot steeper.
The length ,wall plate to knee high load bearing stud wall is 4.2m on top of 450 thick load bearing stone wall, well will be once purling is proped and taken out in sections.
ceiling joists also to be removed.
Building control are on board and structural engineer coming out on Wednesday evening for other things going will check with him as well.
Getting dizzy from looking up and scratching my head and was wondering if any one on here would be kind enough to enlighten me so I don't sound like a dork when I'm talking to the bc officer.
Now proposing to put 9x2 along side existing 4x2.screwed nailed bolted drilled for airflow ,insulated. Double up around Windows. Bc approved
Now the bit I'm confused with is do the new 9x2 have to have a bird that would require taking the roof off and I really don't want to Be doing that.
Or would the bc just accept seat cut on 9x2 and bolted fixed to bird mouth cut 4x2. Also for detail could I put a birds mouth on the inner face of the wall plate rather than cutting the seat cut all the way through level.
Or would that cause to much pressure on the inner wall. Thank you in advance for any advice
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