Opinion Required - Floor Joist Requirement Dormer Extension

[Seasailor]

I want to understand the reasoning of why my structural engineer is  insisting on installing substantial new first floor joists on my dormer bungalow double extension?

There will be one long extension and a shorter one on the other side to give more floor area to the existing two bedrooms and bathroom.

For aesthetic appearance reasons the two extensions will be set in from the existing inner wall plate and therefore the whole weight of the new structures (Roof,front & cheeks) will be supported by a steel frame sitting on the inner load bearing wall. The steel beams comprise of two 11 m purling (UB) beams and a  11m UB at first floor level for the long extension. There are shorter steel beams at the other side (8.8 UC) for the short extension. The structural engineer has insisted on doubling up the existing (FF) joists with 75mm x 150mm joists at 400mm centres and for the first floor area which spans the downstairs living room he has additionally specified 150 mm x 150mm wooden beams alongside the existing beams at 400mm centres (Nightmare for installing new services due to limited space.)  The upstairs before the conversion has two first floor bedrooms from the original build and the original upstairs floor joists are the same size and spacing as the downstairs floor beams. I would appreciate someone pointing out if there is something that I am missing?

 

 

 

Edited 22 hours ago by Seasailor
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