So we are working on a 1930's building in London, the land is owned by Network Rail and building by the Landlord who is looking to modify the building to change the current 1st floor and add a further two levels to the building, to provide suitable modern living space. The project is currently in the planning stages.
As structural engineers we have to consider several issues: the added wind load from the increased height of the building, and the additional dead load from the new fabric being added and live loads from the additional accommodation floor and roof structure.
We have been asked to consider CLT (great material, but a lot of timber added with limited structural benefit in this case) structure for the roof extension.
Unfortunately we just cannot justify the weight increase, so we have come up with removing the concrete roof and replace the roof with a combination of hot rolled steel and cold rolled frame structure (could be timber stud system).
We are retaining the original external envelope, but changing the floor to provide a suitable load to work within for the additional floor, to keep the load in the foundations as close to the original loading as possible.
This is typical roof extension and conversion of commercial space into residential.