Garage is being converted to workshop. Internal size is 2.7 x 5.3 M. Joist dimensions 100 x 50 mm x 2800mm span @ 600mm centres, which are too small for that span but it's been there for 45yrs with very little deflection approx 1 cm measured.
So as not to disturb the integrity of the felting and therefore the waterproofing I want to externally insulate the felt covered roof by using "Jablite inverted roof" insulation:
The steel plates , say 100mm x 6mm x 2.7M, and second joists would not rest on the supporting walls and would be approx 100mm shorter than existing joists so lateral support would be needed at both ends and probably in the middle. Would this be strong enough?
Can anyone help with structural information? and or practical experience which they can pass on please.
Regards Don