Calculate the maximum medium$-$term imposed load that a flat roof can support assuming the following construction details:

roof joists are $50mm225mm$ of strength class C$16$ at $600m\frac{m}{c}$

effective span is $4\mathrm{.}2$ m $-$ unit weight of woodwool $(50$ mm thick$)$ is $0\mathrm{.}3k\frac{N}{m^2}$

unit weight of boarding, bitumen and roofing felt is $0\mathrm{.}45k\frac{N}{m^2}$

unit weight of plasterboard and skim is $0\mathrm{.}22k\frac{N}{m^2}$

Complete steps pls Use bs$5268$