Can I use wider, shorter joists for flat roof to gain height

I am constructing a garden office at present with monopitch flat roof. As I had to build close to boundary I have to keep overall height at 2.5m max. I am using C24 timber and the span between walls is 2.45 meters. In order to gain a few cm here and there I wondered if it is OK for me to use a wider but 'shorter' roof joist e.g. 60mm x 97mm rather than a narrower but 'taller' one like a 47mm x 120mm? Span table doesn't suggest there is a structural problem with this approach - and I am making a warm roof so space for insulation internally is not a consideration.