Roof for new extension

[ragj195]

Hi

I'm just starting to think about the design of a full width rear extension. I know I'll need an structural engineer but I have an question about the roof. Because of a pre 1938 small 2m out x 4m across extension which will be removed I may want to go 5 maybe 6m back from the rear wall meaning the extension would be 5or 6m x 8m. I'd like a lean too roof to give the interior a high ceiling height but does going over 5m mean that I'll need a purlin across the 8m width because the rafters will need more support due to their length? Would having a flat roof make having an 5/6 x 8m

If I kept the extension down to 4m x 8m am I right in thinking that I could then have a lean too with rafters short enough that they won't need any additional support?

Thx
Rich

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[oldbutnotdead]

Depends what you're using for rafters, roof covering, pitdh, snow load etc- I'm half way through a 7m span monopitch roof using JJI beams (300 x 100)- they aren't cheap but they will do the job. May well be cheaper to go the purlin/smaller rafters but thats a design/engineering decision, well above my pay grade