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

 

[tony1851]

A single-storey extension, right?

Going out that distance, the rafters would be a relatively small issue.
Your real problem might be that you end up going above your upper window cills to get the required minimum slope.

For the average house, a 4m deep extension is about the limit for keeping below the cills.

 

[ragj195]

Thanks, I've measured 4m out and it's plenty enough to be honest so I'll stick to that.