Lean to shed roof long and narrow

[mike42]

Hello All

So I'm planning my shed at the moment which will be a lean to off the house wall. Its 2.8M along the wall then a long 4.8M projection. So the rafters will likely be at least 5M.

Like everyone, I want to keep the costs down. A purlin or purlins might be the answer? Would that be a case of fitting a fairly big purlin across the width (wall plate to wall plate) in the middle of the projection ? How would I know how big to use and how would it fix to the wall plates?

And say the rafters turn out to be 5M - would having one purlin in the middle mean I could consult the spanning tables for 2.5M ? I have a feeling its not as simple as that.....

Roof material not 100% decided upon but probably something light like felt shingle.

 

[Notch7]

And say the rafters turn out to be 5M - would having one purlin in the middle mean I could consult the spanning tables for 2.5M

Yes

A purlin or purlins might be the answer? Would that be a case of fitting a fairly big purlin across the width (wall plate to wall plate) in the middle of the projection

Maybe you could get away with a dbl 6x2 as a purlin and 4 x 2 for the rafters

It would be wise to lay out the side frame studwork so there is a stud below the purlin

 

[mike42]

Thanks Notch. If I did that, would the purlin be horizantal from side to side resting on the wall plates? Something like the attached ?
Supposed to be side view so only showing one of the rafters.

 

[Notch7]

Thanks Notch. If I did that, would the purlin be horizantal from side to side resting on the wall plates? Something like the attached ?
Supposed to be side view so only showing one of the rafters.

If you are doing it as per your sketch the purlin would normally be set at the roof pitch as well as the purlin props - but If it’s a shed, I’d make the framework to be the underside of the rafters, then set the purlin into the framework, in which case set the purlin vertically

I suppose it depends on the pitch, but if you are doing this under permitted development it can’t be very high

 

[mike42]

Thanks a lot for taking the time to help me out.

I've updated my back-of-an-envelope and put the purlin directly supporting the rafters. Not sure I fully understood you but am I closer to what you are suggesting or have I got it totally wrong (!) ?

Won't get into planning etc just yet but pitch will be as low as I can but could be as high as 15 or 20 degrees.