If you don't want to square off the timber you need the section moment I (that's a capital i) for a round cross section. It is pi/64 times d^4 where d is the log diameter. I took d to be 0.18 metres, in which case I (carefully) make the moment to be 5.10^-5 m^4 or 5.10^7 mm^4

y is the distance of the wood fibres furthest from the log centre, so that would be 0.09 metres

With your pitch of 1.4m I get a deflection of 19mm and a max stress of 4.7N/mm^2, not much below the max graded stress quoted for larch of 5.2

Edit:- Using .18m for the dia might be a bit conservative. Use .2m and the section moment increases to 7.5.10^-5m^4 and so reduce the sag and max stress a lot.

Design and Make

Must have been at school at the same time as me.
I made, then designed, that way the drawings matched the final product perfectly

Canada and the US..... as now!

In fact it [snow] totally dominates

.

Not when I did my roof calcs (for a flat roof in East Anglia, using eurocode5) it didn't. The point loading (for someone on the roof for maintenance) was the major cause of deflection and thus the thing that determined the timber sizes.