What centres for roof spars

[thyristor44]

Hello
I am slating an outhouse with slates, 10 x 18 thin blue slates. The span of the building is 3m to the outside. It is very low but is used for access to a garage so headroom is at a premium. I have some 4" x 4" spars I want to use but the question is how far apart should they be. I intend to cover with 11mm osb, counterbatten, membrane then laths for the slates. Building is 3.8m long and I have 10 off 4x4s. What centres please?
Thanks
thyristor44

 

[noseall]

What is the pitch of the roof and what will be the clear span of each rafter?
What are you using for ceiling/floor joists?

 

[thyristor44]

What is the pitch of the roof and what will be the clear span of each rafter?
What are you using for ceiling/floor joists?

There are no joists, max height is only 2m, its a lean to. Slope is 20 degrees and clear span is 2.4m.
Thanks

 

[noseall]

A standard 100mm x 50mm section will span around 2m at that pitch/loading. Doubling the width section is not as beneficial as increasing the depth, but I guess it must count for something.
I'd say you may be ok at 400mm c/c but someone like Tony (S.E.) will need to advise.

 

[thyristor44]

Thanks noseall
That gives me some idea.

 

[catlad]

I would say you on the edge of what the span tables would say
but I would agree with Noseall if you are using thin slates.

 

[thyristor44]

Yes they are thin swo I will give it a go.
Thanks for all the help mates.
All the best
thyristor44

 

[noseall]

Yes they are thin swo I will give it a go.

Good luck birds-mouthing a 4" x 4".