You can use them for roof floor and even wall framing but few sheds will need trusses for walls.
Ok to use 2 x 4 for for roof rafters.
If you live in an area that requires a shed roof rafter to support a heavy load then an spf rafter can only span 7.
Rather than having 2 4 purlins every 12 it would make far more sense economically to use 2 6 purlins every 24.
I think you will be fine with 2x4s so long as they are closer than 48 apart.
As mentioned above a 2 4 spf shed roof rafter can span 11.
You can frame a whole house with nothing but 2x4 but it s easier to do a shed as you won t need engineering probably.
The secret word is truss.
Structures such as sheds or barns can be frames with rafters every 24 again make sure to consult local codes.
Front and sides will sit on deck on a 2x4 wall with studs 24 oc and screened in.
Rafters to 2x4 with joist hangers.
Both of these are for light roof loads.
Also if you want to make things cheaper you can do a single slope of the roof and basically cut out the eve cost.
Somehow i am in a traditionalist camp 16 oc for a 12 foot wall will add only 6 extra 2x4 s or 3 extra if you buy 10 foot lengths for the rafters and 6 extra 2x4 s for studs if you use only a single layer top plate on the bearing walls.
It is always better to have a large rafter rather than be concerned with a sagging roof.
Makes construction easier too.
At the pitch range you are looking at the 2x4 is just fine up to 8 6 depending on the lumber grade.
An syp rafter can span 11.