Sailcloth weight for Surf

Not certain yet, but it looks like I'll be building a Surf. Once it's
certain, I'll order the plans.

Starting to scrounge already, though.

Can anyone tell me what weight of sailcloth is used for the Surf sails? We
have a tarpaulin and tent outlet here which occasionally has ends of
sailcloth rolls at reasonable rates. If I start looking now, I might be
lucky by winter.

I'm thinking that even if I build something else, sail cloth weight in a
sail about that size is likely to be similar. . . . I hope!

Thanks for the help.