roof will be 9' x 10', with 1/2 in. cdx sheathing. plus felt and either roll roofing or 'regular' asphalt shingles. i think 2 x 6 would be better; but if i could bring cost down and have 2 x 4 work well, i'd like to do that. but i don't want to risk having it sag over time........ ?