Expert: Dave Johnson - 6/25/2008

Question
QUESTION: Will a roof with 4 x 2 rafters ,5 metres long,at 2 ft intervals,be strong enough to suppport standard 8 x 4 plywood sheets covered in roofing felt.1 end of rafter to be attached to a wall plate,the other end to a 4 x 4 beam supported on brick pillars

ANSWER: No, You will need at least 2"x8" at 16" o.c. to span 16feet.

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QUESTION: would 4 x 4 planed,or unplaned, timber work, and what spacing would I need.I live in Spain and the woodyards have a limited range of sizes .Alternatively , and only as second best,could I use another 4 x 4 beam at 2.5 metres as a brace.however beam only supported at each end

Answer
hi keith,

The span (length) is what is the problem, you need more depth to your joists. if all you can get are 4" depths, use 2 and put one on top of the other... but they will need to be attached to each other with some fasteners. anything less like 2"x4" will surely sag under just the weight of the plywood.