Flat roof

Hi lads just in need of some info i had a chip is to build a shed/office (cus of time) and has only built the frame the was told by the local police that he is in nik and i do not need to pay him because he stole the wood. the police told me that i can keep the wood.


he has not put the bored for the roof in and need to know what is the best sort of board to get (NOTE: is a flat roof) and where is the cheapest to get building goods from?

Thanks lads