Ok so i know that C24 timber is obviously a better grade than the normal C16 but what is the advantage of useing C24 4"x2" as roof timbers over the normall 6"x2" spec on a mono pitch extension i am doing, this is the spec from the drawings but just wonder why? anyone else come across this before? Also C 24 is a special order from my local builders merchants And the local timber yard said they could simply go through the standard timber, pick out the good bits and regrade it from C16 to C24 and charge extra for it!!!