if you own a rafter square you can find a common multiplier that will get you your lengths. Take 1/2 the span of the building and multiply it by the multiplier. This will give you your length material. Also add in for the tail for the overhang. The multiplier will be found underneath the coresponding pitch on the rafter square. The numbers fall beneth the inch incraments on the body ( fat part) of the square.