The required number of roof rafters depends on the length of your awning. The recommended number of awnings is as follows:
Awnings under 12' — At least 1 roof rafter
Awnings 12' - 16' feet —At least 2 roof rafters
Awnings 17' - 20' feet — At least 3 roof rafters
Awnings 21' and above — At least 4 roof rafters