How many rafters do i need for a 20 foot span 

How many rafters do i need for a 20 foot span
To determine how many rafters you need for a 20-foot span, you should consider the spacing between each rafter. The most common spacings are 12 inches, 16 inches, or 24 inches on center (OC). Here’s how you can calculate it for each spacing:
1) 12 inches OC: number of rafters = (20 feet×12 inches/foot/ 12 inches) + 1 = 20+1=21 rafters
2) 16 inches OC: number of rafters = (20 feet×12 inches/foot/ 16 inches) + 1 = 15+1=16 rafters
3) 24 inches OC: number of rafters = (20 feet×12 inches/foot/ 24 inches) + 1 = 10+1= 11 rafters
Therefore, For one side roof, you will need:21 rafters if spaced 12 inches OC,16 rafters if spaced 16 inches OC, 11 rafters if spaced 24 inches OC.
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