Hey Im new to this . Im gona build a friend a leen to shed for his Boat Storage,ap16-20 X 20-30 I would like a 4-12 pitch maybe 5-12. ? How much longer of a board do I need for 16foot run at 4-12pitch. I know that there is a formula for this I lost my book any help out there THANKS DH hutd08@yahoo
Here are a couple sites I use, hope it helps
A 5 on 12 pitch for a 16 foot run calculates to 17. 33 feet not counting any overhang.
a 4 on 12 pitch for a 16 foot run calculates to 16. 86 feet not counting any overhang.
Either way you will end up probably needing 18 to 20 foot rafters.
As a matter of interest, a 4 on 12 pitch is considered the absolute flattest pitch for a shingled roof. I would go with the 5 on 12 if it were me.
The way to figure this is to multiple the run in feet (16) by the rise in inches (4) and divide the answer by 12. That gives you the total rise in feet.
Multiple that number by its self and add it to 16 times16 and take the square root of the sum. That will be the long side of the triangle or the rafter length not counting the overhang.
Let’s say you will have a 16 ft span.
For exactly 16 ft span, slope length at 4-12 pitch is 16 ft 10. 5 in. For exactly 16 ft span, slope length at 5-12 pitch is 17 ft 3 in.
However, you need to take into consideration any roof overhang, widith of support at beams, etc.
A good way to approach this is to start with the standard lengths you will find at the lumber yard. If I were you, I would choose the standard 16 ft board length, then work back to get a reasonable span and slope. I don’t know anything about your design, but you probably don’t want to span more than 16 ft supported by standard 2x rafters.
A lot better than leaving the boat out in the rain like I do.
Good luck.
Bill