If any terms or wording I have used is confusing at all or anything seems unclear, please feel free to ask a question. I don't check everyday, but I do check in often, and would be happy to answer any questions or edit something that is unclear for you.....
These instructions should enable the moderately skilled handy-person to produce a set of common rafters for any building. These instructions are for the moderately skilled. I have omitted basic steps on the tools needed.
Rafter stock will be determined by geographic location, the width of your building, and local building codes. . . .
A pencil, tape-measure, circular-saw, speed-square, framing square, and saw-horses.
Risk of serious injury or death is possible!
If you are unfamiliar with the listed tools and the dangers they possess, these instructions are not for you.
These instructions are formatted in such a way that the peak of the rafter should be to your left, the tail should be to your right, roof side away from you, ceiling side close, laying on the saw-horses. Please read the entire instructions before beginning and then follow them step by step.
Step 1: Determine your run.
(1) Divide the width of your building in half, and remember that number. This is your run. If your building is an even foot-measurement, you can skip the edit below and go to the next step.
If your building is Feet and/or Inches, the edit below shows how to convert the inches and fractions of an inch to decimal values of a foot so the math in a later step will work.
Edit per hangdawg01's excellent suggestion...
If half of your building measurement is not an even foot measurement like 10' 3&7/16" this presents a problem because the Customary Foot Measurement System is a 12 based numbering system. We need to get it to Decimal Notation form which is a much more "math friendly" 10 based numbering system. A couple steps of division, and then add your answers together. It's pretty easy after you see how and do it once or twice..
Lets do this....
The formula is as follows.....
Even feet + [even inch divided by 12] + [ numerator of fraction divided by denominator of fraction, divided by 12]=R. Or using 10' 3&7/16" for our example, 10'+ [3/12] + [7/16/12]= 10.286 feet.
Now that we have this fraction in decimal form, we can multiply in a later step.
We solve for what is in the brackets first, so....
************ 3 / 12 = 0.250
7 / 16 = 0.437/12 = 0.036
********** Feet = + 10.000
************ Run = 10.286 Feet .
For full inches out of a foot, like 3" it is just a division. 3"/12" or 3" divided by 12" = N ( Answer N = 0.25 feet )
Now, for a fraction of an inch out of a foot, it is double division. Lets use 7/16" for example...
We have to divide 7 by 16 to get our decimal inch value, and then divide that by 12, to get our decimal foot value..
It reads like this in mathematical formula 7/16/12.
So 7/16= .437 inches. And then, .437/12=.036 feet. So 7/16 out of 1 foot is .036 feet.
And now we add the 3" decimal of a foot from above to it...
.25+.036= .286 feet. So 3 and 7/16 out of 1 foot is .286 feet, and now we would simply add the 10 feet to it.
I hope I've written this out so that it makes sense and is not overwhelming. It really is much easier and simpler than it looks in writing. Give it a try a couple times and you'll see.