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RAFTER SPAN CODE
Andrew check out my article Rafter Cutting Basics JLC Aug 1999, it explains all this mumbo jumbo about HAP, using the calculator etc. Per the prescriptive tables found in Chapter 8 of the International Residential Code (IRC), the standard roof rafter spacing used is 12, 16, 19.2, or 24 inches on center. Of course in some cases the ridge may already be established in the case of some remodel or quirky set up and then you will have to deduct the Hap from the existing Ridge height before you plug that in as the rise. The best way to figure rafter is use the run and the pitch, not the height of the ridge. This is called the HAP or height above plate. When used as a ceiling joist or a roof rafter, a 2×4 can span up to 7 3 spaced at 16, and 6 4 when spaced 24 apart. Now not to go any further but this would require the backing off for 1/2 the thickness of the ridge to get the proper length, not including the fact that the ridge height incorporates the amount of rafter left after the seat or birds mouth is made. 2×4 rafters spaced 12 can span as much as 11’ 3 at a slope of 3:12, but the load can drastically reduce that allowable span. In addition, rafter sizing is expressed as a ratio of the roof’s rise to the run (half of the distance) or a fraction of the rise over the run. Rafters also have a slope that can also impact the weight the load rests on the rafters. The roof rafter span is commonly measured by the distance of the rafter projection horizontally from the ridge to the supporting outside wall plate’s outer edge. Now back to the original question: If you have a 20' distance from the outside plate to the center of the ridge (this would be called the run) and you have a 5' rise of the ridge above the plate, you will need a rafter that is 20' 7" 3/8 with a 3/12 pitch on the plumb cut. Span for rafters measures from the outside face of the supporting wall to the center of the ridge board. But I must say this that the book has several pages in front that detail framing square cut points that are essential in understanding how rafters are figured and cut.
This can cause some confusion for the beginner.
RAFTER SPAN FULL
The A Riechers rafter table book is how I started in roof cutting, but it is called the full Lenth framer because it uses the "span" from outside plate to outside plate to derive the rafter length. Today we call pitch Rise over 12 inches of run) then punch diagonal to get rafter length. Not to be confused by the older text method such as 8/24 as they refered to once as 1/4 pitch.
One enters the Run, the dimension I stated earlier, the pitch, (which in todays world of carpentry refers to the Rise per Foot of Run.
There are several ways to get this, the simplest for me is the Construction Master calculator. This is the diagonal or hypotenuse of the right triangle. The rafter length is what you want when cutting rafters.
This horizontal distance from the outside of the plate to the center of the ridge is the key dimension used to find rafter lengths. The distance to the center between these opposing walls should be considered the run. For the record, Span should be considered the distance from opposing walls.
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