Calculating the Common Rafter length (crl)
Method 1:
The common rafter length can be determined by the use of the Pythagorean Theorem . . .c2 = a2 + b2 . Where a² is the building rise and b² is the building run in inches.
- So, if you plug in the numbers; you get
- (((square root) crl) = rz2 + r2 ) or
- (((square root) crl) = 1282 + 1922 or
- (((square root) crl) = 16384+ 36864 or
- (((square root) 53248) finally c² or crl = 230.755 (230-3/4").
- It's important to note that any inexpensive calculator can help you determine these numbers. It does not need any fancy sci-fi functions on it, just +, -, x, /, squared and square root. If you should want a cool calculator to do the job you could always think about getting a Construction Master Pro Trig Calculator.
- The reason for using this method is that you do not have to "adjust" your rafter lengths while marking, this was taken care of when you made your calculation.
Method 2:
This is known as the "Step Off Method" and its for an 8/12 pitch roof.
Common Ridge Plumb Cut.
Once You now the "mathematical" length of your common rafter you can being to make what is known as the "Ridge" plumb cut. Set for rafter "stops" to 8" on the tongue and 12" on the body of the rafter square respectively.
First "crown" the rafter stock and place the crowned side away from you. This will be the top side of the rafter. I work from my right to my left when laying out rafters, but you can do what is most comfortable for you.
Place the square so the stops ride on the top side of the rafter. The 8" stop should be to the right and the 12" stop to the left. The "V" of the square should be pointing towards you.
Move the stop to about 1" from the end of the stock and mark a line along the tongue where the 8" stop is. You may need to extend the line depending on how wide your rafter stock is. This is now the Ridge plumb cut line.
Common Length Adjustment.
This number is pretty close to the actual lenght of the common rafter form building line plumb cut to the top of ridge plumb cut. The last that you would need to do is adjust the length of this cut by the thickness of 1/2 (rt). In the cast of your model ridge that will be 3/4".
This is accomplished by measuring 3/4" perpendicular for the ridge plumb cut line. Then marking a new plumb cut line and in effect "shortening" the rafter.
Marking off the Common Rafter Length.
Now measure form the ridge plumb cut line 230-75". This is the point where you will mark the building plumb line. This line is the vertical line of the birdsmouth cut.
Marking the Birdsmouth (heel/seat and building-line plumb cuts)
The next step is to measure down from this point the value of the H.A.P., in this case 6". From this point draw a line perpendicular to the build plumb cut line in the direction of the ridge to mark the "seat" cut or level line of the birdsmouth.
You can accomplish this by using a square as shown in the graphic.
Marking off the Common Rafter Tail.
This is the amount that the rafter extends past the building line. This is also measured square ( perpendicular ) to the building line. This will create that is cased the “soffit” or eave and this is where the fascia board and gutter are attached to.
The sub-fascia is also a combination of a plumb and level cut. The length of the sub-fascia is the location of the level cut. This is the same as with the birds mouth. In this case we want a 6" ( 5-1/2") finished fascia. This means that the sub-fascia will be 5-1/2" - 3/4" (for the soffit material) - 1/4" for a drip edge. This makes the sub-fascia length 4-1/2" measured from the top side of the rafter down.
©JOSEPHFUSCO.ORG
1997-2010
All written, photographic and drawn materials property of and copyrighted by josephfusco.org 1997-2010.
No materials may be used in any way without the written permission of Joseph Fusco
