Need help want to cut rafters for pump house?
Question:i think the formula is run times pitch then i am lost
Answers:
There's a couple of formulas the square root method
and the secant method .I'll explain square root .I'm assuming its a gable
when your cutting rafters think of the roof as a right triangle
take the building width which is the span divide that in half
for example say the building is 20 feet wide 20/2=10 that's the base of your right triangle now you need to find the other leg of the right triangle which is the rise lets say are roof is gonna be a 8/12 roof so to get the rise is simple the roof rises 8 inches for every foot of run remember your base was 10 foot so 8*10=80 inches is your rise .So to get the rafter length is easy 120*120+80*80=20800 the square root of that is 144.222051 on your calculator subtract 144 and multiply .222051*16=3.55 round off to 4 so your Rafter length is
144 1/4 inch.your ridge is 80+h.a.p(height above plate)
Make sure you subtract 1/2 of what ever ridge your using
or your rafter will be to long.
To get the length of your overhang think of it like a small right triangle say you want 18" of overhang .First convert 18" to feet
18/12=1.5' this is the base of your right triangle next find the
rise. The roof is a 8/12 so 8*1.5=12 so 12*12+18*18=21.6333 subtract 21 and .6333 remains
multiply that by 16 and you get 10.128 which is 5/8 so add
21 5/8 to your rafter length.
Give me a email and i will explain in greater detail
First, find the base of the measuring triangle (the run of the roof). Measure between the bearing walls (for example 12ft) and subtract the width of the ridge beam, then divide by 2.
Second, find the altitude of the measuring triangle(ridge height). Divide the base of the measuring triangle by 12 and multiply the result by the rise of the roof pitch. For a 12-in-12 pitch, the base and the altitude are the same. (you can round the numbers up)
Third, find the hypotenuse (rafter length). Divide the base of the triangle by 12 and multiply the result by the hypotenuse of the roof pitch, which is listed as length of common rafter of the rafter square.
Use a framing square. Convert the feet of your wall and the height of your ridge into inches. Put a mark at both of these numbers and then measure the distance between them. Convert that back to feet. Add a bit at the bottom for the overhang (eave), and there you go. I'm assuming that you know something about carpentry.
There is a table of numbers on the face side of the steel square; this is called the rafter table. The rafter table allows the carpenter to make quick calculations based on the Pythagorean theorem. The table is organized by columns that correspond to various slopes of the roof. Each column describes a different roof inclination (pitch) and contains the following information
This table shows five different types of rafter calculations and one table for marking an angle called the side cut or cheek cut.
This is a common rafter with the two different cuts. The plumb cut fits in the ridge board and the bottom cut fits on the wall plate.
Common rafter per foot of run The common rafter connects the peak of a roof (the ridge) to the base of a roof (the plate). This number gives the length (hypotenuse) of the common rafter per twelve units of horizontal distance (run).
Go to the source listed below. Good Luck!
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