If the home measures 20 x 40 and has a 7 12 roof then to calculate the slope of the roof follow these steps.
Roof rafter multiplier.
Divide the rise by the run the run is 12.
Now solving for c in the equation above gives.
Students should use the sin cos and tan funtion rather then the multiplier table to calculate the length of the roof members.
And for a roof with a slope of 6 12 that number 1 118 is your roof slope multiplier.
It takes some time to get use to these functions.
Read the rafter size in the left column of table 10 1 for the 3 3 m span chosen.
Here are the equations you may use to solve for the length of the slope and the rafter.
The hip and valley factor is a number that is multiplied by the actual run horizontal distance of a common rafter to determine the length of a hip or valley rafter.
The slope of the roof provides the rise and the run to be plugged into the equation.
800 multiplied by 1 16.
Take the area of the roof s footprint say it s 5 000 square feet and multiply 5 000 by 1 118.
Calculate the length of a rafter from the roof slope ratio of inches per foot and a building width measurement.
For roof slopes expressed as x in 12 rise in run the roof pitch multiplier is determined by finding the square root of rise run 1.
Pitch rise run 1 5 6 25.
If you add an eaves overhang dimension then the amount the rafter sticks passed the wall will be added to the rafter length.
The next option in table 10 1 is 3 3 m.
Where a is the rise of the roof b is the run of the roof and c is the rafter hypotenuse of the triangle with side lengths a and b.
Calculate the roof pitch as the proportion of rise and run.
Remember practice makes perfect.
You can also calculate roof pitch even without using a roof slope calculator.
Use the following formula which also describes a right triangle.
Multiply step 1 and step 2 i e.
A roof pitch of 4 in 12 4 12 has a rise of 4 and a run of 12.
That gives you 5 590 square feet which is the actual area of the surface of the roof.
Rafter rise run this formula is from the pythagorean theorem.
Multiply 20 and 40 which equals 800.
The more you practise the more familiar you become using these functions.
This exceeds the rafter span proposed without using the multiplier so is suitable.
Find the square root of the result.
This table provides the hip and valley factor for common roof pitches as well as the underlying equation for the factor.
The formula for the roof pitch multiplier comes from the famous pythagorean theorem.
2 5 1 1 multiplier 2 75 m does not give the required span of 3 000 m.
Find the value of 7 12 from the roof slope multiplier table which is 1 16.
This indicates a 190 45 mm rafter size.
Calculate the rafter length substituting these values into the following formula.
Rafter rise run 1 5 6 2 25 36 38 25rafter 38 25 6 18 m.
Step 5 rafter size.
Let s say it is equal to 1 5 m.
