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