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