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