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Estimating Sheet

How to work out the length of the roof rafter

Construction Software

In this informative civil engineering video tutorial, you will learn how to work out the length of roof rafters by applying angles.

For calculation purpose, pythagoras theorem is followed here.

Suppose the breadth of the rafter is 9 meter and height is 5 meter. The angle is 20 degree.

Suppose, there is a triangle with sides A, B and C. The side opposite of perpendicular is taken as hypotenuse.

As per Pythagoras theorem,

Hypotenuse2 = P2 + B2 (Here, P = Perpendicular and B = Base)

or, Hypotenuse = √ P2 + B2

The side opposite of angle is taken as perpendicular.

Now, it is given that,

AB side of triangle = Base = 5 meter (as height is 5 meter)

AC = Hypotenuse = Unknown

It is known that,

Sin θ = P / H; Cos θ = B / H; tan θ = P / B

Now, it is clear that the calculation can be done with Cos θ = B / H

Also Read: Types of roofing materials used in roof construction

After putting the value of θ, we get the following :-

Cos 200 = 5 / H

H = 5 / Cos 200 = 5 / 0.9397 = 5.321 m

So, the length of the roof rafter =5.321 meter

To get more clear idea, go through the following video tutorial.

Video Source: L & T - Learning Technology

How to work out the length of the roof rafter