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Beginner Rafter Layout | Speed Square | Common Rafter

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- Today, we're learning how to lay out and cut

common rafters using a speed square and a basic calculator.

Gabled roofs like our mock-up here

are made up of common rafters,

and in order to lay out those rafters,

you'll need to know the slope of the roof first.

In order to get the slope

you'll need to measure and understand four things:

span, total run, unit run, and unit rise.

First is the span.

The span is the total measurement

of the building from its outside edges.

So the span of our mock-up here is four feet.

Next is the total run.

This is an easy number to get

because it's half the distance of the span,

or in our case, two feet.

To understand unit rise and unit run,

we're gonna use this visual aid.

Unit run, which is represented by this side of the triangle,

is always 12 inches