Constructing a traditional cut roof

Level 3 Diploma in Site Carpentry

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PowerPoint presentation

Constructing a traditional cut roof

Unit 306: Carry out first fix roofing and stairs



IntroductionThe aim of this session will be:

• to understand how to construct a traditional cut roof.

The learning outcomes will be to enable the learner to:

• state the information required to calculate the length and angle of the rafters

• explain the methods of determining rafter length

• explain the methods of determining angles of rafter cuts.



Terminology

Source: BarkingDog



Definitions• Roof span (effective): This is the distance measured from the

outside to the outside of the wall plates.

• Rise: This is the vertical height measured from the top of the wall plates to the intersection of the setting-out line. When measuring rafters, its true length is taken as a straight line from the setting-out line.

• Roof pitch: This is the angle or slope of the roof. It is expressed in degrees, or as a fraction or ratio that is found by dividing the rise by the span.



ExampleIf a roof has a span of 3m and a rise of 1m, then the pitch would be calculated as follows:

Pitch = Rise (1m) / Span (3m)

Therefore, the pitch = 1/3

Since the rise is a third of the span, the angle of the roof would be 33°.



Rafters

Source: BarkingDog



Roof geometry

Source: Tim Taylor



Using a roofing squareA roofing square is used to determine the lengths and angles of rafters.

• A roofing or framing square is a metal square which consists of two arms positioned at right angles to each other. The longer, wider arm is known as the blade; the shorter, thinner arm is known as the tongue. A separate piece of wood is used as a fence for marking out the rafter.

• Once an understanding of geometry is achieved, then geometric principles can be applied when using a roofing square.



Using a roofing square • The length of the blade is 620mm; the length of the tongue is 450mm.

• The roofing square is calibrated in millimetres and degrees, and both sides contain a set of tables which provide both rafter and hip lengths (in metres) for various increases in degrees.

• To use the square, the rise of the roof is set on the tongue and the run of the rafter is set on the blade.



A roofing square

Source: BarkingDog



ExampleConsider a common rafter for a roof with a rise of 4m and a rafter run of 5.5m.

To accommodate the use of the roofing square, the sizes are usually scaled down, or reduced, by 10.

Therefore:

A rise of 4m ÷ 10 = 400mm

A run of 5.5m ÷ 10 = 550mm



Marking out raftersOnce the measurements have been scaled, these can be used to mark out the rafters.

Source: BarkingDog



Marking out raftersOnce the measurements have been scaled, then the sequence of positions can be marked out:

Source: BarkingDog



Any questions?

Education

Constructing a traditional cut roof