Proper use of a roofing square

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One of the most useful and important tools in the carpenter's arsenal is the roofing square or the framing square. It is one of the most essential tools for wood frame construction, and some version of the tool has been used in construction for hundreds of years. Whether you are building a shed, doghouse or a home, the best way to learn the proper use of a roofing square is to learn some basic information and practice cutting rafters on pieces of scrap timber.


Generally, the roofing square is a flat L-shaped steel or aluminium tool that has two components. The longer and wider arm, or blade, is 30 cm (24 inches) long and 5 cm (2 inches) wide. The other end, which is 40 cm (16 inches) long and 3.7 cm (1 1/2 inches) wide, is often referred to as the tongue.

Layout rafters

Rafters are the structural components that provide support for the weight of the roof. There are various types of rafters, such as common, hip, valley and jack rafters. Although there are multiple methods in which to determine the layout of rafters, the simplest technique involves using a combination of two numbers: the pitch and span and roofing square.

The span is the distance between two exterior side walls. The pitch or slope denotes how much the roof rises for every 30 cm (12 inches) it runs and is written as 8/12 or 8-12.

One of the most popular uses for the roofing square is to layout roof rafters using four tables on the face of the tool: length of common rafter per foot run; length of hip or valley rafters per foot run; length of jack rafters 40 cm (16 inches) on centre and length of jack rafters 60 cm (24 inches) on centre.

Common rafter length

Look on the first line of the roofing square to determine the length of common rafters per foot run. For example, if the pitch of the roof is 7/12, look up the number 7 on the square. Under the number seven is the number 13.89, which means 35.28 cm (13.89 inches) for each 30 cm (1 foot) of the run. Remember, the total run equals 50 per cent of the span. If the span in this example is 9.75 m (32 feet), multiply 35.28 cm (13.89 inches) by 4.8 m (16 feet) and you get 565 cm (222.24) inches or a rafter length or 5.65 m (18.52 feet).

Valley or hip rafters

Generally, hip and valley rafters usually run parallel to one another between roof sections and at a 45 degree angles to the exterior walls of a house. Use the second row of numbers on the roofing square to determine the length of the hip or valley rafters. Find the inch line on the roofing square and the get the number directly below. If the inch line is 8, the number below it is 47.65 cm (18.76 inches). Multiply it by the total run of the common rafter.

Jack rafters

Jack rafters are a shortened version of the common rafter. The jack rafter's length depends on whether the rafters are 40 or 60 cm (16 or 24 inch) on centre.The jack rafter gets shorter, or longer, as you move along the structure, from longest jack rafter to the shortest component or vice versa.

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