Directions for Cutting Roof Rafters

Written by john landers
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Directions for Cutting Roof Rafters
(cabin image by Jim Dubois from Fotolia.com)

Directions for cutting roof rafters may be challenging for anyone who has never had the opportunity to lay out a roof and make the required cuts. The majority of homes that are built today are constructed with trusses, thus many professional and do-it-yourself enthusiasts often avoid this aspect of the framing process alike. The best way to learn the techniques of cutting rafters is to start with a small test project, such as a 24-by-32-inch box. Practice until you are reasonably comfortable making the cuts. Once you have the method down, move on to the real thing.

Considerations

The actual task of cutting roof rafters begin with making sure that the walls are level and square. Square wall sections will ease the task of fitting rafters. There are basically five types of rafters: common, hip, valley, jack and end rafters. You also need to know two measurements: the span of the roof and the slope. The span is the total width of the building. The pitch or slope is the angle of the roof or how steep the roof is. The figure is usually denoted as 6/12 or 6-12. The numbers mean that a roof rises six inches for every 12 inches it runs.

Framing Square

The main tool for cutting roof rafters is the framing square. The tool is actually a right triangle that is very versatile and not only used for laying out rafters and staircase stringers. The framing square has measurements and tables that guide you in cutting rafters the correct length. The wide part of the framing square is the "body." The narrow part of the framing square is referred to as the "tongue."

Use a construction calculator to compute the length of the rafters. Just enter basic data into the calculator and it will give you the proper measurement for the type of rafters you are cutting.

Lay Out Rafters

Common rafters are the main rafters that run from the ridge board to the walls they sit on. Cuts that are made on rafters will be toward the top edge of the rafters. For a roof with a pitch of 7-12, position the square so that the tongue is on seven inches. The body of the tool is on 12 inches. Make three plumb cuts in the rafter. The first cut is where the rafter is secured to the ridge board.

The ridge board is the horizontal board that runs down the centre of the roof at the peak. The second plumb cut, the heel, is the point where the rafter rests on the top plate of the wall. The third plumb is called the birdsmouth. The cut is made toward the bottom end of the rafter, which extends over the wall.

In the above example, the run is 12 inches. This means the process has to be completed 12 times as you work down the rafter. The 12th time is where you mark the rafter for the "heel" cut. Make the birdsmouth cut. Go back to the top of the rafter and cut off ¾ inch, which is half the thickness of the 1-1/2-inch ridge board.

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