A rafter table is used by carpenters to cut roof rafters correctly. The table includes the pitch and width of the roof, and is used to determine how to cut jack rafters and calculate the bevel required for jacking beams.
A rafter table is a set of numbers used by carpenters to determine how to cut roof rafters. In roof construction it is imperative that all angles and dimensions are cut correctly or the roof will be off axis. If the dimensional error is large enough, the rafters will not adequately support the weight and may fail when heavy snow or ice lands on the roof.
The dimensions of the joist are normally stamped on a steel square which is made at a right angle or 90 degree angle. Similar in width to a conventional yardstick, it is typically only 24 inches (about 61 cm) long. Other names for this steel square are carpenter square, beam square, or framing square.
When designing rafters, the desired roof slope is the first number to determine when calculating rafter sizing. The pitch of the roof, or slope, on the joist table is noted as the amount of rise over the ride. The rise is the vertical angle and the travel is the horizontal length. Stroke, or horizontal length, is always described in 12 inch (about 30.5 cm) increments.
The second element of beam construction using a beam table is understanding the span. The width is the length of the space you are trying to cover with the roof. To calculate the building light, measurements are taken from the outside corner to the corner. This measurement method will include the thickness of building materials such as brick, stucco or wood.
Once the pitch and width are determined, the carpenters consult the rafter table to determine how to cut the jack rafters. Jack beams are the shortest pieces of wood used to support the main length of the beam. The end beam is roughly triangular in shape; jack beams are attached to the inside of the triangle at corresponding intervals. The jack beams shorten as they fan out from the center and terminate just short of each end of the main beam.
The final use a carpenter has for a beam table is to calculate the bevel required for jacking beams. Since the jack beams fan out inside the main beam at an angle, each end must be chamfered to fit properly. The carpenter places the rafter table over the jack beam and draws a line that is based on the measurements printed on the rafter table. This provides the correct chamfer that matches the pitch and length of the main beam.
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