Since the cosine of an angle in a right triangle is equal to the length of the adjacent side divided by the hypotenuse, we can derive the following formula to calculate the rafter length: Now that you know the run and the angle in degrees, you can use trigonometry to calculate the rafter length. Run = (total width ÷ 2) – (beam width ÷ 2) Step Four: Calculate the Rafter Length Then, account for the beam or ridge board by subtracting half of its width from the run. To calculate the run, divide the total width of the building in half. The rafter’s run is the horizontal distance between the end of the rafter and where it meets the beam, which is known as the rise. Measure the span by measuring the width of the building using a tape measure, then add the overhang on each side to the measurement. You’ll also need to account for the overhang beyond the building on either side. The roof’s span is the total length of the roof. You can also use our rise over run calculator to find this. The run is half the measure of the roof span. You can use the following formula to get the rise: The rise is also the term frequently used for the vertical beam that the rafters will butt against. If you have this figure, you can use it plus the run, to get the rafter length. Pitch can be used to help calculate the rise. Table showing the angle in degrees for various roof pitches. You can also find the angle for your roof pitch in the table below. The angle in degrees is equal to the inverse tangent of the pitch of the roof. But, in order to calculate the rafter length, we need to calculate the pitch in degrees. Extra blocking will be required.įor more information such as proper connections and bracing contact tech support.The roof pitch is the angle of the roof and can be measured in several ways, but is most commonly expressed in rise over a standard 12-inch run. Ripper Strips on TJI® Joist Top Flange: Rip dimensional lumber diagonally to the desired slope and fasten to the top flange of the TJI® joist with nails or Simpson Strong-Tie® Ripper Clips. Contact insulation manufacturer for thermal and installation information. Tapered Rigid Insulation: Often seen on large scale projects, tapered rigid insulation can be used to create minimum slope on top of a TJI® joist roof system. Taper Cut Trus Joist Structural Composite Lumber: A designer of record may specify 1- 3/4" (or wider) TimberStrand® LSL, Microllam® LVL, or Parallam® PSL roof joists to be field modified with a taper cut. Raised Bearing: At one end of the joist raise the plate height and add a vertical 2x to the side of the TJI bottom flange to provide a flat ceiling surface.To achieve a minimum 1/4" per foot of slope, bearing wall elevations need to be raised the thickness of a 2x plate for every 6 foot of joist span. Weyerhaeuser suggests the following variations for creating slope with Trus Joist products. It is the responsibility of the designer of record to address items like ponding, snow/drift loads, lateral loads, wind loads, bracing, ventilation, and insulation when adding a slope to roofs. The International Residential Code (IRC) states that roofs must have a minimum of 1/4:12 slope that allows water to drain properly (Chapter 9)įlat Roofs with a slight slope can be achieved using Trus Joist® TJI® joists and solid section products. When designing and installing a flat roof, it is essential to provide for drainage to avoid leakage problems and pooling water on roofs.
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