![]() Where preformed insulation board is used as the air-impermeable insulation layer, it shall be sealed at the perimeter of each individual sheet interior surface to form a continuous layer. The vapor diffusion port shall protect the attic against the entrance of rain and snow. The vapor diffusion port shall serve as an air barrier between the attic and the exterior of the building. The vapor-permeable membrane in the vapor diffusion port shall have a vapor permeance rating of greater than or equal to 20 perms when tested in accordance with Procedure A of ASTM E96. Where there are multiple ports in the attic, the sum of the port areas shall be greater than or equal to the area requirement. The port area shall be greater than or equal to 1:600 of the ceiling area. An approved vapor diffusion port shall be installed not more than 12 inches (305 mm) from the highest point of the roof, measured vertically from the highest point of the roof to the lower edge of the port. Table R802.5.1(2) Rafter Spans For Common Lumber Species (Roof live load 20 psf, ceiling attached to rafters, L/ 240). In Climate Zones 1, 2 and 3, air-permeable insulation installed in unvented attics on the top of the attic floor or on top of the ceiling shall meet the following requirements: For calculation purposes, an interior air temperature of 68☏ (20☌) is assumed and the exterior air temperature is assumed to be the monthly average outside air temperature of the three coldest months.ĥ.2. Alternatively, sufficient rigid board or sheet insulation shall be installed directly above the structural roof sheathing to maintain the monthly average temperature of the underside of the structural roof sheathing above 45☏ (7☌). The air-permeable insulation shall be installed directly under the air-impermeable insulation. Where both air-impermeable and air-permeable insulation are provided, the air-impermeable insulation shall be applied in direct contact with the underside of the structural roof sheathing in accordance with Item 5.1.1 and shall be in accordance with the R-values in Table R806.5 for condensation control. In addition to the air-permeable insulation installed directly below the structural sheathing, rigid board or sheet insulation shall be installed directly above the structural roof sheathing in accordance with the R-values in Table R806.5 for condensation control. Where air-permeable insulation is provided inside the building thermal envelope, it shall be installed in accordance with Section 5.1.1. Where only air-impermeable insulation is provided, it shall be applied in direct contact with the underside of the structural roof sheathing. Item 5.1.1, 5.1.2, 5.1.3 or 5.1.4 shall be met, depending on the air permeability of the insulation directly under the structural roof sheathing. The wider the space between girders commonly resultate in rafters of huge depth to. Depending on all contributing factors though, 16 press 24 are commonly used for most architecture. To calculate the maximum spans of species not shown above, use the Span Calculator or the Span Tables for Joists and Rafters on the American Wood Council website.5.1. Standard industry roof rafter spacing lives 12, 16, 19.2, and 24 on-center, however other spacing may be used by ampere Structural Engineer. To calculate maximum rafter spans using different design criteria (load, snow load, spacing, grade, etc.) for these common lumber species, see the International Residential Code (IRC). The span values (displayed above) are from the American Softwood Lumber standard sizes. ![]() ![]() Rafters with ceiling not attached to rafters, ground snow load = 50 Psf, dead load = 20 Psf, deflection limit L/180 The following span table uses a moderate snow load of 50 Psf, but yours could be more or less. Consult your local building code authority to determine the snow load in your area. Note: Snow load factors can be specific to the regional location of a structure. Rafter Span Tables Rafters with ceiling not attached to rafters, live load = 20 Psf, dead load = 20 Psf, deflection limit L/180 The braces need to be supported by a bearing wall, shown in the diagram above.Įxample: In the rafter span table below, the highlighted cell (13-0) indicates that a 2" x 8" Douglas Fir rafter, with a grade of #2, spaced 24" apart, can have a maximum span of 13 feet - 0 inches (13-0) if designed for a live load of 20 Psf, and dead load of 20 Psf. Note also that you can break up the span of a rafter by adding a purlin and bracing to the underside. The span of each rafter shall be measured along the horizontal projection of the rafter. For other grades and species and for other loading conditions, refer to the AF&PA Span Tables for Joists and Rafters. When calculating the maximum span of a rafter, use the horizontal distance between two vertical supports. Allowable rafter Spans (R802.5): Spans for rafters shall be in accordance with Tables R802.5.1(1) and R802.5.1(2).
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