That load takes an increasingly deep rafter to resist. The rafter is already under a bending load from snow or wind outside and now there is a connection pulling on that rafter in span from the inside. That takes care of the heeljoint problem.Īs the tie is raised it is also placing a bending point load on the rafter at the tie's connection. This calc give the forces in a raised tie ![]() This calc give the thrust forces in a roof at the plate A raised tie is trying to restrain those increased forces. Stick the nut up near at the hinge and squeeze, your force is multiplied many times. Put the nut out there under your hand and squeeze the handles, the nut won't crack. This calc gives the forces in a conventional flat ceilinged roof, if you do some comparisons to IRC tables, this engineering professor was pretty conservative.Īs the tie is raised the forces in the tie increase. Withdrawal is a function of diameter, density AND depth of penetration. A smaller diameter nail requires less penetration to develop its' full shear values, use more of them, think of a truss plate, many small nails. On nails in shear 10 diameters penetration into the main member develops full shear values. HR = Height of roof ridge measured vertically above the top of the rafter support walls.So raising the rafter tie to 1/3 puts more pressure on the connection and you need to put 50% more nails? HC/HR Heel Joint Connection Adjustment Factor When ceiling joists or rafterties are located higher in the attic, heel joint connection requirements shall be increased by the following factors: Tabulated heel joint connection requirements assume that ceiling joists or rafterties are located at the bottom of the attic space. Is this because the weight of the ceiling is bearing more on rafters? Now if I take that 124 inch "rafter spans" and "shall be multiplied by the factors given below", which is. 2 12" O/C can span 124 inches according to the chart. So, If I moved the rafter tie from the bottom of the rafter to 1/3 up from the bottom of the rafter I decrease the rafters span?Įxample: TABLE R802.5.1(1) 2x4 spf no. HR = Height of roof ridge measured vertically above the top of the rafter support walls. HC = Height of ceiling joists or rafter ties measured vertically above the top of the rafter support walls. When ceiling joists or rafter ties are located higher in the attic space, the rafter spans shall be multiplied by the factors given below: The tabulated rafter spans assume that ceiling joists are located at the bottom of the attic space or that some other method of resisting the outward push of the rafters on the bearing walls, such as rafter ties, is provided at that location. With the cold weather, I've been spending a lot of time reading the charts and practicing design ideas.Īll rafterspan tables have a Footnote a to them: 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.Ok. 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. Rafter Span Maximum tabulated or 26 Footnote b Table R802.5(1)-(8) Ceiling Joist Span Maximum tabulated or 26 Footnote b Table R802.4(1) & (2) Rafter/Ceiling Joist Spacing 24' o.c. 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. Joists that do not resist thrust shall be permitted to be nailed in accordance with Table R602.3(1). Rafter Span Tables Rafters with ceiling not attached to rafters, live load = 20 Psf, dead load = 20 Psf, deflection limit L/180 Where ceiling joists are used to provide resistance to rafter thrust, lapped joists shall be nailed together in accordance with Table R802.5.2 and butted joists shall be tied together in a manner to resist such thrust. 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. When calculating the maximum span of a rafter, use the horizontal distance between two vertical supports.
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