4/24/2024 0 Comments Flat roof rafters span table 2x4A post for each story sits on the floor surface up to support the floor beam above it. In this way, you can continue your floor beam structural posts all the way from the basement up to the roof beam. If your span table shows the roof beam cannot support the roof rafters for the whole length of your house, you will want to choose a built-up roof beam whose maximum spans are the same as (or slightly longer than) your chosen floor beam spans. In the same way you may need posts to support your roof beam. Now recall from Part 3 of this module, Beam Span Tables, we discussed that we may need structural posts to support the floor beams. So this table could accommodate a house with a total rafter span (or house width) of 32 feet. The spans in this table are based on a maximum supported rafter span of 16 feet, where the supported span is the total horizontal distance spanned by the roof rafters divided in half (NOT the diagonal distance or the length of the rafter). Spruce, Pine or Fir, No.1 or No.2 with 31.3 psf of Snow Loading Sample Maximum Roof Beam Span Table (feet-inches) The table below shows built up beams for 31.3 psf of snow loading. Once again, you will need to look at your local charts and use your local snow load factor to determine the maximum span for a given wood rafter size. Typically, the roof beam is made by nailing together a few pieces of dimensional lumber. You will need a roof beam as a nailing surface to support the rafters. If you are building a one or two-story home but may someday convert the attic to living space, make sure your design will allow enough space for adequate roof insulation. It is wise to research the types of insulation you are considering and the R-factor you hope to obtain and then calculate how much depth you need in the rafter cavity. This is generally cheaper than building with a larger dimension of lumber. It is possible, however, to add pieces of wood onto the rafters to fur them out to allow for more insulation. So if your rafters are only 5 1/2" deep, that is all the space you have for insulation. Roof insulation for 1 1/2 story homes (a home where you have living space in the attic) is typically installed in the rafter spaces. In making your choice, consider how you will insulate your insulate your roof. Another option are 2 X 8 rafters spaced 24 inches o.c. The table states that 2 X 6 rafters spaced 16 inches on center (o.c) can span a maximum distance of 13 feet 5 inches. Looking through the center of the table, we can find a couple of options. Let's use this example and find a size of rafter that will work. So if your house is 26 feet wide and the roof ridge beam is centered on the house, you will need a rafter that can span 13 feet. The maximum spans in the table above are for the horizontal distance the rafter can span (NOT the diagonal distance or actual length of the rafter). This table is simply a sample and may not be valid for your region. Sample Maximum Spans (feet-inches) Douglas Fir or Larch, No.1 or No.2 with 31.3 psf of Snow Loading Let's look at the sample table below which shows maximum rafter spans for a snow load of 31.3 pounds per square foot (psf).įraming Table for Maximum Roof Rafter Spans If you live in a region that gets winter snowfall, contact your municipal building department to find out what maximum snow loads you need to design for.ĭepending on your region, you may need to design for a maximum snow load of anything between zero to 65 pounds per square foot (or roughly 0-3 kPa). All roof framing tables are organized by maximum snow load. In most regions, the first thing to consider before sizing the wood framing members for a roof is the maximum snow load that the roof will have to bear. Initially, we will discuss the framing of gable roofs so that we can learn the basics of how to size a roof rafter, beam or joist. Each rafter is in turn fastened to the top of the exterior structural walls.Īs we discussed in Exterior House Designs there are many different house roof types. Along the top center of the house there is a beam to which are attached the rafters. The image below shows a cross-section of our simple 12' X 13' house from the joist span tables section of this tutorial. Let's consider a basic gable wood roof framing design. Continuing on from Part 3: Floor Beam Span Tables of Residential Structural Design, we will now look at roof rafter and beam design.
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