Roof Rafter, Joist and Beam Design
Part 4 of Residential Structural Design Tutorial
On this page we will explain how to design with roof rafter span tables and how to read all types of joist and roof span tables for residential framing.
This page is part of the Design Your Own House online tutorial. If you are just starting out with your house design, you may want to go to our tutorial site map to go through the tutorial in order, or to jump around it as it suits you.
Wood Roof Framing
Continuing on from Part 3: Floor Beam Span Tables of Residential Structural Design, we will now look at roof rafter and beam design.
Let's consider a basic gable wood roof framing design. The image below shows a cross-section of our simple 12' X 13' house from the joist span tables section of this tutorial. Here we have a wood framed gable roof. Along the top center of the house there is a beam to which are attached the rafters. Each rafter is in turn fastened to the top of the exterior structural walls.
As we discussed in Exterior House Designs there are many different house roof types. 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.
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. All roof framing tables are organized by maximum snow load. 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.
Depending 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).
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).
Framing Table for Maximum Roof Rafter Spans
Sample Maximum Spans (feet-inches) Douglas Fir or Larch, No.1 or No.2 with 31.3 psf of Snow Loading
This table is simply a sample and may not be valid for your region.
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). 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. Let's use this example and find a size of rafter that will work. Looking through the center of the table, we can find a couple of options. 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. Another option are 2 X 8 rafters spaced 24 inches o.c.
In making your choice, consider how you will insulate your insulate your roof. 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. So if your rafters are only 5 1/2" deep, that is all the space you have for insulation. It is possible, however, to add pieces of wood onto the rafters to fur them out to allow for more insulation. This is generally cheaper than building with a larger dimension of lumber. 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.
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.
Built-up Roof Beams
You will need a roof beam as a nailing surface to support the rafters. Typically, the roof beam is made by nailing together a few pieces of dimensional lumber. 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. The table below shows built up beams for 31.3 psf of snow loading.
Sample Maximum Roof Beam Span Table (feet-inches)
Spruce, Pine or Fir, No.1 or No.2 with 31.3 psf of Snow Loading
Maximum Rafter Span of 16 feet
This table is simply a sample and may not be valid for your region.
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). So this table could accommodate a house with a total rafter span (or house width) of 32 feet.
Now recall from Part 3 of this module, Beam Span Tables, we discussed that we may need structural posts to support the floor beams. In the same way you may need posts to support your 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 this way, you can continue your floor beam structural posts all the way from the basement up to the roof beam. It is not a continuous post however. A post for each story sits on the floor surface up to support the floor beam above it. The next post sits atop this floor beam (directly above the lower post) and runs up to the next floor beam or to the roof beam.
Summing Up
In this tutorial module, Residential Structural Design, we've covered basic structural loads for wood framed homes, sizing floor joists, floor beams and roof beams. For a timber frame home or a house design that uses glu-lams or LVLs, the principles are the same but you will use different tables. Your local building inspector can direct you to sources for the tables required in your region.
Next Module—Drawing Floor Plans
It's finally time to draw your floor plans. Let's move on to:
Module 8: Draw Floor Plan
If you haven't followed the Design Your Own House Plans tutorial from the beginning you may want to go to the Tutorial Introduction before moving on to drawing floor plans.
For a more technical discussion on designing with wood roof framing see the span tables tutorial from the American Wood Council.
