How to Know the Rafter Size for Your Attic

Existing Homes

• If you are making a repair in an existing home--such as with a leaking roof that has damage to the rafters that needs to be repaired--it’s fairly easy to determine the size of your rafters. You can enter the attic and simply measure the rafters with a tape measure to determine the size. Different houses have different sizes, so a physical measurement is required to know for sure what kind you have.

Span

• There are a number of different spans to use, and it all depends on the size of the rafters themselves. A common span is 16 inches on center, which means the rafters are spaced out 16 inches from each other, using the center of each rafter as a base measurement. However, there are dozens of calculations that can affect the overall span of the rafters, not the least of which is the actual size of the rafters and the weight they will be holding up.

Size

• Rafters can be purchased in a variety of sizes, although most of them are 2 inches thick. You can find 2-by-10-inch-pieces, 2-by8s, 2-by-12s, and so on and so forth. The reality is that you cannot plan on the exact size of the rafter that is necessary for your attic roof because the calculations needed can only be made by a licensed professional. The type of shingles you are using, the part of the world you live in, the soil conditions and the type of foundation you have can all factor into the size you need for your attic roof.

Load Calculations

• Load refers to the weight that the roof must hold over the years, and it is more than just the weight of the roof materials themselves. While the shingles and the subsequent wood sheeting installed on top of the rafters is certainly important, the structural engineer also has to factor in the weather conditions. Will there be hurricanes? Do you live in a heavy ice area? Does it snow 3 feet or more per year? There are dozens of load calculations that can affect the size of your rafters.