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Beam (10)

Typically, beams are made by nailing together multiple 2x boards. (Note: "2x" stands for boards with a "nominal" size of 2 inches. For examples, 2x4, 2x6, 2x8, 2x10, and 2x12 are the common "two-by" boards used in construction - 2x boards are really only 1.5" thick). Nailing 2x boards together to make a beam is cheaper than buying a solid beam and is stronger than a solid piece of lumber of the same size. Beams are generally used at the middle of larger decks and at the far end. When they're used at the far end, we call them rim joists. Sizing beams is a three step process.

Step1: In the first step, the area of the deck that the beam supports is figured out. We call this the "tributary area". Finding the tributary area is a relatively simple process because each beam carries a section of floor defined by the distance between the posts and half the length of the floor joists.

Step2: The second step is to convert the tributary area into the number of pounds per linear foot (plf) the beam must carry. We do this by looking in a Conversion Table for the multiplication factor. This factor is based on the amount of snow that the area gets. One of the great features of a Conversion Table is that it already incorporates code requirements concerning load combinations and drifting snow load effects making your job easier. Note: If you live in an area that doesn't get snow, the conversion table has entries for you too.

For this example, the beam is at the edge of the deck with floor joists attached to the side with metal hangers. If we live in an area of the country that gets up to 60 pounds per square foot (psf) of snow, the conversion table says we need to multiply the tributary area by 78 and then divide by the length of the beam (8 ft.). For a 42 sqft. area of floor, the pounds per foot (plf) works out to (42x78)/8=410plf. That's it.

By the way, if you are wondering why we multiply by 78 and not 60, it's because building codes require adjustments that take into account combinations of dead, live, snow, and wind loads. You don't have to learn about all of this to use the tables; we've built these combinations in for you. However, if you're interested, all these topics and more are covered in the lumber sizing sections.

Step 3: The final step is to look up how big the beam needs to be. Using the tables, we see that a beam made up by nailing together three treated 2x8 boards is strong enough to carry up to 470plf when the supporting posts are spaced eight feet apart. Since our example beam only needs to carry 410plf, this beam will do just fine.

The tables on BestDeckSite have all been specifically designed to make it easy for deck builders to determine lumber sizes. These tables automatically incorporate building code requirements for load combinations, outdoor wet service conditions, the type of lumber, and any drifting snow load conditions. To size a beam, builders figure out the area of the deck resting on the beam and the tables do the rest.

If you'd like to learn all about how to size beams, properly nail them together, correctly attach them to the posts, and lots more, there is just a ton of information in the Password Area to help you get the job done right.