Many older decks were built using massive timbers—4x6s or 4x8s—for beams. Some builders still do it that way; using a one-piece beam can save time. But large timbers are heavy and hard to handle. They often warp beyond remedy, and they almost surely will crack over the years.
On the other hand, fastening multiple pieces of 2x lumber together actually creates a beam that is stronger than a single piece of 4x or 6x lumber. Our beams take about 2 hours to build (depending on size), and can be cut to length before they're installed. For this project, you'll need to know how to measure and cut a board, and fasten screws.
When determining location for deck beams, be sure to assemble the beam so the 2x10s have the crown sides up. Miter-cut ends of plywood spacers and point them upward.
Stack the three 2x10 beam members on top of each other with their crowns facing the same direction. Square the ends and cut them to length. If you choose to do so, cut a decorative angle at one or both ends.
Cut pointed spacers from a sheet of 1/2-inch pressure-treated plywood (the points prevent rainwater from sitting on the plywood and soaking in). Fasten the spacers, points up, to two beam lengths with 1-1/4-inch screws. Attach the spacers at 16-inch intervals.
Stack two beam pieces with the spacers sandwiched between. Align the ends and clamp them together. Drive 16d galvanized box nails or 3-inch screws every 16 inches, alternating the distance from the edge of the beam. Add the third beam piece and repeat the process.
Sandwiching posts between two 2x10s is a common way to build a beam. Begin by cutting your posts about 1 foot longer than needed. Set them in place with temporary support. Position the members and fasten them with carriage bolts.
Starting about 2 inches from the edges of the beam and at 16-inch intervals, counterbore two holes on the inside (or the least-visible side) of the beam for carriage bolt nuts. Drill through the beam with a long bit the same diameter as the bolts.
Tap carriage bolts through the holes. On the flip side, add a washer and nut and tighten the nut with a socket wrench.
Low decks might not provide enough vertical space for joists attached to the top of a beam. The solution is a flush beam, which is essentially a header that has been doubled for strength. Cut two headers (the boards that will be perpendicular to the joists) and mark lines on them indicating where the joists will go. Fasten them to the posts so they are level. Double up the headers to make them flush beams. Drive a pair of nails or screws every 16 inches to bind the pieces together firmly. The second piece is 1-1/2 inches longer than the header on each end to accommodate rim joists on both sides of the frame.