How to Build a Deck Beam
When determining the location for deck beams, be sure to assemble the beam so the 2x10s have the crown sides up. Miter-cut the ends of plywood spacers and point them upward.
What You Need
Tape measureCircular sawDrillLong bitHammerClampsAdjustable or socket wrenchLumber for the beam pieces (2x10s in this case)Pressure-treated plywood1 1/4-inch deck screws3-inch deck screws3/8 x 6-inch carriage bolts with nuts and washers
Step 1: Stack and Square Deck Beams
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 desired, cut a decorative angle at one or both ends.
Step 2: Cut and Attach Spacers
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 ($11 for one pound, Home Depot). Attach the spacers at 16-inch intervals.
Step 3: Clamp and Screw Deck Beams Together
Stack two beam pieces with the spacers sandwiched between. Align the ends and clamp them together. Drive 16d galvanized box nails ($7 for 1 pound, Home Depot) or 3-inch screws ($37 for 5 pounds, Home Depot) every 16 inches, alternating the distance from the edge of the beam. Add the third beam piece and repeat the process.
Optional: Sandwich a Post Between Beams
Instead of stacking three beams on top of each other, sandwiching deck posts between two 2x10s is another 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.
Step 4: Drill Holes for Bolts
Starting about 2 inches from the edges of the beam, 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. Repeat this process at 16-inch intervals across the length of the beam.
Step 5: Tap in Bolts
Tap carriage bolts through the holes. On the flip side, add a washer and nut and tighten the nut with a socket wrench. Once the beams are built, you can secure them to posts to continue building your deck.
What to Do if the Deck Is Too Close to the Ground
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.
