How to Build a Stronger Deck Beam for a Sturdy Outdoor Space

Follow these steps to create a stronger, longer-lasting beam for your DIY deck.

Beams are an essential part of a deck's support system, so if you're building your own deck, you want to make sure these elements are as sturdy and long-lasting as possible. Many older decks were built using massive timbers, such as 4x6s or 4x8s, for beams, which support the underside of the deck horizontally. Some builders still do it that way, as using a one-piece beam can save time. But large timbers are heavy and hard to handle, and over time, exposure to the elements can cause them to warp or crack beyond repair. Alternatively, fastening multiple pieces of 2-inch-thick lumber together can create a beam that is stronger than a single piece of 4-inch or 6-inch lumber. Following these instructions, you can build deck beams in about 2 hours (depending on size) and they can be cut to length before they're installed. Before you begin this home improvement project, you'll need to know how to measure and cut a board and how to fasten screws.

back of taupe house with upper deck and lower patio sitting area
Peter Krumhardt

What You'll Need

Equipment / Tools

  • Tape measure
  • Circular saw
  • Drill
  • Long bit
  • Hammer
  • Clamps
  • Adjustable or socket wrench


  • Pressure-treated plywood
  • 1 1/4-inch deck screws
  • 3/8 x 6-inch carriage bolts with nuts and washers


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.

  1. Step 1: Stack and Square
    Dave Toht

    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.

  2. Step 2: Cut and Attach Spacers
    Dave Toht

    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.

  3. Step 3: Clamp and Screw
    Dave Toht

    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.

  4. Step 4: Drill Holes for Bolts
    Dave Toht

    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.

  5. Step 5: Tap in Bolts
    Dave Toht

    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.

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