Beams are a crucial part of any deck, so it's imperative that they're properly installed. A beam runs across the posts, parallel to the ledger, and supports the joists.
First consider your construction options: The beam can be a single piece of lumber, a built-up piece with or without spacers, or two pieces of lumber fastened to the sides of the posts. Local codes may help you narrow your options. Some localities consider a beam with spacers stronger than a single board, and others don't. After code compliance your chief concern is how much the beam will show (and therefore how you want it to look) and the amount of space your plan allows for it.
A cantilevered front edge on the deck helps hide the beam unless you're building a raised deck. Then you can hide it with skirting. A low deck might not leave you much vertical space for a beam so you would have to use smaller stock and install an additional post. Instead of digging more holes, you could build a beam that doubles as the header joist, then hang the deck joists between the ledger and the beam.
Expect to spend about three hours installing a 16-foot beam constructed from two 2x10s. Before you begin, set and cut posts, and brush up on your measuring, cutting, drilling, and caulking skills.
Sight down each member of your beam and make sure you have the crowns side by side. Cut them to length and clamp them with all edges flush. Place the assembly on a firm work surface and drive three fasteners every 12 inches.
Install post/beam connectors to each of the posts, orienting them in the same direction. Move the completed beam assembly to the deck site and lower the beam onto the connectors. Center the beam on the deck site.
Stake a 1x4 brace into the ground and with a helper holding a 2-foot level, plumb the face of the beam and keep it in place by driving a toenailed screw through the brace and into the beam.
Install another staked brace on the opposite side and add braces to secure the beam plumb along its length.
With the beam braced, secure it to the post/beam connectors with screws or nails. Screws will be less apt to push the beam out of alignment, but may not be permitted by some local codes. Remove the braces when you hang the joists.
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