This story covers the construction options and process for installing beams, which run across the posts, parallel to the deck ledger.
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.
About three hours for a 16-foot beam constructed from two 2x10s
Tape measure, circular saw, framing hammer, caulk gun, cordless drill
Measuring, cutting, driving fasteners, drilling, caulking
Set and cut posts
2x lumber or beam stock, deck screws, beam framing hardware, bracing lumber, caulk