What size beam and post does my deck need?

Beam size is not a judgement call. It is a lookup with two inputs: how far the joists span, and how far apart you want the posts. Set both below and the span table answers.

The short answer

The beam follows from the joist span and the post spacing. For the common case, spruce pine fir carrying a 12 ft joist span on posts 6 ft apart, the selection in the table is a 2 ply 2x6. Push the posts to 8 ft and the same deck needs a 2 ply 2x10.

A 4x4 post is allowed only under a 2 ply beam, on a post up to about 6 ft 6 in, under a deck no more than 1.8 m (5 ft 10 13/16 in) above grade. Otherwise it is a 6x6, up to 12 ft. A 3 ply beam always needs a 6x6 to get full bearing.

The beam bears on top of the posts. It is never bolted to the side of one as a prescriptive solution.

How the lookup works

Two inputs. The joist span, which is the depth of the deck off the house, plus any cantilever past the beam. And the post spacing, which is how far apart you want the holes in the ground.

There are two tables, and picking the wrong one is a real error. A single span beam carries joists from one side only, which is the normal deck with a ledger at the house and a beam at the front. A two span beam carries joists landing on both sides, which happens on a wide freestanding deck with a beam down the middle, and it needs a bigger member for the same joist span.

The trade you are actually making

Joist span	Posts 4 ft	Posts 6 ft	Posts 8 ft
8 ft	1 ply 2x6	2 ply 2x6	2 ply 2x8
10 ft	1 ply 2x6	2 ply 2x6	2 ply 2x8
12 ft	1 ply 2x6	2 ply 2x6	2 ply 2x10
14 ft	1 ply 2x6	2 ply 2x6	2 ply 2x10
16 ft	2 ply 2x6	2 ply 2x8	2 ply 2x10

Spruce pine fir, not incised, single span. Use the demonstration for incised lumber, other species and the two span case.

Read down a row and the trade is clear: every 2 ft you add to the post spacing costs you beam depth, and every post you delete costs a hole, a footing, a bag of concrete and an afternoon. On a 20 ft wide deck, posts at 6 ft means four posts and a 2 ply 2x6. Posts at 8 ft means three posts and a 2 ply 2x10. There is no right answer, only a cost you choose.

When a 4x4 is allowed

Three conditions, and all three have to hold:

It is under a 2 ply beam. A 3 ply beam is wider than a 4x4, so it cannot get full bearing.
The post is no more than about 6 ft 6 in long.
The deck is no more than 1.8 m (5 ft 10 13/16 in) above grade.

Past any of those it is a 6x6, which truly measures about 5 1/2 in square, and 6x6 is good up to about 12 ft of post. A round post has to be at least 7 1/4 in in diameter.

A built up post is allowed: full length pieces at least 1 1/2 in thick, nailed with two rows of 3 in nails no more than 12 in apart, or bolted with 3/8 in bolts no more than 18 in apart.

The beam sits on top. Always.

This is the detail that separates a deck that stands from a deck that sags. The beam bears on top of posts that are at least as wide as the beam. It is not bolted to the side of a post with through bolts, and it is not hung off another beam, unless an engineer has designed the connection.

Alberta gives no direction at all for a split beam with plies bolted either side of a post. That means it is not a prescriptive solution. If you must build one, either get an engineering detail or block the beam members fully down to the foundation at every post.

Building up the beam

Nailed: two rows of 3 1/2 in nails no more than 18 in apart, with nails 4 in to 6 in from the end of every piece.
Bolted: 1/2 in bolts with washers no more than 4 ft apart, and within 2 ft of each end.
Joints: in a 2 ply beam on three posts, the single joint goes over the middle post. If it cannot, build a 3 ply beam instead. In a 3 ply beam on three posts, one joint per span, near the quarter point from the middle post.

Drop beam or flush beam

A drop beam sits under the joists and lets them cantilever past it, which hides the beam, the posts and the footings behind the fascia. It is the common choice and the easier build.

A flush beam has the joists framed into its side with hangers. It needs a beam at least as deep as the joists, proper metal hangers, and the specific fasteners the hanger manufacturer names. Not the screws in your apron: the ones in the instructions. If hangers are not available, a deeper beam with a 2x3 ledger strip nailed to its side with two 3 1/4 in nails per joist is an accepted alternative, or a 2x2 strip if each joist also gets four 3 1/2 in nails into the beam.

Beam cantilever

Not prescribed in the Alberta code, which means strictly it is engineering. The rule of thumb Edmonton reports as having worked is one and a half times the beam depth past the centre of the post: 9 in on a 6 in beam, 12 in on an 8 in beam, 15 in on a 10 in beam, 18 in on a 12 in beam. Treat that as a limit, not a target.

Where this comes from

Canadian Wood Council, Prescriptive Residential Exterior Wood Deck Span Guide, 2016, Tables 4b and 5b for a beam supporting a single span and Tables 6b and 7b for two spans, imperial, with note 3 on equivalent builds and note 5 on post sizes.
City of Edmonton, Uncovered Deck Design Guide, Section 2, Posts and Beams, for the 4x4 allowance, the built up post and beam nailing and bolting patterns, the joint locations, the flush beam alternatives and the beam cantilever rule of thumb.

Nothing on this page is an approval or a design. Code editions change and every authority having jurisdiction has the final word. Check your numbers with yours before you build.