A code-compliant post to beam connection puts the beam on top of the post so the wood bears directly on wood. For a deck, that means notching the post so the beam sits in the notch with two through-bolts, or using a metal post cap; bolting the beam to the side of the post is not allowed.
Those rules come from the American Wood Council’s deck guide and the International Residential Code, and they exist so the beam’s load never depends on bolts alone. Below are the options, the details that make them work, and how the same idea carries over to steel posts and beams in a basement.
Why the Beam Has to Bear on the Post
A beam carries the joists and everything on them. That load has to get into the post. There are two ways to do it:
- Bearing: the beam sits on the post, and the load passes straight down through wood touching wood (or wood touching a steel cap).
- Fasteners in shear: the beam is bolted to the side of the post, and the bolts carry the whole load sideways through their shanks.
Bearing is far more reliable outdoors. AWC’s commentary in DCA 6 says that designing fasteners for wet-service conditions requires significant capacity reductions, and that prohibiting side attachment ensures wood-to-wood bearing. Bolts in wet, treated lumber also corrode, and the wood around them shrinks and splits over time.
The American Wood Council’s FAQ on how deck beams or girders are attached to posts (read October 2026) explains that starting with the 2015 IRC, beams and girders supporting deck joists must have full bearing at posts and may no longer straddle posts on through-bolts alone. The beam still has to be attached so it cannot slide sideways off the post, which is what the notch or the cap does.
The Three Details You Will See
The AWC Prescriptive Residential Wood Deck Construction Guide (DCA 6), 2015 IRC edition (read October 2026) shows two approved post-to-beam details and one prohibited one.
1. Notched post (let-in) with through-bolts
The top of a 6×6 post is notched so a 2-ply beam drops into the notch and rests on the cut shoulder. The beam is then through-bolted to the remaining tongue of the post. DCA 6 uses 1/2 inch diameter through-bolts throughout, with washers under both the head and the nut, and pilot holes of 17/32 to 9/16 inch.
The guide also requires cut ends and notches of treated posts to be field-treated with an approved preservative such as copper naphthenate. Cutting a treated post exposes untreated wood in the core, and that cut is exactly where water collects.
2. Metal post cap
A post cap is a galvanized or stainless steel saddle that fits over the top of the post and wraps the bottom of the beam. The beam bears on the full top of the post, and the cap’s nails or screws hold it against sliding and uplift. DCA 6 makes the cap mandatory for 3-ply beams, because notching a 6×6 deep enough for three plies leaves too little post. Use the fasteners the cap manufacturer lists, not whatever is in the bucket.
3. Beam bolted to the side of the post (prohibited)
This is the detail on many older decks: one or two 2x beams bolted to the faces of the post, sometimes on both sides. DCA 6 Figure 9 prohibits attachment of the beam to the side of the post without notching. If your deck is built this way, it is worth having it inspected.
Notch vs Post Cap: Which to Use
| Notched 6×6 post | Post cap | |
|---|---|---|
| Beam it suits | 2-ply 2x beam | 2-ply or 3-ply beam, solid or engineered lumber |
| Post size in DCA 6 | 6×6 minimum | 6×6 minimum (4×4 only with an approved cap by alternate approval) |
| Hardware | Two 1/2 in through-bolts with washers | Manufacturer cap and listed fasteners |
| Look | Wood only, no visible hardware except bolt heads | Visible metal saddle |
| Weak point | Cut wood must be field-treated | Cap must be corrosion-rated for treated lumber |
Post Size: Why 6×6 and Not 4×4
The IRC itself sets a 4×4 minimum for wood columns, but DCA 6 requires 6×6 posts. Its commentary says a 4×4 would often be overstressed in the deck sizes the guide covers and that 6×6 gives enough bearing for the beam. A 4×4 also has too little wood to notch. If a building official approves a 4×4 as an alternative, it has to use a post cap, and caps for 4x4s are generally limited to 2-ply or 4 inch nominal beams.
Fasteners and Corrosion
Preservative-treated lumber attacks plain steel. Citing IRC R317.3, DCA 6 requires bolts, nuts, washers, screws and nails used with treated wood to be hot-dip galvanized, stainless steel, silicon bronze or copper, and connectors such as post caps to be galvanized or stainless. Hot-dip galvanized bolts over 3/8 inch must meet ASTM A153 Class C. Match the cap and its fasteners so you do not pair stainless with galvanized. Bolts that rust inside the post are invisible until the connection loosens.
Bracing at the Post
The post-to-beam joint alone does not stop a tall deck from racking. DCA 6 calls for diagonal bracing parallel to the beam at each corner post taller than 2 feet, fastened with 1/2 inch lag screws to the post at one end and the beam at the other, and it prohibits diagonal bracing on center posts. Freestanding decks have their own rules in the guide.
Steel Posts and Beams Indoors
The same principle, beam bearing on post, applies to a basement girder on steel columns. A lally column or adjustable steel post has a steel cap plate on top. The wood or steel beam sits on the plate, and the plate is fastened to the beam so the beam cannot slide off. Our guide to the basement support beam covers how these beams are sized and replaced.
For structural steel columns under a steel beam, the beam’s bottom flange bears on a cap plate welded to the column and is bolted through the flange. Where a wood beam needs more capacity without more depth, a flitch beam with a steel plate between the plies is one option, and its bearing at the post has to be detailed by the engineer, because the steel plate and the wood plies share the load.
At the other end of the joists, joist hangers do a similar job where joists frame into the side of a beam: the joist sits in a steel seat rather than hanging on nails through its end.
Common Mistakes
- Bolting a beam to the side of the post with no notch or cap.
- Using a post cap sized for a 2-ply beam on a 3-ply beam, or the wrong nails in the cap.
- Notching too deep or using a 4×4 for a notched detail.
- Leaving the cut end of a treated post untreated.
- Using plain steel or indoor-rated hardware with treated lumber.
- Shimming a short post with stacked scraps instead of cutting a proper post.
DCA 6 is a prescriptive guide for typical decks within its limits. Anything outside them, including heavy hot tubs, roofs on the deck, tall decks or unusual spans, needs a design by a licensed structural engineer, and your local building department has the final say on which details are accepted.
Post to Beam Connection FAQ
Can I bolt a deck beam to the side of a post?
Not under DCA 6 or the 2015 and later IRC. The beam has to bear on the post through a notch or a post cap. Side bolts alone are prohibited.
How deep should I notch a 6×6 post for a beam?
Deep enough to take a 2-ply 2x beam, as shown in DCA 6 Figure 8A, leaving the rest of the post as the tongue for the bolts. For a 3-ply beam, do not notch; use a post cap.
What size bolts connect a beam to a post?
DCA 6 uses two 1/2 inch diameter through-bolts with washers under the head and the nut, in pilot holes of 17/32 to 9/16 inch. Use hot-dip galvanized or stainless bolts with treated lumber.
Is a post cap better than a notch?
Both are approved. A cap keeps the full post section and adds uplift resistance, and it is required for 3-ply beams. A notch avoids visible hardware on a 2-ply beam.
Can I use a 4×4 post for a deck?
DCA 6 requires 6×6. A 4×4 needs building official approval and a post cap, and it is limited in the beams it can carry.
The Short Version
Set the beam on the post, never just beside it. Notch a 6×6 for a 2-ply beam with two 1/2 inch through-bolts, or use a corrosion-rated post cap, which is required for 3-ply beams. Treat every cut, brace tall corner posts, and get an engineer for anything outside the prescriptive guide.
Reference information only, not engineering advice. Disclaimer.