Beam-Over-Post vs Notched-Post Detail per IRC R507.5
Why this matters
The post-to-beam connection is the load path that keeps the deck from collapsing onto the patio. The 2015 IRC tightened this detail and the 2021 cycle locked it down further. Through-bolting a beam to the side of a 6x6 post is no longer code-compliant on permitted residential decks, and crews still doing it are getting red-tagged.
What IRC R507.5 actually says
The 2021 IRC R507.5 prescribes that deck beams shall bear on the top of a post (beam-over-post) or in a notch cut into the top of the post, with a connector rated for the load. Side-bolting a beam to the side face of a post is allowed only on engineered designs that account for the eccentric load and that use a tested hardware assembly. The prescriptive path is one of two details.
Detail A: Beam over post (top mount)
The beam sits on top of the post, full bearing on the post end-grain, connected with a post cap. This is the strongest and simplest connection.
- Connector: Simpson BC, BCS, CC, ECC, or AC series cap appropriate for the post and beam dimensions
- Fasteners: hot-dip galvanized 10d common or structural screws to the manufacturer's nailing schedule (no exceptions for substitute fasteners)
- Post height: measure to the top of the post and cut clean; out-of-square cuts shed bearing area and the cap rocks
- Beam orientation: crown up, with the joint over a post unless using a properly engineered splice
Top-mounted beams produce a deck with the joists sitting above the beam, which raises the finished deck elevation by one beam depth. Plan ledger height and stair count around it.
Detail B: Notched post
The post is notched at the top, the beam drops into the notch, and the remaining post side resists the load. IRC R507.5.1 limits this to 6x6 nominal posts (5.5 by 5.5 inches actual). The notch removes wood from the post, so the remaining material must meet:
- Minimum remaining post dimension at the notch: 3.5 inches
- Notch depth equal to the beam thickness (typically 3 inches for a doubled 2x stock beam)
- Through-bolt the beam to the remaining post leg with two 1/2-inch diameter hot-dip galvanized through-bolts with washers under the head and nut, per IRC R507.5.1
A notched 4x4 post is not allowed under the prescriptive path. A 4x4 post that is sized for the load can only top-mount the beam.
Why side-bolting fails
A beam bolted to the side of a post puts the load on the bolts in shear and the post in bending. Wood does not handle eccentric loads gracefully; cyclical wet-dry movement loosens the bolts, the bolt holes ovalize, and the beam sags away from the post over years. This was the detail behind several high-profile residential deck collapses and is why the IRC prescriptive path no longer permits it.
Hardware coatings and treated lumber
ACQ and CA pressure treatment is highly corrosive to bright zinc and standard galvanized hardware. The minimum acceptable on a residential deck:
- Connectors: G185 hot-dip galvanized per ASTM A653, or stainless 304/316
- Through-bolts: hot-dip galvanized per ASTM A153 Class C, or stainless
- Within 1 mile of saltwater spray or in a saltwater-pool deck application: 316 stainless only
Mixing coatings on a connection (stainless bolt in galvanized cap) is allowed but mixing bright zinc with anything else on treated lumber is not.
A post cap that is sized correctly for the post but undersized for the beam will fail the inspection. A 6x6 post cap on a tripled 2x10 beam needs the larger cap variant (Simpson BC6 for single 2x members vs BC60 series for the doubled or tripled assemblies). Match the cap to the entire connection, not just the post.
When to engineer rather than prescribe
Send the connection to an engineer when any of the following are true:
- Post height exceeds 12 feet
- Beam supports a roof, hot tub, or other concentrated load not anticipated by R507
- Site is in seismic design category D2 or higher
- The deck is freestanding (no ledger) and depends on the posts for lateral stability
The engineer can specify a hold-down or a moment connection that the prescriptive code does not cover.
References
- 2021 International Residential Code, Section R507.5 and R507.5.1 (deck beam to post connections)
- 2021 IRC Figure R507.5.1(1) and (2) (acceptable post-to-beam details)
- Simpson Strong-Tie Wood Construction Connectors Catalog C-C-2024, BC, BCS, CC, ECC, AC, AC4Z post cap series
- ASTM A153/A153M Standard Specification for Zinc Coating (Hot-Dip) on Iron and Steel Hardware
- American Wood Council DCA 6-2015, Prescriptive Residential Wood Deck Construction Guide, beam-to-post details