Spot the Violation

FramingDecksLevel 22018 IRC

Beam line under a walkout deck

Standing on the lawn under a high walkout deck, looking along the outer beam line toward a row of posts. The beam is two brown-treated 2x10s nailed together; at the nearest post its end grain shows two plies, and it is fastened to the yard-side face of a 6x6 post with two round-headed carriage bolts. The post runs up beside the beam to the joists, and nothing sits under the beam. Brown-treated 2x8 joists sit on top of the beam with galvanized ties and carry the decking above. Each post stands in a galvanized base on a concrete pier at the grass, and the lower-level siding and a patio door of the house are in the background.
Under a walkout deck, outer beam and posts

What's wrong here?

  1. The beam hangs on bolts through the side of each post instead of bearing on top of it
  2. Deck posts must be 4x4; a 6x6 is too heavy for the post bases
  3. The two beam plies must be bolted together; nailing plies is not allowed
  4. Nothing. This installation is compliant
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Show the answer and the code reference

The answer

The beam is side-mounted: it is through-bolted to the face of each post with nothing under it, so the deck load rides on the bolts in shear instead of bearing on the posts.

Correct choice: 1. The beam hangs on bolts through the side of each post instead of bearing on top of it

R507.5.1 requires deck beams to have full bearing: at least 1-1/2 in. on wood or metal at the ends, and where a beam runs over intermediate posts, each ply must bear fully on the post. R507.5.2 requires the beam-to-post connection to carry vertical load and resist displacement, following the IRC's post-to-beam details: the beam sits on top of the post in a post cap, or in a notch cut in a 6x6 with through-bolts holding it there. A side-mounted beam puts the deck load on two bolts per post, and the wood around them splits, especially as the lumber dries and the bolts loosen. That is a common cause of deck collapses. The post size, nailed plies and joists on top of the beam are all fine.

How to fix it

Shore the deck, then put the beam on the posts: cut the posts down and set the beam on top in galvanized post caps sized for the post and beam, or notch each 6x6 so both plies bear in the notch and through-bolt with washers per the IRC detail. Keep or re-drill the bolts only as the lateral connection, not as the support.

Code references

  • IRC 2018 R507.5.1 — Deck beam bearing

    The ends of beams must have at least 1-1/2 in. of bearing on wood or metal and at least 3 in. on concrete or masonry for the entire width of the beam, and where multiple-span beams bear on intermediate posts, each ply must have full bearing on the post.

    Drills citing R507.5.1

  • IRC 2018 R507.5.2 — Deck beam connection to supports

    Deck beams must be attached to their supports in a way that transfers vertical loads and resists horizontal displacement; connections to wood posts follow the IRC beam-to-post figures (beam on top of the post or in a notched post), manufactured post-to-beam connectors must be sized for the post and beam, and bolts need washers under the head and nut.

    Drills citing R507.5.2

  • IRC 2018 R507.5 — Deck beams

    Deck beam spans follow Table R507.5. Beam plies are fastened with two rows of 10d (3 in. x 0.128 in.) nails at 16 in. on center along each edge, and beams may cantilever up to one-fourth of the allowable beam span at each end.

    Drills citing R507.5

2024 IRC:not yet checked Confidence: low

Minnesota

MN adopts this rule as writtenConfidence: medium

Minnesota adopts the 2018 IRC with Minnesota amendments as the Minnesota Residential Code (Minn. Rules ch. 1309). No Minnesota amendment to R507.5.1 or R507.5.2 is known. DLI is adopting the 2024 IRC as the next edition; see the 2024 IRC note.

About the Minnesota Residential Code

Citation under review. Verified from the verifier's knowledge of the 2018 IRC; code text could not be fetched. Confirm the 2018 wording of R507.5.1 (1-1/2 in. of bearing at beam ends on wood or metal; each ply fully bearing on intermediate posts) and R507.5.2 (connection transfers vertical load and resists displacement; wood-post connections per the IRC beam-to-post figures; washers under bolt head and nut), and the R507.5 ply-nailing sentence used in a distractor. The drill does not depend on figure numbers.

Why the other choices are wrong

  • Deck posts must be 4x4; a 6x6 is too heavy for the post bases. Table R507.4 sets deck post sizes from 4x4 up to 8x8, with taller allowable heights for the larger sizes. Nothing caps the size, and post bases and caps are made for 6x6 posts.
  • The two beam plies must be bolted together; nailing plies is not allowed. R507.5 fastens built-up deck beam plies with two rows of 10d nails at 16 in. on center along each edge; nailed plies are the prescriptive detail.
  • Nothing. This installation is compliant. With no bearing under it, the whole deck load goes through two bolts per post in shear and into the wood around them, which splits or crushes. The IRC requires the beam to bear on the post.
Safety hazardLevel 2 · Journeyman#deck#beam#post#bearing#bolts#exterior

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