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Deck Girders and Beams

The illustration above depicts the minimum distance of untreated support members from grade. Untreated joists should be at least 18 inches away from the ground. Girders and beam plies should be 12 inches away from the ground. However, in many situations, exceptions are made where the elevation of the home does not provide for these minimum distances and the climate is very dry.

Beam plies should be fastened together with two rows of 10d (3-inch) nails at a minimum of 16 inches (406 mm) on center along each edge.

2021 IRC Table R507.5 includes single-ply beams in all listed species, including redwood and cedar. Single-ply beams are useful for lighter loads and shorter spans. Single plie beams also eliminate decay caused by water being trapped between two or more beam plies.

The image above depicts a girder improperly relying on the shear strength of lag bolts. Girders should bear directly on posts.

The image above depicts a girder properly resting on a notched post. Girders should bear directly on posts. The remainder of the notched post should be at least 2 inches thick.

The post should be notched for full beam bearing on the notch.

The illustration above depicts a girder or double plie properly resting on a post. Girders should bear directly on posts.

Girders supporting joists should not be supported by deck ledgers or band joists.

The illustration above depicts a butt joint or splice improperly located within a girder span. Butt joints in a girder span are generally not permitted unless specially engineered. A butt joint or splice should be positioned to land at the posts.

The illustration above depicts notches in a supporting beam.

Notches in solid lumber joists and beams:

  • must not be greater than one-sixth the depth (D/6),

  • must not be longer than one-third of the depth (D/3), and

  • must not be located in the middle one-third of the span (L/3).

Notches at the ends must be less than one-quarter the depth (D/4).

The tension side (bottom side) of beams 4 inches (102 mm) or greater in nominal thickness must NOT be notched except at the ends of the beams.

The diameter of holes bored or cut into joists or beams must not exceed one-third the depth.

Holes must not be closer than 2 inches (51 mm) to the top or bottom of the joist or beam, or to any other hole located in the member, or to any notch in the beam or joist.

The illustration above depicts a level being used to check for beam sag. Even with a carpenter's level, it can be difficult to see beam sag from the front.

The image above depicts beam sag being eyed up. Often, it is easier to detect beam sag by the eye than with a level by looking along the bottom edge of the beam.

Information used with permission by

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