Calculating spans and cantilevers for deck beams and joists is beyond the scope of a home inspection. This section of the course will go over examples of how to calculate span and use a general rule of thumb. This is another topic that is beyond the scope of a home inspection: calculating allowed spans and cantilevers. But a home inspector should know what is a beam span, a joist span, a cantilever, and how to divide by 4.
Joist Span Does Not Include Bearing
A joist span is shown by the illustration inside the IRC in the 2018 IRC Code Commentary. That illustration is below. There are conflicting illustrations about the measurement for joist span. This illustration makes it clear that the joist span does not include any bearing area, but only open space that is not bearing upon the beam. Span is a measurement of the open space below the bottom edge of the spanning joist. Joist span does not include bearing length. For joists bearing on a wood beam, at least 1-1/2 inches of bearing length of the joist is required.
Beam Spans
Maximum allowable spans for wood deck beams are described in the 2021 IRC Figure R507.5 and Tables R507.5(1) through R507.5(4). Wood deck beams can cantilever at each end up to one-fourth of the actual beam span.
Using the 2021 IRC Table R507.5(1) below, let's assume the beam species is Spruce-pine-fir, the beam size is two 2x10s, and the effective deck joist span is 10 feet in length. ("Effective" deck joist span is not the same as "actual" joist span. Effective is the actual joist span length multiplied by the joist span factor in the code). The maximum deck beam span is 7' 9" in length.
Beam plies, like the two 2x10s, must be fastened together with two rows of 10d (3-inch × 0.128-inch) nails minimum at 16 inches (406 mm) on center along each edge.
Beam Cantilever Is "Beam Span/4"
Beams are permitted to cantilever at each end up to one-fourth of the actual beam span. Refer to the illustration below. Cantilevers are always based on the actual span of the beam or joist.
The illustration below is from 2021 IRC, and it is a flush beam where the ends of the deck joists are connected to the deck's horizontal beam resting upon posts. Joist framing into the side of a beam or ledger board must be supported by approved joist hangers. The beam span is identified in the illustration below.
Assuming the maximum deck beam span is 7' 9" in length. Then the beams are permitted to cantilever at each end up to one-fourth of the actual beam span, which is (7' 9" / 4) 1 foot and 11 1/4 inches.
The “one-fourth-the-beam-span” or "Beam Span/4" rule for beam cantilevers is permitted by code.
Joist Spans
The maximum spans for wood deck joists are shown in Figure R507.6 and described in Table R507.6 of the 2021 IRC. A common type of deck design observed by home inspectors is displayed in the illustration below. It's an attached deck with a flush or dropped beam upon which spanning deck joists are resting and cantilevered out. The ends of the cantilevered joists are connected to a rim joist.
The “one-fourth-the-joist-span," "Joist Span/4," " or "L/4" for deck joist cantilevers has been replaced in the 2021 IRC with a detailed table full of maximum allowable deck joist spans, cantilever, and joist back spans for each common joist span. The previous codes limited the cantilever based on joist spacing, as opposed to joist span, which was an error.
Each joist type now has a maximum joist span and maximum cantilever length. This change in the building code provides for more flexibility in design and more accuracy in the sizes and spans.
Deck beams can cantilever at each end up to one-fourth of the actual beam span.
Deck joist cantilevers are calculated in accordance with the table of spans and cantilevers, not "L/4."
Home inspectors may refer to the old general rule of thumb during a home inspection, because home inspectors are not code inspectors and are not required to refer to and quote code, including this section about deck joist cantilevers.
The illustrations above show a cantilevered deck. The arrows are indicating generalized measurements of the L and L/4 sections. They are not detailed or specific measurements. The old general rule is that cantilevered joists should not be more than one-quarter of the joist length and three times the joist width (nominal depth). Both conditions can be true. But L/4 is no longer used to determine deck joist cantilevers according to 2021 IRC R507.6.
In reference to 2021 IRC Table R507.6 above, let's assume the deck is built with a 40 psf live load design, the joist species is Spruce-pine-fir, the joist size is 2x10, the allowable joist span is 13' 7" with joist spaced 16 inches on center.
Wood-framed decks should be designed and built for the live load or the ground snow load listed in the building code, whichever is greater.
Let's be conservative and round down from 13' 7" to 12 feet at the joist back span. The maximum cantilever is 3 feet. That's also L/4 (12 feet / 4 = 3 feet). The old general rule of "one-fourth-the-back-span" or "L/4" rule actually still works in this example.
The illustration above shows a deck joist cantilever in the front of the deck and deck beam cantilevers on each side of the deck posts. Beams are permitted to cantilever at each end up to one-fourth of the actual beam span (beam span / 4). The deck joists should be cantilevered according to the 2021 IRC Table R507.6 for maximum deck joist spans and maximum cantilevers.
Information used with permission of nachi.org