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In designing a deck I've been looking at this Deck Construction Guide (PDF)

Table 2 lists the max joist spans (Lj) for joists that are cantilevered by a maximum of Lj/4.

How do I calculate Lj/4? For example, say I have a 12 foot joist span and use southern Pine 2x12, could I have a cantilever of 4.5 feet(18 feet / 4), or 3 feet (12 feet /4)?

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The joist span between supports is Lj. You use that same value to compute the maximum overhang (Lj/4). So with a 12 foot span you could have at most a 3 foot overhang. Or conversely, if you wanted a 4.5 foot overhang you would need an 18 foot backspan. –  Henry Jackson Jul 1 '13 at 3:27

2 Answers 2

Directly from the document that you linked the following figure shows that the Lj and Lj/4 dimensions are additive.

enter image description here

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That wasn't the question, but maybe I asked it wrong. When it says Lj/4 maximum overhang is Lj the joist span that I choose to use, or the max joist span for the joist size I choose. For example I have a 12 foot joist span, but I want a 4 foot over hang, can I just choose a joist that has a max joist span of 16 feet or greater, or do I need to increase my joist span? –  Michael Jun 30 '13 at 21:18
    
You do not need to increase your joist span just because. You can always reduce the span for materials and spacings specified in the table 2. If table 2 shows a Lj of 12 feet you can see in the figure above that you can extend overall length of the joists out to 12 + 12/4 = 15 feet provided the materials used are equal to or better than the table 2 values. Table 2 simply specifies joist SPAN, not overall joist length. In your selection of a 12 foot span using southern pine you can concentrate on 2x10's that can be 12", 16" or 24" on centers and up to 15 feet in length. 2x8's are too small. –  Michael Karas Jun 30 '13 at 22:20

To have a 4 ft. overhang, you need to have both a joist size that can handle the Lj/4 of 4ft (so Table 2 needs to read 16 ft. or better) and you need to have an actual span of 16 ft. or better. Both are required.

The second requirement prevents the creation of "diving board" situations where you could have a 1 foot span and a 4 foot overhang as an extreme example.

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