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American Wood Council

In Prescriptive Residential Wood Deck Construction Guide (DCA 6) Table 2, why, for a given joist size, does the overhang span sometimes increase as joist spacing increases?


For this table, a uniform load on the joist never determines the allowable span of the overhang. Only deflection due to the point load or ¼ the length of the main span (whichever controls) limit the length of the overhang. Under a single point load, deflection at the overhang decreases as the main span decreases. Thus for many cases in the table, allowable overhang spans are shorter because the allowable main spans are longer. Where it appears that the overhang spans are inconsistent with the joist spacing, the increased deflection of the overhang is controlling. Where the overhang deflection does not control, the overhang spans are limited to 1/4 the main span and appear consistent with the joist spacing.

For example, the three joists below are the maximum allowable spans for Southern Pine 2x6 joists, which are all deflection controlled. While the allowable overhang span increases as joist spacing increases (widens), it is because the main span significantly decrease in length, which adds stiffness at the overhang. Overall deck length is increased by closer spacing.

DCA6 Overhang


 
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