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Deck Beam Length Question

453 Views 5 Replies 3 Participants Last post by  thanks4thehelp
Hi all. New to the forum but I'm an above average handyman. Having just finished our basement, the next project was to extend the deck that was installed by the previous owner.

The main part of the deck is approximately 12'x12', supported by 2 6x6" posts and a ledger board (properly attached to the house). The beam consists of 3-2X10" joists sistered together. The joists span 140" (11-2/3') and the beam spans 132" (11').

The village here provides a nice guide for DIY'ers on how to build a deck to code and according the section on beams, the maximum span allowed for a 3 ply, 2x10" beam is 9' 2" for southern pine. Since it appeared that my deck post was not to code, I checked with the village to confirm if the previous owner had pulled a permit and, unfortunately, he did not.

Extending the deck is no longer an option so now I'm trying to decide what to do. The chart that the village provides shows joist spans of greater or equal to 12' and, being at 140" or 4" under 12', I realize that the beam max in my case is slightly longer than the 9' 2" that appears on the chart... There are no cracks in the beam or signs that it is failing but we've also never had more than 8 people on the deck prior. Is there a calculator somewhere that will tell me how much weight the deck will support based on these dimensions?

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The deck load is 40 psf Live Load + 10 psf Dead Load. Deck loading is shared by both the beam and the ledger board. You would need to determine the area of the deck that is supported by the beam. This is affected by the location of the beam in relation to the end of the deck and in relation to the ledger board. For example, if the beam is located 1 ft. inboard of the deck end, then the beam carries more load than the ledger board carries.

The city chart you mentioned can be useful, but only if your deck is configured exactly like their assumptions. Otherwise, the chart is either conservative or non-conservative.

If you have concerns on whether the existing condition meets Code, I suggest getting an engineer to do a proper beam calculation using exact dimensions and spans, and correct beam allowables for live and dead loads. Or, just add a mid-span post to the existing beam to shorten the beam span.
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