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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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Thanks Nealtw,

We are in a Milwaukee suburb with an annual average snowfall of 52". The posts are in-ground but there are no signs of heaving (or sinking) so I believe the footings are fine. The siding was cut to attach the ledger board directly to the house rim using staggered lag bolts and flashing.

The current surface area of the deck is 144 sq feet which, to be to code would support a total of 5,760 lbs (live weight at 40 lbs/sq ft). If the beam was the correct length, the deck surface area would be just 109 sq ft which would support a total of 4,360 lbs. So I'm actually at 30 lbs per sq foot (or a bit over because my joists are 4" shorter than 12'). However, I'm not sure I'm calculating this correctly since the beam will support exponentially less weight the longer it is, correct? So, again, I'm just looking to calculate how much load my current beam will handle so I can sleep well at night.
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Thanks House Designer. The beam is located at the end of the deck (no cantilever) so the load is equally distributed. Also, the deck is above a concrete patio slab and the posts were located at the corners of the slab (which is probably why the beam length was pushed to the limit). I'll be sure to hire an engineer if we end up deciding to add an extra post to allow us to extend the deck, however, at this time, I'm just looking for a fairly accurate estimate on deck's current load limit.
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