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The deck construction guide for my area contains this table:

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Look at the "Allowable Overhang" columns. It's odd that a wider joint spacing (24") permits a longer overhang.

For example, look at the row for 2x12 fir joists. At 12" O.C. spacing, overhang can be 4'4". At 24" spacing, this decreases to 3'3".

Now look at 2x8 fir joists. At 12" spacing, overhang can be 1'8". Increasing the spacing to 24" increases the allowable overhang to 2'2"!

This doesn't make sense: I'd think closer joist spacing means more load bearing capacity, allowing for longer overhangs. Even more confusing is there's no consistency between joist sizes. What gives?

7

Looks like data entry error to me. The values were reversed for several rows at some point.

Considering that they spelled 'fir' as 'fire' in one case, one can assume that accuracy was not a high priority here. That's not a flush beam, either, nor is '2x8' a 'length'.

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It’s probably because you are adding weight to the overhang by spacing the joists closer together. I would seriously doubt there are data entry errors on this American wood council doc that is constantly updated

2
  • No. The weight of the joists themselves is trivial compared to the potential load of the deck. Also, the fact that the values are reversed for just some rows indicates error, as do the terminology and spelling errors that I mentioned. – isherwood Jul 9 '20 at 14:25
  • the document reads like a spam email from a Prince, it is the problem – Ack Jul 9 '20 at 14:49

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