Joist Spacing for the Dummy Freelancer

Joist span and load

Whether you're a professional or a well-informed amateur, odds are good that you've learned the vocabulary of your chosen field. Take DIY projects: not every wrench is an "adjustable wrench," for instance, and not every framing member of a building is a "beam." When we see those weasel words in a how-to, it's usually a giveaway that the writer wasn't familiar with the task in question. Today's nominee is a freelancer whose failure to grasp construction terminology caught a staffer's eye, but it's the content of the Malcolm Tatum post "What are the Best Ways to Calculate Joist Spacing?" (at WiseGEEK niche site AboutMechanics.com) that put him over the top.

We were a bit taken aback at the notion that anyone had to calculate joist spacing, since the usual problem for floor design is balancing joint width with the length of the unsupported span. You can, of course factor in spacing, but there are generally only three spacing options for the DIY carpenter: 12, 16, and 24 inches on-center. If you have an unlimited supply of lumber of a specific dimension, though, you might be forced to play with the spacing to make certain you didn't have to buy any different lumber. Could be... probably isn't.
Late in his post, Tatum advises his readers to use a joist calculator for this purpose. We looked at several online calculators, and all that we saw were intended to calculate the maximum span of joists given inputs such as live and dead load, wood species, joist spacing, and joist width. Malcolm's research seems to have led him to be quite concerned about wood species and the dimensions of the lumber. That's because, according to Malcolm,

"Ideally, choosing a wood that harder [sic] and is likely to undergo less expansion and retraction [sic] as the seasons change should be used... [and] Joists that are longer and somewhat thinner, such as those used for an outside deck, will need to be spaced closer together in order to bear the weight of the flooring with more efficiency."

We suspect Mal meant contraction, not "retraction," and we sort of get the meaning of "longer and somewhat thinner"; though we have no idea why he thinks that's because they're used in exterior settings. And then there's this rather confusing passage:

"When calculating joist spacing, it’s also a good idea to consider the design of the flooring that the joist framework will support. For more intricate designs that make use of smaller sections of wood, the joists should be placed closer together."

"Intricate"? He really said that's a factor? Sheesh...

Here's Malcolm's biggest problem, though: nowhere does he suggest that the load on the floor must  be taken into account, yet that is one of the most important inputs to joist calculators. He simply ignores it, other than some babble about "efficiency"! We have to admit that we can see no reason other than ignorance to neglect that aspect of determining joist spacing.
Just for fun, we ran some numbers with one calculator, holding the species (southern pine), spacing (16" OC),  joist size (2 x 8), and dead load (10 lb/ft²) constant; changing only the live load. At 40 lb/ft², the maximum joist span is 13'-7"; at 50 lb/ft² it's 12'-7"; and at 60 lb/ft² it's 11'-11". If you change the species to douglas fir, that last is the same; change the joist spacing to 12" OC and it becomes 13'-1". So, load may well be the most important factor, which makes sense to us.

In other words, Tatum is richly deserving of his award, the Dumbass of the Day.

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