It's an American thing for spacing out joists and studs. The standard length
of flooring or partitioning board there is 8ft (96 inches). The higher the
load bearing capacity you need the more closely spaced the joists have to
be. Dividing 96" into the commonly used integer spacings gives 8 x 12", 6 x
16", 5 x 19.2" and 4 x 24". The only one of those that isn't a nice round
number is 19.2" so if you're spacing out at 5 joists per floor panel you can
follow the diamond marks.
Trim joist spacing:
It is customary to place traditional joists at 16-inches on-center.
However, since the TrimJoist has a 3.5-inch wide flange, many contractors
choose a 19.2-inch spacing to maximize the price/performance ratio of the
product. Most contractor-grade tape measures now have a marker every 19.2
inches for just this very purpose. This spacing will also work out correctly
for your plywood subfloor. By using a 19.2-inch spacing, the flanges of the
TrimJoist are still only 15.7-inches apart. This makes the plywood subfloor
behave like traditional joists at 16-inch centers.
I have some '99 dot-com schwag that's just such a custom ruler. It
looks like some bright spark at a hosting company went and had them
run off by a vinyl sign printer, with their logo on.
Of course it appeals more to the box wrangler who might use it than
the marketroid who places the hosting contract.
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