I've watched some videos and talked to a contractor buddy about how to do floor framing. "16 inches on center" is the phrase I keep hearing, and I understand that that will ensure a layout divisible by 4 feet.

However, if they were really all 16 inches on center, there would be an extra width of 1 1/2 inches (assuming 2-by framing material) because the outer joists would be a factor of 16 inches in distance on center. Based on reading and watching the proper method, it appears that the outer joists are actually 15 1/4 inches on center from the second joists since they start the second joist at 16 inches on center from the end of the rim joist. Is that correct?

By the way, this is just a tiny shed and I'm not messing around with double joists. But my current best guess it that the outer joists are simply ignored in the N-on-center scheme.

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    Strictly mathematically, the outer joists could be 1-by, as they support only one edge of a flooring sheet, while the inner joists must each support two edges. You can consider the extra inch as stiffening for the outer inch, which would otherwise warp or bow. – A. I. Breveleri Apr 10 '16 at 22:43

You are correct.

Framing a wall or floor the two outer studs or joists are moved in to compensate for the end of the interval. If you are going for an exact dimension divisible by 4'.

This makes placing 4'x8' sheets of OSB come out nice and even with no waste.

Good luck!

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