So I’m looking at the Home Depot catalog and some of the wood items, like plywood siding, are described as 4" on center, or 8" on center. I’ve also seen this term used in deck construction/planking. What exactly is the meaning of “on center”?
Thanks!
Although I’ve never seen it used in the context you describe, “on-center” is generally accepted to refer to the positioning of one piece of wood in relation to another.
For example, wall studs are usually layed out 16 inches on-center. Meaning that the center of one stud is exactly 16 inches from the center of the next.
Should be in multiples of four.
For real flooring boards, it would be 16" on center. That’s the spacing of the supports. 4" on center would imply that the material isn’t very heavy duty.
Decks are typically, like floors, 16" on center, meaning all the joists are 16" apart.
For these new age deck materials, which are more flexible, you might need to run supports under the deck planks every 12"…or every 12" on center.
Basically, as the others have pointed, it’s a way of laying out construction.
Since lumber isn’t always cut exactly down to the fraction of an inch, it’s best to measure “on-center” (distance from the center of one board to the center of the next) so that things don’t get screwy.
Excellent, thank you all!
The 4- and 8-inch figures came from the plywood siding, so as Philster surmised, it’s really not intended to be heavy duty at all. In regards to the decking, I believe 16 inches on center (and some smaller number if it was laid out diagonally) was the number I saw there, but it was a while ago. It was certainly greater than 4-8 inches though.
Thanks again!
Ummm Toad?
I’ve done a lot of construction. I framed for a while and did exterior trim. We don’t measure from the ‘center’ of one board to the ‘center’ of the other. 
Outside of warping, you may be surprised just how consistant lumber is.
I think photog has the best answer. But we actuall will measure from the edge of one to the edge of the other. Inside to inside, or outside to outside.
I’m not sure what you mean by “inside to inside” but if you’re measuring from the left hand edge of one board to the right hand edge of the other (or vice-versa) you will find yourself off quite a bit on a long enough run. In fact that’s the whole point of the “on center” phrase. When they say 16" apart on-center they are making it clear that that is not the same as leaving a 16" space between the boards.
On centre is used because sometimes members of varying thicknesses are used to frame a particular project, and even when you use 2x4, 4x4 and 4x6 the sheet goods should fit centre to centre.
Sometimes 4’ or 6’ O.C is used to describe the grooving pattern on plywood siding.
If you were looking at plywood siding, chances are it was grooved in intervals to look like board-and-batten siding. Most people in the trades refer to this as “T-111” siding.
The four-inch-on-center and eight-inch-on-center measurements refer to the frequency of the grooves.
I think what you mean to say is that you measure from the left edge of one to the left edge of the next. (or right to right, whatever) Which is the same distance as on center, as you say.
It’s important all right. We put a new roof on our garage last fall. When we ripped the old stuff off, we discovered that the ningnong who built it had put the trusses in with the space between them at 24 inches.
Since the trusses were 2 inches wide, this made them over 25 inches on center. Which means that the 4x8 foot sheathing we got had to be all cut up to fit in 25 inch increments instead of fitting right into nice, neat 2 foot ones. Morons.
Yes, of course. The left to left, or right to right is a much clearer way to put it. I never really even think about it, just do it.
Wow.