Does anyone understand what HomeDepot deck builder app is trying to explain to me in terms on-center joist spacing?

Deck builder jopist layout

The thing that confuses me is, reading the joist left to right. The first joist is 6" OC, but how there is no reference to a zero. The bottom left most corner of the deck is 0 point of the grid system in the deck CAD app. Whis is reflected where the mark their base point enter image description here

I assume zero is the outside edge of the left-most rim joist. Am I reading this wrong?

Where do I start measuring joist spacing from ?

  • The 1st 6" dimension is referring to the spacing between the 1st and 2nd joists, not to the 1st joist by itself.
    – brhans
    Commented Jun 27, 2022 at 12:22

2 Answers 2


I agree this is a pretty funky layout. Best I can tell is it is laid out from left to right in 2 separate sections- as shown in drawing below. This is probably done to minimize the number of joists you would need if laid out all the way from one side. The additional joists for the "picture frame" and the jog in your deck throw off a regular single layout and you would end up with too many joists (unnecessary ones).

enter image description here

The truth is that it does not really matter how you do the layout. As long as there is no more than 16" center to center for any joists and as long as you provide the "picture frame" joists and/or blocking to support your deck edge boards properly.


The first 6" OC is actually too far to the left on your drawing. The distance between the first joist at the left edge to the 2nd joist will be 6" OC between them, and then 16" OC between most of the rest of the deck joists (except near the middle where you change deck depth, and then on the right side). In looking at your drawing, if you shift all of the lower OC measurements a bit to the right, it should make more sense how they're supposed to be spaced.

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