Are these joists or trusses? enter image description here

My confusion is that I am reading spacing between joists of between 18inches and 24 inches. However when I measure the spacing I am getting somewhere in the region of 12 inches (center to center spacing is more like 12.5 inches).

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Can this attic be used for storage if I put some plywood on top? I heard about a product called AtticDek but it seems it can only be used with spacing of 18 or 24 inches. Should I considered this product or something else to make crawling around easier?

  • I assume your attic was designed for 10 psf uniform live load required by the code for uninhabitable space without storage. If changes to light storage use, it requires a live load capacity of 20 psf, and if a fixed stair is provided, 30 psf. I suggest having a structural engineer look into this matter (conversion) for safety.
    – r13
    Jun 6, 2021 at 0:35

2 Answers 2


Those are joists and the measurements center to center can be all over the place. I'd stay away from AtticDek and install plywood instead. If your attic access allows you to get 4' x 8' sheets up there, great. If the boards are too short by an inch or two going across the joists, sister a 2x4 to the joist. If too long, then cut off the excess. If your opening is small, just measure and cut the plywood down below, then bring it up. With those short distances, 1/2" plywood will do the trick. To secure the plywood down, use screws to avoid pounding and possibly cracking plaster or drywall seams. Be careful not to screw through any cables.


The code requires ceiling joists support a “live load” of 5 lbs. per square foot plus (psf) a “dead load” to support the joists, insulation, lights, etc. This usually adds up to 8 - 10 psf depending on the size of the joists, depth of insulation, quantity of lights, etc.

Just looking at your attic I’d guess you’re at the maximum span already. If you give more info we could calculate how much storage could be added. Like: size of joists, span of joists, species (or where you’re located).

  • The joists look like standard 2x4s (they measure slightly smaller than that). The span is about 12 inches. Not sure what species is? This is california.
    – Jeff T
    Jun 6, 2021 at 1:50

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