I am trying to add floor joists to an attic and intend for getting decent amount of support for plywood flooring and storage.

The existing truss is 13' across and 24'' span between 2x4 ceiling joists. these won't hold a pillowcase from what I've been told, even with the added support boards binding them to the rafters.

My question is what I can do about adding the support to joists so that I don't accidentally overload the wall itself. This garage wall is open but filled with cabinetry and would be a pain to add in extra studs. Do these added joists need to be above the studs period? If I don't sister the existing ceiling joists, will that be a mistake? I'm aware that the joists need to lay on the load baring walls but I'm not entirely certain how intense the requirements are for a bare plywood floor for storage space are.


  • Where is the existing truss located? How tall is it? What is it made of? What kind of connections? What is the span of the existing ceiling joists?
    – Lee Sam
    Apr 10, 2019 at 20:31
  • the existing truss is on top of the garage. I haven't exactly measured it, I would guess around 4 feet tall or so. It's not a place you could stand to your full height, connections are fastened to the rafters with nails on two points, one right by the joists end and another about halfway through. The span as I said was 24'' between joists. and the length is 13' thereabouts between the walls. I'm not sure what the wood is made of, nor how to really find out.
    – Anon
    Apr 10, 2019 at 21:53

1 Answer 1


It sounds like the existing 2x4’s are 24” oc and span 13’. (The truss has nothing to do with the existing ceiling or future floor of attic.)

Yes, the existing 2x4’s can barely carry the existing ceiling material.

To create an attic floor, I’d add 2x6’s at 24” oc spanning 13’ to support about 25 - 30 lbs. per square foot, depending on the species and grade of lumber. (This is good for Christmas decorations, luggage, etc.)

For more support, I’d add 2x8’s 24” oc spanning 13’ to support about 40 - 45 lbs. per square foot. (This will support books, people, etc.)

I doubt the existing walls will need to be modified, if they are bearing walls now.

  • Can you answer if the position of the studs in the wall and the floor joists need to line up or if I need to sister the joists? As for the truss, my understanding was that over loading the 2x4's would, even if they did not snap, sag the roofing.
    – Anon
    Apr 11, 2019 at 2:20
  • @Anon The joists do not need to align with the studs. Also, the truss is independent of the existing ceiling joists (and future floor joists) unless the existing 2x4 ceiling joists are attached to the trusses in some manner.
    – Lee Sam
    Apr 11, 2019 at 2:32
  • They are tied to the rafters using spare flooring slats.
    – Anon
    Apr 11, 2019 at 2:51
  • @Anon Hmmm...a picture would help. I don’t understand “tied to rafters”.
    – Lee Sam
    Apr 11, 2019 at 2:57
  • youtube.com/watch?v=JQI1eXcTGLs I dont have pictures but essentially the two vertical pieces of wood on the left and right side rather than the center on every rafter. Maybe the other person I talked to briefly might not know what a truss is and I dont have one, but I know even less so I just used the word blindly, but this is the video that gave me the idea of how it could damage the roof if I put weight on those joists.
    – Anon
    Apr 11, 2019 at 3:32

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