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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.

https://i.stack.imgur.com/bPw4q.jpg

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Anon
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  • 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 '19 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 '19 at 21:53

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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.

Lee Sam
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  • 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 '19 at 02:20
  • @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 '19 at 02:32
  • They are tied to the rafters using spare flooring slats. – Anon Apr 11 '19 at 02:51
  • @Anon Hmmm...a picture would help. I don’t understand “tied to rafters”. – Lee Sam Apr 11 '19 at 02:57
  • https://www.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 '19 at 03:32
  • @Anon Do you have diagonal 2x4’s extending from the lower 2x4 ceiling joists up to the roof or truss above? – Lee Sam Apr 11 '19 at 05:11
  • The beams and walls and rafters are all pretty much identical to the video, which is why I used it. the only difference is there is only one brace along the center, top for some reason, of the joists, and no rafter ties in the center, only on the left and right sides of all joists. The rafter ties themselves are not 2x4s but unused flooring slats as I described. https://thumbs.dreamstime.com/z/ipe-decking-installation-wood-slats-flooring-background-55485287.jpg – Anon Apr 11 '19 at 06:01
  • @Anon Pretty much like the video, except for ..., makes it difficult without a picture. Your terminology of construction terms is not matching mine. Posting a picture is not that difficult. Without a picture, my advice is worthless.... – Lee Sam Apr 11 '19 at 07:27
  • I added pictures. – Anon Apr 11 '19 at 16:32
  • @Anon If you add ceiling framing, as I’ve outlined, the additional load from adding storage will not affect your existing roof, provided: 1) you do not remove any existing vertical braces, 2) the new ceiling framing sits on top of the existing walls and not framed into the side of the walls, 3) the new flooring (plywood) you add in the attic is nailed to the new framing. please note: I see the sole plate in one wall has an anchor bolt to the concrete foundation. If you live in a seismic zone or high wind area, that bolt and shear wall is not adequate when there is a lot of load overhead. – Lee Sam Apr 11 '19 at 17:58