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I'd like to double my ceiling joists to make my attic space easier to work in (for electrical/plumbing) and for lightweight storage.

Status Quo: 2x4's (actual size) spaced 16" on-center running the width of the building uninterrupted, with the ends resting on top of the wall top plate. Each joist is strong enough to support my weight (~175lbs ) concentrated in a ~1' segment of the joist, and there isn't much deflection either.

Goal: Reinforce the joists and lay down a wood subfloor (OSB, ply, etc.) so I'm not tiptoeing on joists whenever I'm up there.

Plan: "Sister" or double the ceiling joists with 2x6's or 2x8's with the ends resting on the wall top plates then lay the subfloor on the new joists.

Question: I understand that there are requirements for doubling joists, including how frequently they need to be fastened, but does this plan make sense overall?

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  • When you say, "2x4's (actual size)," can we assume it's really 1½ by 3½ (i.e. actual vs. nominal)?
    – BillDOe
    Mar 27 '17 at 20:02
  • No, I said "actual size" because the true, measured dimensions are 2x4. Otherwise, I would have not said "actual," or possibly specified it as "nominal."
    – Hari Ganti
    Mar 27 '17 at 20:16
  • What kind of electrics or other utilities in the way to set new framing?
    – Jack
    Mar 27 '17 at 21:20
  • @Jack I'm not sure I understand you. By new framing, do you mean removal of the old joists and replacement with new joists, or do you mean to ask if there is anything in the way of doubling the joists?
    – Hari Ganti
    Mar 27 '17 at 21:22
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    @BillOertell There was a golden age when 2x4's were actually 2x4, not to mention rough-sawn. Much better wood, too.
    – Harper - Reinstate Monica
    Mar 28 '17 at 3:52
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If you have no sag in the ceiling just screw 3/4 inch plywood on on top of the ceiling joist. This will add strength to them and keep your headroom. Be sure to run the length of the plywood across the joist.

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Your plan is sound. I would be certain the joists that are added are tight to existing and use a deadman or "jimmy bar" as I call it to ease the existing joists up a bit in unison to help relieve some of the existing sag that is in the ceiling. The use 2 1/2"to 3" long screws to tie it to the new joists every few feet or so, mainly in the middle span, the ends are not critical. Using nails will create too much vibration and may cause cracking or worst, your existing ceiling to come down.

Depending on the span overall and the location of the mid support, determines the size of new joists, or just go with 2X8s for better insulation under the floor. I would up the immediate area under the plywood another level of 2X4 or more to get a total of 12-15" of insulation on the ceiling.

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  • Thanks Jack! As for the height addition, I need to keep it minimal because my maximum working height is already fairly low at ~5'. Out here in the Bay Area (CA), we don't quite need 15" of insulation, and at that point, I'd just be losing heat from the walls instead. I'd rather keep the extra headroom (literally) and suffice with 6" of insulation. Any more would just be a waste.
    – Hari Ganti
    Mar 27 '17 at 23:00
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Add plywood to the bottom of the 2X4 , with deck screws . Plywood on top is a floor and adds to the strength making something like an "H" beam. You lose very little height this way.

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  • I'm going to assume there's already a ceiling (presumably plasterboard/drywall) on the bottom of the 2x4s, considering it's a ceiling joist... Or are you planning on adding plywood between the 2x4s, directly above the ceiling material, connected sideways into the 2x4s? Sounds like a lot of effort if so.
    – AndyT
    Aug 16 '17 at 11:26

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