Between the first and second floor of a friend's house he has about 4' of dead space between floors that he wants me to help convert into storage (this is in San Francisco, so every square foot counts!). He got the idea from a similar space in my house.

Currently the ceiling of the first floor consists of 16" o.c. 2x4s, generally running across a span of about 10' for each room.

I do not want to rip out and replace all of the ceilings in the first floor, so my plan is to strengthen the 2x4s in some way and add either 1/2" or 3/4" flooring so that I can put some crates up there and leave "crawlways" to get around.

Here are the solutions I've come up with and my thoughts on them (I'm not a builder, but I've built lots of ad-hoc structures and so far nobody has died.. :) ). I should also mention that I plan on using screws instead of nails everywhere, so hopefully that improves stiffening.

  1. Just put 1/2" flooring on the 2x4s. I think this would probably work for a little while - I had a house many years ago that had standing attic space that was only on top of 2x4 joists, though I recognize that 2x4s really shouldn't be used for a 10' span, even for an attic.

  2. Sister the 2x4s with another 2x4. I believe this only improves the span by about 25%, but that gets us within a foot or so of 10'

  3. Sister the 2x4s with 2x6s and put the floor on top of that. This should be strong enough, but unfortunately the way things are laid out that would take a lot of extra work with making everything fit properly (because of the location of stringers and where the floor would go and working around a pipe on one side and etc.. etc..)

So those were my options. I kind of felt like 1&2 might work but would be on the weaker side, and 3 would definitely work but create a bunch of labor. Ideally something in between the two would be enough for handling attic storage that was 4 feet high and that's when I thought of this crackpot idea:

  1. Place a new set of 2x4s on their side going across (90 degrees to) the current 2x4s. I know that 2x4s aren't that great on their side, but this only adds 1.75" to height, like the sistered 2x6s would, and with it all screwed together, wouldn't this add a decent amount of stiffness with the flooring on top? But I can't quite figure out how to guesstimate how much strength this will really win me and if it beats #2 above. I don't want to put the 2x4s vertically on top of the other 2x4s, because there's only so much space in there..

Any thoughts?

If the answer is to get a structural engineer, then that's out of their budget, so we'll just probably be forced to put in the 2x6s, though I bet the 2x4s or sistered 2x4s would actually be enough for such a low attic space...

  • Actually talking to my brother just came up with a new idea - add 2 or 3 posts that tie the (weak) attic floor to the (load bearing) ceiling above, and then just be happy with a bunch of 2x4s for most of the weight bearing, knowing that the posts can cut the span in half.
    – daveola
    Mar 1, 2020 at 23:55
  • What's overhead? The best structural bang for the buck if you can stand some pieces going through the space is to truss the 2x4's to the floor joists above them. Writing comments at the same time, I see.
    – Ecnerwal
    Mar 1, 2020 at 23:57
  • Right - overhead is 2x8s that make up the floor above, which presumably can handle plenty of load. :) So how many trusses would I need (that interfere with my attic space) so that I don't need to add 2x6s? Or perhaps even sister 2x4s?
    – daveola
    Mar 2, 2020 at 0:07
  • 1
    How is it that there's four feet of space between floors? Was the ceiling originally higher?
    – isherwood
    Mar 2, 2020 at 3:28
  • @isherwood - yup! They dropped the ceiling at some point for some reason...
    – daveola
    Mar 2, 2020 at 18:24

1 Answer 1


Wow, this is quite an effort, but I know how valuable space is in SF.

I’ll make some assumptions and you let me know if I’m wrong. First, Loads are divided into Live Loads (storage boxes, books, Christmas decorations, etc.) plus Dead Loads (gypsum board ceiling, ceiling joists, 1/2” plywood subfloor, etc.) Generally we add about 3 psf for miscellaneous loads like lights, ductwork, etc. into the dead load. So, here’s your Dead Load:

Gypsum board ceiling: 2.5 psf, plus 2x4’s @ 16” oc: 1.0 psf, plus 1/2” subfloor on 2x4’s: 1.5 psf, plus misc.: 3 psf...for a total Dead Load of 8.0 psf.

Second, Live Loads are considered uniformly distributed across the storage space...AND loads in the middle third of the span affects the deflection the most AND can cause cracks in the gypsum board ceiling when you exceed the allowable loads.

Here are your options:

Option 1: Just add subfloor on existing 2x4 @ 16” oc.

Depending on the species and grade of ceiling joists, 2x4’s @ 16” oc spanning 10’ can support about 12 psf Live Load plus 8 psf Dead Load.

This is certainly the easiest option. I’d suggest you lay out some items you want to store and see if it exceeds 12 psf. (Remember, it’s 12 psf over the span of several ceiling joists. That is to say, if part of the area is 16 psf and an adjacent area ON THE SAME JOISTS weighs 8 psf...you’re probably ok.)

Option 1A: Same as Option 1, but don’t store anything in the middle third of the span:

With this option, you can store about 20 psf Live Load in the first 1/3 on each side of each span...but nothing in the middle third.

Option 2: Add 1 - 2x4 between each existing 2x4. (You can sister it to the existing 2x4’s or secure it anywhere between the existing joists.)

This will double the allowable loads discussed in Option 1 and 1A.

Option 3: Sister 2x6’s to existing 2x4’s.

2x6’s @ 16” oc spanning 10’ will support about 60 psf Live Load. If you add that to the existing 2x4’s you can support about 72 psf. However, this is based on the fact that you fasten the 2x6’s to the 2x4’s with adequate nails/screws. (Of course, if they are not fastened together then the allowable Live Load would be 60 psf.)

Option 4: Install 2x4 flat to top of each 2x4 ceiling joist.

This is not recommended. The extreme fiber in bending on the tension side (on the bottom) is still limited by the lumbers fiber strength for a 2x4 on edge.

Option 5: Provide supports from ceiling joists up to existing 2x8’s @ 16” on center spanning the same 10’.

2x8’s @ 16” spanning 10’ can support about 120 psf Live Load. Adding a support (2x4) at mid span will increase the allowable Live Load on the existing 2x4’s up to about 110 psf AND add a point load to the 2x8’s....which could over stress the 2x8’s depending on what the actual load is above the ceiling and what it’s supporting on the second floor.

If you have a refrigerator or a waterbed above this area I would not do it. Otherwise, the Code requires 40 psf Live Load plus 10 psf for Dead Load for a living space. In this case, I think your 2x8’s would be fine, depending on the species and grade.

I think your next move is to consider what will be stored above the ceiling and how much it weighs...on average over a 10’ span.

  • That's super helpful - though I was under the impression that sistering the 2x4s wouldn't actually double the load but only improve the span by about 25%... Or maybe that works out the same way. I could only find load calculators that worked for 30/40 psf, how did you calculate the 12 psf?
    – daveola
    Mar 2, 2020 at 8:10
  • And to answer your question on the bottom - the upstairs does not have any heavy furniture/waterbed/appliances - it's just a large bedroom.
    – daveola
    Mar 2, 2020 at 8:11
  • Which makes me think that Option 2 might actually work, because 24 psf seems like it would be fine for a small attic space? Option 5 was not meant to be vertical posts every 16", but just maybe 2 or 3 in the room, so every 4-6' or so. So maybe a combination of two posts + sistered 2x4s and I'll be fine with reasonable storage. Though then I have to consider earthquakes...
    – daveola
    Mar 2, 2020 at 8:14
  • 2
    Don't forget to check your local codes. San Fran is in an active earthquake zone, and load requirements may be impacted by local ordinance. If there's a quake and the house comes down, the insurance company may have questions about how this additional storage space was constructed. If they find it doesn't meet code, they could use that as grounds to deny the entire claim.
    – FreeMan
    Mar 2, 2020 at 15:47
  • 1
    @daveola You said you weren’t expecting to install a vertical support at each joist at 16” on center, so I’m suggesting installing a vertical support at about 4’ on center and attaching it to a strongback across the ceiling joists. However, remember that it will support one-half of the load on each joist times 4’ on center. So, the connection at the floor joists must support that load plus the normal floor Live and Dead Load. (Perhaps a strongback on the bottom of floor joists too...in order to distribute the load out on all the floor joists.)
    – Lee Sam
    Mar 2, 2020 at 22:04

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