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.
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.
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'
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:
- 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..
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...