Q. I need to convert an attic to livable space with minimal disruption to first-floor habitation. The existing ceiling joists are 2x6s, clear-spanning about 11 feet 3 inches. Rather than sistering in deeper members, which would involve moving wiring, can I stack new 2x6s on top of the existing ones? This creates a 2x12 joist depth, but will I achieve an actual structural benefit from stacking members? What if I gusseted them together at regular intervals?
A. Chris DeBlois, a structural engineer in Roswell, Ga., responds: Yes, you can in fact achieve some structural benefit, depending on how you tie the upper and lower joists together. The next logical questions are: How much benefit and is it enough? And what else do you need to do to ensure the stability of the new floor system? That's where the answers get interesting.
The maximum structural improvement you can get by adding new 2x6s on top of the old ones is the same as the maximum you would get if you sistered them side by side - exactly double. As a frame of reference, a 2x12 has about twice the bending strength and four times the stiffness of the two 2x6s you'd get if you ripped it in half. Unfortunately, now that you're essentially stuck with the two 2x6s, there's no practical way to recapture the original strength and stiffness of the full-depth 2x12. The problem has to do with creating a "no slip" connection between the upper and lower pieces.
But you can still double the capacity of the existing 2x6s in the attic floor, so first let's make sure that's enough for your situation. My calculations tell me (assuming #2 southern yellow pine joists, which are common in my area) that 2x6s 16 inches on-center will span 9 feet 9 inches, with allowable deflection being the governing factor. If you double those joists, you can span 12 feet 3 inches.
To achieve this doubling effect in strength and stiffness, you will indeed need to gusset the upper and lower joists together. If we use strong enough gussets and fasteners, we know that the top and bottom joists will deflect by exactly the same amount when loaded. That's key to analyzing the behavior of the stacked framing.
I'd suggest you first run a continuous 1-by or 2-by plate across the ends of the existing joists to provide a stable base for the new joists. (That also gives you a fighting chance at a level floor. If you simply set the new joists on the old they probably won't mate well, and any slope in the old floor will translate through to the new.) Next, gusset the upper and lower 2x6s together with 2x4s or 3„4-inch plywood. Gussets on one side only can work, but I prefer gussets on both sides so the load is transferred concentrically, preventing twisting in the joists. I recommend three gusset pairs per joist at the quarter points - one about every 3 feet. Where they connect to both the top and bottom joists, each gusset pair will transfer approximately 100 pounds - that's 50 psf of floor load times the 3-foot gusset spacing times the 16-inch joist spacing divided by the two joists. Use six 10d nails with plywood gussets, or six 16d nails with 2x4s, half into each 2x6 joist. That's conservative nailing in terms of the capacity of the fasteners, but it also keeps the gussets from twisting.
You may wonder how I decided on three sets of gussets per span. The more gussets you have, the more uniformly the load is shared from top to bottom; ideally, we'd like the joists to share the load equally. In this case, three gets us pretty close to a 50/50 split. The stress in the lower joist will be about 7.5 percent higher than that in the upper joist - not enough difference to justify more gussets. Let's say for comparison that you went nuts and used nine gussets per span: The lower joist would still have 1.3 percent more stress than the upper - too much work for not much improvement, in my book. By contrast, if you used only one gusset at midspan, the stress in the lower joist would be a whopping two-thirds higher. The problem in this scenario is that all the load in the lower joist is introduced right in the middle of the span, the worst place to put it. So that's why I chose three per span: It gets pretty close to splitting the load.
The last question I raised has to do with the stability of the floor system. Stacking joists is not nearly as stable as sistering, which braces the joists against twisting. Not so with stacked joists. Think of it like this: You can probably walk across the edge of a 2x6 joist, but if you stack the joists two high and try to walk across, your last name better be Wallenda. Blocking solves the problem. Put 2x8 blocks at the third points, set at the midheight of the assembly and bridging the horizontal joint between top and bottom joists. Toenail the blocks or end-nail through the joists to tie everything together. For slightly better bracing across the entire depth of the assembly, you could instead use 2x12 blocking or double 2x6 blocks, one layer for the top joists and one for the bottom. But 2x8s should work fine and will save labor and materials.
Finally, to simplify the analysis, I've ignored the fact that some of the load - namely, the weight of the ceiling - is actually applied directly to the bottom joists and won't be shared with the upper joists unless you remove and reinstall the ceiling (which is pretty much what you were trying to avoid in the first place). But this won't make much difference to the calculations or recommendations unless you have a very heavy plaster ceiling. If you do have an unusually large ceiling load - or an ornamental plaster ceiling you want to protect - you may need to consider doubling some of the new joists.