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...

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