We want to reinsulate and install flooring and pull-down stairs in our attic, but the joists are only 2x4. What's the best way to reinforce? Can we use 2x4 sister joists, or do we need to use 2x6 sister joists and somehow attach 2x2s to the existing 2x4s to match up?
The 2x4 joists that you have now are for ceilings only. Even if you add 2x4 "sister" joists, the arrangement would not be correct for a floor load. A 2x4 joist may in fact span the distance and carry the load but it wil very likely deflect so much that the ceiling drywall will crack. Also, 2x4 floor joists are VERY springy. The size and spacing of the joists is a building code item that is determined by the span of the joists and the intended use of the room (bedroom, living, storage). You would be wise to consult a local building official, framer, or builder for advise on the proper joist size and spacing. They need to actually see the house to provide reliable advise. You can leave the 2x4's in place and add the new joist the the sides of them. Spike the two together so that they act in tandem.
Attic Truss Reinforcement
Another Attic Storage Question, But Perhaps a Soluti...
insulating for a finished attic
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