I have a century home that has an unfinished attic that has some nice space potential so I decided to finish it, adding insulation, electric, floor, etc. After a few days of working up there I noticed the floor was sagging and that the ceiling in the bedroom below had seriously cracked and bowed.

It was then I realised the attic joists are 2x4s spanning ~16 feet that were nailed thru the sides into the rafters. The rafters continue down to the 2nd floor exterior wall structure (the bed room has a sloped ceiling on either side).

The answer is to call a contractor, however, I'd like to be able to discuss this intelligently with them. So what are the options? Not much head room to spare -- sister with 2x6's? Build the knee walls 3 feet out in such a way as to "hang" the joists from the rafters, sort of like a truss? That would effectively shorten the span to 10 feet, transferring some of the load to the rafters.

 

Changes to structure require approval from the building inspector and a permit. Work done without permit may have to be removed at time of resale. Attic floor must be strong enough to adequately support people and furniture.

Ceiling joists are generally not intended to support heavy loads. There are height restrictions that may affect the project. Generally if a house has
2-by-8-inch attic floor joists an attic conversion can be done. Anything, less tends to be overspanned. Then, the options are to replace all the joists or add to the sides of them. This will require removing the wiring in ceiling below and rewiring.

If you replace the joists, the stairs from the floor below will have to be replaced because when the larger joists are added, it will make the top step much taller than the others. Stairs and headroom above must meet codes.

 

Yes, I plan on using a contractor who will pull the applicable permits. I just need to be able to discuss intelligently when hiring.

The 2x4s are definitely overspanned (~16' unsupported) for any use. The up side is that the area is completely devoid of obstructions such as wiring, etc. This makes sistering / replacement that much easier. The two tricky things is that I didn't realize the joists were only 2x4 until they'd already sagged and seriously cracked the plaster below. So replace or jack? The other thing is that the joists do not sit on the second floor wall structure and are tacked thru the rafters several feet above it. This going to be a roof load issue?

Ideally I want to use this space for storage and as a walk up closet -- as you are aware, century homes are typically very short on closet space. The stairs are right off the master bedroom so this would be a nice extension.

Part 2 is the other 3/4 of the attic which I'd like to use as additional storage and office. This area has interior walls underneath it so it much better supported. Still would need reinforcing I am sure.

As for head room, there is 8' at the center and although it is a narrow space, I can live with it as long as the City can.