Engineering Dilemma

Step up from the Living Room to the Dining Room

The living room in the new house is a step down from the kitchen. My wife got the idea that we should raise the floor so that everything on the main floor is at the same level, no steps. I'm generally in favor of flat floors. Steps might make it more visually appealing, but do you really want your home to contain traps for the unwary? I mean, does every time you walk into the living room need to be a dramatic entrance? Besides, if we raise the floor, we can get rid of the soffit covering a heating duct in the master bedroom. Maybe.

The living room is kind of large, and it is just over the master bedroom, which is nearly as large. Yes, the house is a little odd. The entrance and the living room are on the top level, all the bedrooms are on lower levels.

Typical Two by Twelve Floor Joist Framing

Anyway, we've got (24) 20-foot-long two by twelve's on twelve inch centers spanning the floor. The floor is going to be raised about seven and half inches, the width of a two by eight.  Can we run the two by eights cross wise and on top of the two by twelves that are already there? And can those existing joists support the additional load? We want the stringers in the built up section to be cross wise so we can move the ducts from the soffit. I don't think two by eights, 24 feet long can be counted on to support much weight, and since the new subfloor is going to weigh at least twice as much as the old subfloor, we might have a weight issue. We might have to run the two-by-eights directly inline and above the old joists and splice them together to make stronger joists. That might be stronger, but it would be put a serious dent in my plan to move the heating duct.

I'm good with math, but figuring out how much all this wood would weigh, how much weight the floor is expected to support, and how much weight these different arrangements of sticks can support is something for someone who's familiar with all those numbers. Shoot, this problem might even be odd enough to make it interesting.