There are a couple reasons for the bowed ceiling. As I previously mentioned, both the floor and the ceiling sagged like a saucer. The sag was about 4″ in the center of the room.
Sag reason #1. The old 2×8″ oak joists (roughly 16″ on center) spanned almost 16′. This is undersized for this span (even considering they were oak). Looking at the span tables, the joists should have been 2×10 – 2×12, depending on the species of lumber. I would like to state that the 2×8 oak joists were rough sawn and were a full 2″x8″ (more or less, as things were not exactly “standard” back then).
Sag reason #2. We were told that previous Amish residents from the past had brought the bathroom facilities inside. In order to have “running water”, they had rigged up a gravity system where they stored the water upstairs and piped it down to the bathroom below (much like a water tower). Now with water weighing 8 lbs/gallon it is very easy to see how even having a 500 gallon tank up there would easily weigh 4000 lbs. And, if it leaked, that would cause even further warping
The floor of this room was obviously the ceiling of the 1st floor room below. I had previously leveled that ceiling by removing the plaster and lath in that room. Then, instead of doing anything with the existing undersized oak joists, I went to the low point of the ceiling and “sistered” 2×6″ boards onto the existing 2×8 oak joists. The old joists, over nearly a 100 year time frame had sagged as far as they were going to. By sistering the new 2×6 joists to these, I actually strengthened the span. Once the ceiling was level, we installed full length 3/4″ T&G bead board (which I stained). We had also redone the walls in this room and wallpapered.
A photo of the bead board ceiling (in the room below) is shown above.