It took some time, but the bathroom now has an OSB floor! Or it did, but now you can’t see it because I’m back to tearing off the plaster & lathe walls. This photo shows just one small piece of the floor remaining to be screwed into place.
But the subfloor was not as easy as it sounds. I couldn’t just drop the OSB boards down and start screwing it into place. I tested it with the first piece, and found that the board rocked on one end, and dipped in the middle when I stood on it. That was an indicator that the floor joists were not all the same height. I used masonry string (basically a bright color thin cord) and a string level, and stretched the string from one corner of the room to the other. I simply wrapped the string around the head of a nail and then hammered the nail all the way into the top of the floor joist. Then pulled it to the opposite corner of the room, stretched the string tight, and nailed it into place again. Using the string level, this showed me how the string was touching the floor joists on the edges of the room and made a level line, but the string did not touch the joists in the center of the bathroom.
Why? Well, I’m no structural engineer but I would say from the weight of a 300 # cast iron tub, and the years (and years) of water leaks probably had something to do with it. The joists are very sturdy, but they’ve sagged over the last 98 years from weight and water penetration through the old floor. So how to fix this.
I knew I couldn’t raise the floor joists, so I measured the distance between the sunken joist and the string, and ripped long shims on my table saw from a 2×4. In the center of the room, I needed a shim 5/8″ thick, and then one at 3/8″ for each joist on either side of the first. I laid them in place on each floor joist under the string, and now the string touched each one and was perfectly level. I nailed these shims into place with just a few nails each.
Then I carried the OSB floor back in and laid it down. Voila, it laid flat and level, no rocking and no dipping! Then I screwed it down with 2 1/2 deck screws along each of the floor joists, spacing the screws about 10″ apart. Why so close? I do not want any movement with this floor, as that will mean cracked tiles or grout down the road.
So now with a floor down, I started attacking the walls with a hammer, pry-bar, anything else I could swing the break the plaster off and then tear off the lathe boards. I couldn’t do this step before tearing up the original floor, because that floor was so rotted through it was unsafe to stand on.
Plaster removal is slower and dirtier than I expected. It’s not as quick as Nicole Curtis makes it look. I’ve learned over the last 2 months that all of those fancy HGTV and DIY home improvement shows rely way too heavily on one of two things: time lapse video, and an unseen crew of 20 or more.