I moved briskly along to the hall floor today. I needed to level it and put 15 lb felt paper down - after that, I can dry-lay the floor over the weekend, then rent the nailer Monday and hopefully get it done in a day.
Here's what I learned: the cheap laminate was failing because of the wonky dips in the subfloor. I'd seen one of the ridges as soon as I took up the laminate, and I've been worrying about it every time I walk down the hall for two weeks. The ridge I was looking at was a weird little hump where the hall runs into the front room, by the coat closet. I used the old beadboard to shim up the closet floor and part of the hall floor, then evened everything out with layers and layers and layers of felt paper. But that ridge turned out to be the easy one! There was another high spot in the center of the hall, headed toward the laundry room. The sides of the hall were EACH 1/4" below the middle of it for a good 5' before it evened back out.
I have been deliberately clunking down the hall and bouncing up and down on the subfloor, and nothing squeaks or bounces. That's good - it means the floor joists underneath are in good shape (just like the home inspector promised. Of course it's fine - everything that got inspected is fine, everything that isn't on a standard home inspection is wrong, fucked up, broken, or unsafe.) Now - if the floor joists are fine, what gives with the subfloor?
It's probably just age. Things warp and sag and settle into new slightly off kilter configurations from about the time the first owner moves into the house. I could shim it up and make a flat surface to nail the flooring to, or I could climb under the house and put in some bridges between the joists and jack the bridges up perfectly level and probably level the floor from below. If the joists were creaky, I would go under there and do some work. But as it is - if I go mucking around bridging the joists and jacking them up, I could cause other immediate problems, like shifting the whole house so that the doors don't shut right or the drywall cracks. I am already putting the hardwood down because I tried to fix something and made it worse, so... I just shimmed the subfloor from the top.
I filled in the big spots with the old paneling from the front room. I hadn't gotten around to sawing it into small enough pieces to hide in my trash bin, and I'm glad. (Remodeling really brings out one's hoarder tendencies. You really can reuse that junk!) The old paneling that I took down was plywood and slightly thicker, and the leftover pieces of new paneling were MDF (or maybe HDF) and slightly thinner. Here's what I did.
I got three straight level-things ready - my 24" spirit level, a piece of the hardwood flooring exactly the width of the hall, and a 6' piece of aluminum I found in the garage. (I think it was part of the closet doors we took down. See, I did need it for something!) The long piece checked the flatness of the subfloor lengthwise down the hall, the short hardwood laid on its side checked it side-to-side, and just for laughs I checked the actual level every so often.
I started at the end that was bugging me.
See how the level is sitting on the paneling in the closet, the hump in the floor, and the paneling on the floor at the right? Whole day went like that. I filled in the low spots with paneling, then layers and layers of felt paper to smooth out the transition from the paneling to the subfloor. Crawl 2 feet away, check flatness from both directions, repeat. I used five strips of staples and about 250 sq ft of felt paper (for 100 sq ft of hall!)
The cats were nuts about this crazy thing happening before their eyes. And Cersei was a little nervous, but not too bad. The main casualty was me - my back is in knots, and I'm sure my knees are bruised.