My nightmare of flattening an OSB subfloor

This is my experience of preparing our floor for the installation of Pergo flooring. I will update it as things pop into my head.
The wife and I are installing Pergo XP Highland Hickory laminate flooring from Home Depot. We have cats and they have destroyed our carpets. We plan on having kids also, so carpet is just a losing proposition no matter what. Engineered hardwood would have been our first choice but with animals and children on the way it just isn't going to happen.
Our house was built in 2009 and we purchased it brand new. We have an open floor plan where our kitchen and living room are completely undivided by a wall. Nowadays this is all the rage and it's great for parties. Picture a large rectangular room with a smallish standard kitchen at one end. The living room area was covered in the cheapest carpet and padding they offered and the kitchen had the cheapest vinyl flooring. We always intended to replace both once they had worn out their welcome so why waste the money plus have it on our home loan.
About a month or two after moving in I swore I could feel a dip in the kitchen floor between the refrigerator and dishwasher. One day I put a 4ft straight edge across it and I was able to spin it like a top. While it looks neat, it's a BAD thing. However, when I envisioned what would happen to our house if I complained and had it all ripped up I decided I could live with it. As it turns out this dip is about 3ft wide and 10ft long. It coincides with the direction that the floor joists run. Someone got really lazy when they were installing this particular joist I thought.
Fast forward to present day and we are installing new floating flooring in nearly the entire house. The carpeting was easy to tear up (no surprise there) and only covered about half the square footage. The vinyl flooring was glued to 4 x 8 sheets of 3/16" birch plywood, which was in turn stapled to the OSB subflooring. Before tearing up the vinyl flooring I decided to start checking the subfloor under the carpet using a 4ft straight edge. As I worked my way across the floor parallel with the floor joists it seemed to be incredibly flat. I was super happy and figured this install would go quickly. I slapped down one or two layers of 30# roofing felt (which is about 1/24" thick) where it was low. As I thought about it, it just seemed to good to be true. How could my floor be THIS flat? In reality, it wasn't.
One often overlooked but critically important detail is how flat your floors are. Pergo has some pretty strict requirements in their warranty terms that guarantee almost no one will be able to meet them if they are not a on a concrete slab. Even people on concrete slabs probably need to do some bit of leveling to meet them. Picture a 10ft diameter circle in the center of your floor. From one side of that circle to the other, your floor cannot dip or rise more than 3/16". This is such a small change in height that you won't see it without a straight edge or a tight string. If the circle were 8ft in diameter then you would only be allowed to have 1/8" of up or down change. And if the circle were only 3ft in diameter then you can't have it change by more than 1/16". Get a ruler and see just how small a measurement were talking about. I would imagine 99% of the floors in today's homes do not meet this requirement. I know there is a zero percent chance that I will get the floor to meet this standard but I want to get it as close as possible.
Here's an example of a 10ft diameter circle plotted in my living room. Rather large isn't it?
What are the symptoms of a floor that isn't flat? One of them is separation of the ends of the planks. The rise and fall of the floor as you walk across it, or just the floor having to "bend" across the floor weakens the tongues and they separate. It's so common that Pergo has a recommended repair for their installers. I'd call it more of a band-aid.
Another symptom would be floors that feel soft, spongy or hollow. The XP laminate has 2mm of pre-attached padding so there will always be just a little bit of cushion. However, if you can feel the floor flex as you walk across it, it's because your weight is pushing down on the planks until they contact the subfloor below. According to Pergo nothing but a vapor barrier should ever go underneath laminate with pre-attached padding. Too much padding means too much flex which means damaged planks.
Back to the floor, out of (morbid) curiosity I grabbed some twine and stapled it at one end where the vinyl flooring stopped and ran it across the room to the wall. This was a distance of around 16ft. To my horror I discovered that as the floor approached the wall (and the supporting foundation wall) it made a very steep upward climb. I would say the 12 inches of flooring closest to the wall rose by about 3/16". I soon discovered that what I thought was only a few minor low spots was actually spots that were as low as 3/8" in an 8ft span. It makes sense when you think about the mentality of a housing builder. Why invest time in flattening the floors if you're just putting carpet and vinyl flooring down? Odds are, by the time the owner discovers it you'll be out of business or the warranty has expired.
I stopped what I was doing and turned my attention to the vinyl flooring. The only way to get a true picture of what the floor looked like was to rip it all up and run a masonry line from one end of the room to the other. The 3/16" birch plywood that the vinyl was glued to was held down with 10's of thousands of 1.5" crown staples. To get it pulled up required a sharp crowbar and a flat shovel. The crowbar was to try to lift up a corner and the shovel to pry up the sheet of birch. At the edges of each 4 x 8 sheet they placed a staple roughly every 2 inches. It was kind of crazy. Most of the 4 x 8 sheets did not come up all in one piece and I had to cut them down to 1 x 4 strips before Waste Management would collect them. Because of my OCD the wife and I pulled every single one of those staples out of the floor. I own a heavy duty adjustable seal puller which made it a much easier job. The ends are nice and pointy and the rounded head makes for a nice fulcrum. We could have just hammered the staples into the subfloor but a good number of them had birch wood still lodged underneath and they would have made measuring the low spots difficult.

Ok, so now the flooring is completely ripped up and I'm staring at a nice ugly subfloor made of OSB. If you're wondering, OSB sucks. It flakes apart and is very rough. The recesses make vacuuming it a PITA too. I figured I would create a sort of topographical map of the floor. I hammered finishing nails at the edges of the room into the subfloor and then bent them over toward the wall so they would pinch the masonry line tight against the floor when pulled tight. I put nails in every 4 inches along the kitchen cabinet and the opposite wall. Using two small pieces of 3/16" and 1/8" tempered hardboard I slid them underneath the line until they just started to touch it. I would then stick masking tape underneath the string and mark the depth. I did this for the entire area and started leveling it out using roofing shingles and my #30 roofing felt. (I detail the process more below)
After I got about halfway done I opened the pantry door and got another nice shock of horror. Just like the opposite wall, the floor of the pantry sloped upwards drastically as it approached the basement wall. In this case it was clearly visible to the naked eye but I had never really looked at it closely. So, now how do I fix this? No matter what I need to bring the rest of the floor up to flatten it. I decided to try and "feather" this change in elevation out into the rest of the floor hoping it wouldn't be too bad. And so I spent days working on that. THEN I pulled away the oven (the back of the oven is against the wall) and it too had a sharp incline! In fact, the cabinet installers had to shim the front edge of the kitchen cabinets about 1/4" in order to make them level. I made sure to show it to the wife so she didn't think I was crazy because at this point she is getting antsy.
It was quickly becoming apparent that the sides of the subfloor supported by the basement wall were about 1/4" higher than the rest of the floor. An outer basement wall poured just a 1/4" too high and a lazy builder is all it takes.
So, how did I fix it?
The first thing I did was string a masonry line (available at HD or Lowes) from the wall behind the oven to the opposite wall and tighten it as much as possible. You want it tight because if there is slack the string will sag and give you false measurements. Make sure there isn't any kind of debris underneath the string and nothing resting on top of it. Either one of them will cause you headaches. I usually snap it like a chalk string just to make sure. Using my scrap pieces of 3/16" and 1/8" hardboard I slowly slid them underneath the masonry line. At the same time I would wiggle them side to side so that once they contacted the string it vibrated. The tightness of the string makes it easy to spot the movement. The best way to do it is to visually find the lowest spot and start there (a bright light from the side casting a shadow helps spot them). I worked my way from the middle of the depression to the right until the 3/16" no longer fit under the string. I took a piece of masking tape and stuck it to the floor along the side of 3/16" scrap that touches the string. I wrote 3/16" with an arrow pointing toward the center of the depression. Then I went back to the low spot and worked my way to the left. I now had two pieces of masking tape on the floor and the arrows pointing toward each other. The arrows will indicate where I needed a minimum of 3/16" of material to bring the floor up even with the rest of it. This is what my pieces of masking tape looked like at this point. I repeated the process with the 1/8" piece of scrap but started where the 3/16" pieces of masking tape left off. I worked my way to the right and then to the left. I now had 4 pieces of tape (you can just write on the previous two pieces). I now knew where I needed a minimum 3/16" of material and 1/8" of material. You can repeat this process to whatever degree of accuracy you desire, the sacrifice being time and a lot of masking tape. I have some spots as deep as 3/8" so I go in increments of 3/8", 5/16", 1/4", 3/16", 1/8", 2 layers of #30 roofing felt, and then finally 1 layer of #30 roof felt. Stacking 1/8" on top of 3/16" makes 5/16" and two pieces of 3/16" makes 3/8", etc. Once I finished that line, I moved the string 4 inches on both ends and repeated. Eventually, I worked my way across the entire room and had a whole bunch of masking tape pieces. These were my "plot points" that I used to "connect the dots". Here is a simple example of what your lines might look like. After performing this procedure about 6 times, I could now visualize just how terrible that dip in the floor I always felt was. Using a 6 ft straight edge I set it across some scrap pieces of hardboard so I could extrapolate that height across the front of my kitchen cabinets. An 8 or 10ft straight edge would have been even better. Home Depot sells metal stock in various lengths and widths. While I don't need the floor to be level, I do need it to be flat. As it turns out the floor in the pantry had about the same amount of incline as behind the oven. That isn't too surprising since the back of the panty and the wall behind my kitchen cabinets is supported by the basement wall. So while the floor isn't consistent going in one direction, at least it is when going in the other.
Long story short, I had to nail down 1/4" worth of hardboard across the front of the kitchen cabinets so the height would be "true" to what was required coming out of the pantry, and from behind the oven. Since I had effectively "raised" the height of my floor this meant my topographical map was way off. All the work I had done was nearly futile. While I didn't rip it all up, working around it to adapt to the new height requirements was a royal PITA and doubled the amount of time it would have taken.
Lessons learned for me:
1) Get the entire room emptied and stripped before starting.
2) Find high spots and nail heads and get rid of them.
3) Get appliances out of the way.
4) Use hardboard instead of shingles. Even though shingles are easy to work with I will never use the roofing shingles for flattening again. First, they make a hell of a mess. Second, they are lumpy and not of a consistent thickness. Third, for the square footage they cover and their price I think you get a better deal out of using 1/8" tempered hardboard. Fourth, they take a ton of staples to hold them flat to the floor. Fifth, they crunch when you walk on them.
5) Make sure the wife knows how much work is involved.
6) The $400 install package from Home Depot or Lowes won't buy you this kind of labor.
7) If you get a measurement that seems out of whack when compared to the previous row figure out why. A change from 5/16" to 3/16" in the span of 4 inches is worrisome.
8) Don't start filling depressions until you've finished measuring.
9) Put refrigerator on 1/8" hardboard after installing flooring so as not to damage it.
10) Laying down 1/8" hardboard across the entire floor before creating the topograph would have smoothed out all of those annoying little bumps.
11) Wives will be impatient no matter what. Some recommended videos: http://www.youtube.com/watch?v=Y4-5S6buKfg https://www.youtube.com/watch?v=ZVlL8AID5Lg https://www.youtube.com/watch?v=NuUbUxDmqNE