Installing a hardwood floor isn't just about slapping some wood boards, down banging it together and then nailing it down. The process starts much before that. First off, the "foundation" of the floor is the sub-floor, and making sure that the foundation of the floor (sub-floor) is flat is very critical.
If the sub-floor isn't properly fastened and flat before the floor is installed it can affect your floor forever. That is why the NWFA (National Wood Flooring Association) has a set guidelines to help with proper installation.
The first thing that I do is check the relative humidity (RH %) of the sub-floor. Once the sub-floor reaches an exceptionable range it's time to bring the wood into the house and let the wood acclimate to the surroundings. The reason for this is to make sure the wood acclimates to the climate it will live in, reducing the number of problems that may occur with expanding or shrinking boards.
If the sub-floor and the wood flooring are in an acceptable range of each other, there is no need for acclimation. Once the sub-floor and wood falls within the RH %, it's time to check and make sure the sub-floor is flat.
The flatness of the floor is determined by taking a 6-10 foot straight edge level and checking the flatness of the floor. If there are any edges of the sub-floor that are sticking up they must be ground down, and if there are any spots that are low they will need to be filled in with the appropriate materiel. Making sure the floor is flat is very critical no matter if it's an on-site sanded floor or a pre-finished floor. Making sure the floor is flat is just as important as checking the sub-floor RH %.
The flatness of the sub-floor is important so that when you look across the floor there are no high or low spots and the floor looks even. That being said, sometimes in old houses, you can't fix all the high and low spots!
Once the sub-floor is deemed even, it's time to install the flooring. There are a couple of different ways to install a floor: nailed down or glue down. Glue down is generally done over concrete but can also be used on a wood sub-floor. Nail down is only done on wood sub-floor.
If it's a wide plank floor, that is anything over 5" in width, it's a good idea to both glue down and nail down because the floor boards are wide the glue adds an extra amount of holding power. This also makes sure that as the boards expand and contract there is still good holding power.
At Signature Custom Flooring, one thing that makes us different when we install a floor is that we use the least amount of exposed fasteners possible. This way when you look at the floor you don't see any nail holes on the starter row or any of the flooring installed against the wall, giving it a seamless look across the whole floor.