Cupped floors, also called washboarding, develops gradually across the width of the wood strip where the edges of the hardwood planks raise up and the center of the board sinks down. The cause of cupped floors is a moisture imbalance where there is more water on the bottom side of the wood plank than on the top. The only cure is to balance the humidity levels in your home, and to give the surface time to return back to normal. After the floor has stabilized, you can have a professional sand it flat and re-finish it to perfection. Hardwood Floors
Eliminate any problem spots to make the subfloor level. Sand minor humps down with a hand-held or orbital sander to make the subfloor level. To fill any dips or low spots, use a leveling compound (also known as floor patch). Mix up the compound according to package directions, fill in the dips, then pull your straight piece of lumber back and forth over the spot to flatten and level it with the rest of the subfloor.[7] Denver Hardwood Floors
Another common hardwood floor malady is the development of gaps between planks. The most common causes of gaps between the boards is Mother Nature. Wood shrinks as it dries out, and it expands when it gets wet and humid. That regular expansion and contraction over time is the most common cause of gaps, and is the main reason why most gaps are seasonal in nature. Perhaps your best plan of action is to exercise patience.

Width & Length – Plank flooring comes in a variety of widths and lengths. The most common widths are currently in the 5” – 6” range, but wider widths are rapidly becoming more popular. The length of your floor is also important to consider depending on the size of your space. Hardwood flooring comes in mixed lengths because imperfections are cut out of longer planks during production. Rigid Core flooring is built to size and comes in lengths ranging from 36” up to 80+”. Denver Hardwood Floors
Tongue-and-groove: One side and one end of the plank have a groove, the other side and end have a tongue (protruding wood along an edge's center). The tongue and groove fit snugly together, thus joining or aligning the planks, and are not visible once joined. Tongue-and-groove flooring can be installed by glue-down (both engineered and solid), floating (mostly engineered only), or nail-down (not recommended for most engineered). Hardwood Floors
Glue down engineered hardwood, alternatively. If you don't want to nail down the boards, you do have the option of using adhesive to put engineered hardwood in place. It's a good choice, but won't last as long or be as sturdy as nailed down hardwood. The procedure is very similar to the nail-down process. Follow the adhesive manufacturer's instructions for installation and drying times.[11] Hardwood Floors