Most cracks in hardwood floors are considered normal part of wear and tear. Temperatures and moisture shifts are seasonal and can cause hardwood floors to crack. Most of these cracks are very fine and do not need to be filled as they are a natural characteristic of a wood floor which will tend to expand or contract depending on the season. However, some cracks are larger and are not due to normal wear and tear or the fluctuation of the seasons. When cracks like these show up, then they may need to be filled.
Cleaning the Crack
Firstly, the crack must be thoroughly cleaned. Scraping away and vacuuming debris is very important. This leaves a clean surface for the filler to adhere to. If your floor has been waxed, you will have to test a small area even after cleaning to make sure that the filler sets properly in the crack.
Filling the Crack
Second, apply filler down into the crack to the tongue level. Some fillers can shrink and will need another application especially if the crack is very wide. If this is the case, make sure that you let the filler set and then continue with the second application.
Staining the Crack
Finally, after filling the crack you may determine that it needs to be stained. Screen off the area of the crack and lightly touch up with stain that closely matches your existing finish.
Additional Crack Repair Tips
If you are considering a refinish of your entire floor, time your crack repair before you refinish. Also, try to do this between seasons when it's neither too cold nor too hot. With humidity at a stable level, you'll reduce the impact of the expansion and contraction of the wood while filling your crack. When finished, your repair should not be noticeable and your filler should be firmly set in the crack.