The wood filler putty products sold at home improvement stores are great for doing small repairs in parts of the floor that are relatively stable, such as disguising nail holes or filling in a small chip at the edge of a board. However, filling in gaps between floor boards is a bad use for this product because it can't flex as the boards shrink and swell, resulting in cracked and crumbled material that makes the gaps look dirty and more noticeable than ever. These four techniques for closing up those unsightly hardwood floor gaps provide better and longer-lasting results.

More Humidity

Even the best kiln-dried hardwood flooring loses internal moisture when the indoor air is particularly dry, resulting in shrunken boards that reveal gaps. Depending on your climate and air conditioning equipment, the gaps could worsen in the winter, summer, or appear all year round. The fastest and easiest way to close up these flooring gaps throughout the home is to monitor and control the amount of humidity inside your home.

An inexpensive hygrometer and a whole house humidifier will take care of the problem within a few days. Instead of aiming to always keep your humidity at a static level, try to control the most dramatic changes instead by limiting rising and falling moisture levels to no more than a 20% change. For example, a home that is usually at 40% humidity shouldn't be allowed to fall below 20%. Hardwood flooring is acclimated to your particular climate before it is installed, so sudden drops in air moisture are more likely to trigger shrinking than a specific humidity level.

Flexible Caulk

If running a humidifier won't work because of moisture-sensitive materials or heirlooms in the home, you can still fill the gaps with a material that can flex as the boards shrink and swell naturally. Tinted silicone caulk is a product recommended by many hardwood flooring installers because it won't crumble or become loose after just a few years of being subjected to board movements. Before squeezing caulk into those gaps, prepare the area by

  • Brushing and vacuuming out any dirt or debris left in the gaps
  • Securing loose boards and repairing damaged edges that border the gaps
  • Taping the edges before you apply the caulk since it's very difficult to get the residue off of the surrounding boards if you overfill.

Natural Cork

If you're not a fan of silicone caulk because of how difficult it can be to remove later, try a more natural product instead. Rope has been used as a filler for large gaps in flooring for decades because it's flexible and relatively long-lasting, but it can be noticeably unattractive and may start fraying eventually. Sheets of natural cork are a good option because they're easy to cut into strips that you can layer into each gap until it's tightly packed. The cork will flex with the movements of the wood and takes on any stain you like. While cork doesn't have a natural grain, it still better resembles wood than the filler putties sold in stores.

Real Wood

Finally, the most time-consuming and attractive option is to get packages of wood shims and carefully pound them into the gaps. This filler is the most adaptive because the strips of wood will swell and shrink at exactly the same rates as the rest of the flooring, ensuring no new gaps appear. Of course, it's very easy to stain and sand the wood you insert into the gaps as well, resulting in the most natural looking filler you can get. This method of gap filling does take a lot of time and effort, but it's worth it if you want flawless looking hardwood flooring again.