Hardwood Floor Cupping: The Causes and How to Fix it

By Samuel N •  Updated: 11/04/22 • 

Hardwood floors and water do not get along. When water or moisture interacts with a hardwood floor, the part of the floor closest to water/moisture will expand. When it expands, and the center becomes higher than the edges, this is called crowning. However, when it expands, and the edges become higher than the center, this is called cupping.

Floor cupping creates a concave shape in wood. As the wood continues to cup, it can result in bigger gaps between the planks. With time this causes the wood to buckle away from the subfloor.

Before you fix or attempt to repair a cupped hardwood floor, first, you should learn why cupping is taking place. In this post, I’ll discuss the possible reasons for floor cupping and how to fix it. Enjoy. You can also check out other hardwood problems to expect in this post.

Why Hardwood Floors Cup

Cupping in hardwood floors is caused by changes in the moisture levels in the air. Wood is a fibrous material and will react with moisture around it to be at equilibrium. If the air is humid, wood will absorb more moisture and swell. If the air is dry, wood will lose moisture and might crown.

Because cupping happens mostly because of higher moisture or water, you need to understand where this moisture comes from and take the steps necessary to prevent more damage. Below are some possible sources of moisture/water to check out for in your home.

1. Leaks and Spills

When a leak or spill on a hardwood floor is left for a longer time, the damage will soon follow. Leaks around faulty plumbing sinks, and dishwashers, should be fixed, no matter how small, to avoid gradual wood damage. For liquid spills, make sure to clean them as they happen to avoid them soaking into the wood. If you notice mold or other problems, fix the source before it gets out of hand.

2. Subfloor Moisture

Another common cause of floor cupping is subfloor moisture. Unfortunately, this is harder to identify, and you’ll often notice it when it’s too late. Sources of subfloor moisture include damp and humid basements, flooding, wet concrete slab, and plumbing leaks.

For damp and humid basements and crawlspaces, a dehumidifier is the best option to prevent moisture from rising through the subfloor. The ALORAIR 120 PPD Basement/Crawl Space Dehumidifier is a great option for a great dehumidifier. Place it in your basement or crawlspace, set the humidity level, and turn it on. The dehumidifier drains using gravity or can be fed to a condensate pump. However, you can also get the ALORAIR 120 PPD with a built-in pump.

Damp concrete can also cause floor cupping, but this is rarely the case. If you installed a moisture barrier before installing your hardwood flooring, it should keep excess water from going up. It should be properly installed, and you should ensure all the seams are properly sealed.

If you recently had a flood in your home, you should contact a professional to help save your flooring from water damage. If the water sits for too long, nothing much can be done. However, if inspected and dried on time, a repair can begin.

3. Higher Humidity Levels

Higher humidity in your home makes for an invisible enemy against your hardwood floors. Wood adjusts its moisture content to match the moisture content of the air around it. In higher humidity, wood will absorb moisture causing it to expand and cup. Low humidity will do the opposite. It will cause your hardwood flooring to shrink. Maintaining an ideal humidity level is key to preventing most hardwood flooring problems like cupping.

When the humidity levels rise in the summer season, you can use a dehumidifier to keep them in check.

How to Fix/Repair Cupped Hardwood Floors

The first step in fixing a cupped hardwood floor is locating the source of moisture and fixing it. Once you’re fixed the source of the problem, then you can move on to restore your hardwood floors to a good state.

If the cupping is not severe, it should return to normal and straighten out in the next few weeks. Cupping can also be a seasonal issue and will disappear on its own once the relative humidity of your home stabilizes.

If the cupping is severe and caused by water damage, you might require professional help to dry the hardwood floor. Drying a hardwood floor can take weeks, months, or worse, an entire heating season.

Once the floor is dried properly, you can sand minor bumps to return it to its normal state. Do not attempt to sand down a cupped floor because this will result in a badly crowned floor once the moisture stabilizes. The floor should only be sanded once the drying process is complete.

If properly dried and sanded, you can proceed to apply a protective coat to prevent future damage.

Samuel N

Samuel N is the founder of Improve Floor and has been in the flooring industry since 2005. Since then, his mission has been to make flooring easier for everyone. He helps countless people with flooring installation, finishing, maintenance, and repairs each year.