Wood Floor Cupping (Causes, Fixes and Sanding)


wood floor cupping causes fixes and sanding

Having sanded wooden floors for 18 years, I am no stranger to cupped floors. In fact, it’s one of the most common issues I have to deal with. Before I get right into solutions, let me define what wood floor cupping is.

Cupping in wood flooring is a form of warping where the edges of the wood boards are higher than the center, causing a basin effect running along the boards. This occurs when the underside of the board has a higher moisture content than the top side of the board.

This can be a frustrating problem, as it can make the floor appear uneven and cause tripping hazards. In severe cases, it can also make it difficult to open and close doors. Above all, it just looks odd! However, this doesn’t mean that your wood flooring is ruined. You might be wondering and hoping whether on or not cupped floors will flatten out on their own.

Cupping in wooden floors can flatten out on its own if the cause of the cupping is identified and fixed. Once fixed, it can take 4-12 weeks for the moisture levels to stabilize, allowing the floors to return to their former flat state.

What causes hardwood floors to cup?

There are several factors that can contribute to wood floor cupping.

Humidity levels. One of the most common causes is fluctuations in humidity levels.

When the humidity in a room decreases and begins to suck the moisture out of the surface of the boards, causing the surface to shrink, in contrast to the underside, giving it the cupped effect. Conversely, when the humidity increases, the surface will expand, giving the opposite effect; wood floor crowning.

Water ingress. When moisture levels in the subfloor rise and begin to swell the undersides of the boards.

Improper installation. Another cause of wood floor cupping is a lack of acclimatization before installing the wood floor.

Incorrect installation leads to cupping when the flooring hasn’t had enough time to acclimate.

I would still run through the process outlined below to identify causes.

It’s important to be very clear on the physics because it may allow you to identify the cause that you might otherwise miss.

Since cupping is caused by a contrast in moisture content between the top and bottom of the board, it’s easy to understand that cupping may not be caused by increased moisture from beneath (the most common cause), but by decreased humidity from above.

For a few examples, sometimes homes that have open fireplaces may find their floors cupping in the winter when the fire is in regular use. Maybe the recent installation of air conditioning has caused the relative humidity to drop in the air of the room and extract moisture from the surface of the wood.

cupped floor cupping hardwood floor

Where in your house is the cupping occurring?

The most common cause of wood floor cupping is a leak. If you have found all of a sudden that your wood floors have developed a serious cupping issue, it’s very likely that you have sprung a leak somewhere.

Maybe a dishwasher or washing machine is the culprit, maybe a radiator joint or valve with a corroded connection has finally given way and is slowly leaking water onto or into the subfloor.

Even floods that start off above the floor will usually end up under the floor and cause cupping. This was the case with my now famous court case with the plumber that flooded my client’s floor!

It’s very easy to understand why. The floor gets drenched, you mop that up, however long that takes, and you clear the floor.

The surface dries within a few days (maybe a few hours!), but the water that soaked down between the boards or down the sides of the room is now sitting soaking into the underside of the boards and causing expansion.

On that note, it may be important to learn the right way to clean hardwood floors. Spoiler alert; it’s not with a mop and bucket. I have had many clients whose cleaners have poured incredible amounts of water onto their wood floors and destroyed them.

Cupped floors in front of a sink, exterior door, or kitchen appliance give us a very strong suggestion for the cause. Cupping in the middle of the room or over the whole floor may point to a more general and harder-to-diagnose problem.

How To Fix Wood Floor Cupping

Step 1. Route out the cause. Now that you understand the cause of cupping, it’s easy to create a plan of action. First, identify the cause of the cupping.

Take stock of the changes that may have happened in your home recently. What could have changed the moisture levels, either in the environment or in the subfloor?

If it is something you can change then do so. If it’s something like an open fire or air conditioner, then it may be worth buying a cheap humidifier(Amazon link) just to have running overnight to even out the net humidity and help prevent cupping in the seasons affected.

For many of you, it’s going to be historic cupping and you’re not sure when it started. Maybe the floors were cupped when you moved in. This is the most common case I see during site surveys.

Even for you folk, I would try to identify a cause. There may be no cause, it may just be bad acclimation during a DIY installation.

In which case it may be a case of skipping straight to Step 3. However, before you get aggressive with the sanders, let me just address this one common issue.

Homes that have an air cavity beneath usually have air bricks to allow airflow under the house. The airflow allows moisture to escape, rather than rise into your subfloor.

This is true of millions of homes with floorboards nailed into joists. Floors thought to be glued on to solid cement are actually on block and beam. These homes also have a cavity beneath to allow this airflow.

Dust, debris, and little critters can block the holes of the air bricks. Make sure there is adequate airflow beneath your house. Blow out the air bricks.

Clear any blocked vents out, and get a french drain installed around the house if you’d like. Fix any leaks or address any variations in humidity and move on to Step 2…

Step 2. Wait. You need to give time for the flooring to adjust to the new environment. Wait until either the floor has leveled out or 3 months after your changes to fix the problem.

Step 3. Sand the floor. (if necessary) Ignore this step if you have no intention of sanding the floor and just wanted to fix the cupping issue!  However, if you were already planning on sanding the floor and wanted to fix the cupping first, then now is the time to start sanding.

Though the cupped effect may still be around after the 3 months, you can be confident in sanding the floor flat knowing that you have given it time to flatten out and it hasn’t!

To prevent wood floor cupping, it is important to maintain a consistent humidity level in the room where the wood floor is installed. This can be achieved by using a humidifier in the winter and an air conditioner in the summer.

Whenever I have worked for musicians, they already have this dialed in! It is also important to properly acclimate the wood to the room before installation and to ensure that the subfloor is level.

Cutting the cupped boards out of the floor to allow the subfloor to dry is a faster route to success. Make sure you have replacement boards, as most will be damaged in this process. Sanding the floor may be necessary if the floor is old and the replacement boards don’t match very well.

It is important to note that wood floor cupping can be a complex issue, and it may be necessary to consult with a professional to determine the best course of action. If you are unsure how to address wood floor cupping, it is recommended to seek the advice of a wood flooring specialist or a general contractor to identify your specific issue.


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  1. I learned a lot from reading your article on cupping. My floor has been damaged over a long time due to a slow dishwasher leak. Not sure if I want hardwood again or take up wood and put carpet down. Trying to decide if I should wait to see if cupping subsides and sand.

  2. Very good reading, thanks. Our floor on the 2nd has cupped, flat and crowned boards. We purchased the house recently. The floor doesn’t look worn but hasn’t seen much love either. I have no idea how the floor was cleaned or maintained. Before I fire up the sander i’m going to measure the moist percentage periodically the next couple of weeks. I have a basic moist detector with two spikes that can be poked into the wood. Maybe also will try and lift a couple of boards to see what’s underneath.

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