So you've just decided to put hardwood floors into your home. Great choice, indeed! However, before you dive right in and start fitting the first planks, it is vital to know the effects of moisture on hardwood floors. It is a fact, that the majority of problems affecting the quality of hardwood flooring installation , are related to moisture.

Why? Because wood is a natural, living material, that responds to changes in atmospheric humidity, by expanding and contracting. During the warm months it expands and during winter it contracts.

This is a normal procedure for hardwood flooring when it occurs to a minor extent and to reduce such movement at a minimum, humidity controls needs to be installed before the floor is fitted. Sometimes though, more severe moisture problems can occur, such as cupping, crowning and buckling and in these occasions, the responsibility falls entirely on the installer.

Why does wood expand and contrast?

Wood is a hygroscopic material, which means that when it's exposed to air, it picks up or looses moisture as a result of humidity variations in its surroundings. It absorbs and releases moisture until it is in equilibrium with the humidity and temperature of the air. When it gains moisture it swells and when it releases it shrinks. The difference between wood moisture content and air humidity, will determine whether a hardwood flooring will expand or contrast. Note that expansion and contraction is an ongoing process that will happen to every wood floor as moisture content and environmental conditions change. Real problems begin, when there is a dramatic change of moisture content.

During winter time:

During winter time, when the heater is on and humidity is low, wood flooring releases some of the moisture it contains and as a result it contracts. This implies that thin gaps will appear at the joints between the planks. This is something that you shouldn't worry, as it is sure that will happen. The gaps will close up once the wood gains back its lost moisture, when spring will come and you no longer need to have the heating system on.

What you can do to minimize these gaps or to avoid them completely, is to install a humidifier in the furnace. This will allow you to monitor and control the levels of humidity in the air, during the cold months of the year. The ideal humidity levels range between 40-55% and if you manage to keep it at this level, then no gaps will appear at all.

During summer time:

During the warm months of the year, where the humidity levels are high, it happens exactly the opposite from what is described on the above paragraph. The wood absorbs the extra moisture that exists in the air and as a result it swells or expands. When a hardwood flooring is exposed to high humidity even for a short period of time, it cups. This means that the edges of a plank will come higher than it's centre because of humidity building up in the room. This is normal to occur during summer and it usually doesn't take long for the boards to become flat again.

Now if your wood flooring is exposed to excessive moisture for an extended period of time, it may not return back to it's original size. What is happening in a such extreme case, is that the adjoining planks start pressing against each other and as a result they crack. To eliminate this problem, again you'll have to monitor and control the indoor humidity level in your home, which should never exceed 60%. Having a dehumidifier or an air condition will help. Keep in mind that cupping can also occur when water is spilled onto the floor and absorbed into the boards.

Moisture problems in hardwood floors:


Cupping can be described as when the floor is no longer perfectly flat, but has a slightly undulating effect, with the edges of the planks being slightly raised compare to the center of the plank, making it concave when viewed from above. This is often the result of moisture being gained from the sub-floor. If you notice cupping in your hardwood flooring, first you'll need to identify the possible origin of moisture. Once you find the moisture source, whether that is a leaking pipe, appliances or ventilation, it should be properly fixed. Then, the flooring needs to be allowed to dry to the extend that it is in equilibrium with the natural ambient relative humidity. When the drying process is completed, careful sanding can be carried out to ensure that the floor is perfectly flat again. To find out more about cupping, click here.


This is the opposite from cupping, meaning that the middle of the floor planks are higher and the edges lower. Crowning occurs when the unfinished surface of the flooring boards has been exposed to moisture prior to finishing. Usually when you sand your floor soon after it has cupped. Where flooring has become crowned due to the sanding flat of cupped floor that has then dried out, sanding this flat again can provide the solution.


Buckling is when either the whole or portions of the flooring become separated from the sub-floor. This is a very rare and extreme situation caused by the expansion of the flooring due to a rise in moisture content. To learn how to resolve buckling problems, click here.