About Hardwood Flooring Cracks

I would say the biggest issue with hardwood is to understand hardwood. Wood flooring is a living thing, which is odd since hardwood floors are “dead” trees. But, you will notice if your atmosphere of your houses changes the wood floors change as well. I would say that the #1 call for ANY hardwood flooring business is about wood floor crowning, cupping & cracks – wood movement. This one topic is also the most mysterious for clients as it seems so crazy wood acts the way it does. This article will go over each hardwood flooring issue and break it down. If you have any further questions, feel free to email us. The following examples are made up and could or could not be conversations we have had in the past with customers.


Cracks || Easily the #1 Question & Issue

I would say that most every hardwood flooring installer has had questions and been questioned about hardwood floor cracks. It is hard for people to understand that wood can move fast, sudden and crate cracks if the atmosphere changes.

Myth #1 || Oh my God! Cracks are appearing after you refinished my floor

Well, I hate to say it but most likely the cracks were on your floor prior to the refinish, you just never took the time or had the need to notice the cracks in your hardwood floor. The true reason you probably did not see the hardwood flooring cracks before the wood floor refinish is that the ambering of the oil finish. The finish over time gets darker and darker (just go see a gym floor) and you will see that it is virtually impossible to see the hardwood floor grain let alone cracks. So in short, your cracks were probably there but you never noticed them on the flooring because it was not top of the mind.

Myth #2 || Oh, the floor cracks were never there before – I have pictures, Company X did it.

Well, lets take a deeper dive into the scientificness (made up word) of flooring cracks. Wood can only move because of a few reasons.

– Moisture in the air

– Moisture in the crawlspace

– Water on the floor

– Bad nailing

–  Equilibrium of the flooring is not acclimated to the house (see moisture above basically)

So unless the above things happen – wood will not move.  Wood moisture at a given time is what the wood moisture is. So when a wood floor refinishing company  comes to your house and refinishes the floor, they should take a moisture reading and if there is no cupping/crowning that should be the baseline reading – the one you would need to keep the flooring at to have zero cracks. Wood only moves with a variant of water changing in the woods atmosphere.

Example of a good environment:

Moisture of wood is 7% – over the year it stays between 6-9%  —> air humidity stays within 10% of the initial reading (your hardwood flooring refinisher should have this info – Floor Masters saves it for every job.). Usual humidity is around 30% here in Idaho – so winter you should be at a minimum of 25% and in the summer a max of 35%. The NWFA allows for a 15% swing, but each reader has a +_ on it so 10% is safer.

Example of a House with Cracks

Moisture of the wood is at 7% – Moisture is 35% at time of refinish. When the cracks start appearing the house is at 5% Moisture in the wood and the humidity is 12%.

Why is the above example bad?

The simple version is that wood expands when it captures water and shrinks when water is taken away. In the bad example, the wood is shrinking because the atmosphere is taking the water out of the wood to make every material at equilibrium of 12% moisture. Typical shrinkage does not happen if the wood is within 7.5% down from original reading at the time of the refinishing process.

Well, how in the world is my house changing so much between the seasons?

Great Question! the main reason is the house heater. Heaters really dry out the air going into the house and make everything shrink. In summer the heater is not on and the air has much more moisture. I find it kind of hard to understand as the air in winter is usually pretty high in humidity and summer is lower – but the heat changes the game.

How do I keep my hardwood floors crack free?

The key to crack free floors as stated above is to keep the moisture and humidity in check. So, the real question is how do I keep the humidity and moisture in check?

Step #1 | Go to Home depot and get a temp/humidity display – see here.

Step #2 | Check your reader every morning/night to ensure you are within the normal deviation of 7.5 points up or down from the starting point.

Step #3 | If normal|  Do nothing, keep checking.

Step #3 | If humidity is low | If the humidity is low you need to raise it back to the equilibrium point. Do raise the humidity you need a air humidifier in your central air unit or you can boil water to help raise the moisture.

Step #3 | If humidity is High | If the humidity is high you can use a dehumidifier or run your air conditioning. Lowering the temperature usually lowers the humidity.


The key to not having cracks is being aware of the humidity and moisture. Cracks are not the fault of your hardwood flooring refinisher. Cracks are 100% a result of the atmosphere in your home.


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