The Problem

I was contracted to inspect newly installed, sanded and finished red oak flooring. The flooring cupped shortly after the project was completed.

What Happened

The home in northern Illinois had existing solid 21⁄4 strip flooring. The new flooring was nailed onto an OSB subfloor over a basement and laced into the old flooring. All of the flooring was sanded and finished at the same time in late March. Approximately six weeks after the flooring was finished, the homeowners noticed cupping in only the new flooring, not the existing oak flooring.

The Inspection

At the time of my inspection, the flooring had been in place for a year. When I arrived, I observed that the landscaping was correctly graded away from the foundation. Upon entering the home, I could see approximately 50 percent of the flooring was cupping. Using a straightedge and a feeler gauge, I found the cupping ranged from 0.009–0.015 inches. Moisture meter readings ranged from 6.7–7.3% MC on the tops of the boards and 9.2–9.6% MC in the subfloor in both cupped and uncupped areas. During my inspection the home's RH was 34% with a temperature of 70 degrees Fahrenheit.

The installer said the wood was stored in the home for a week with the HVAC running before installation and that the flooring MC before installation was 5.5% throughout.

I determined the cupping found in the new flooring to be consistent only with incorrect acclimation before installation, sand and finishing. The flooring was installed when the MC of the flooring was below 6%, but the target EMC of wood flooring in the Chicagoland area is between 7–9%. After the flooring was installed and once the flooring acclimated to the proper EMC, the flooring expanded. Because the flooring was installed and had expanded in place, it had nowhere to go but up, which is considered acclimation cupping. This cupping did not and will not go away with the seasons. (Cupping that does go away with seasonal changes would be considered "normal" cupping.)

RELATED: How to Prevent Cupping and Worse in Summer Months

How to Fix the Floor

To correct the cupping, all of the flooring must be re-sanded and refinished. This should happen when the flooring is close to a mid-point of the MC levels it experiences throughout the year.

In the Future

Depending on the time of year, leaving flooring on a job site to acclimate can be problematic. In this case, the flooring dried to a level too low for the typical moisture levels throughout the year. Before wood flooring is installed, installers must measure the flooring MC and determine if it is acceptable for the year-round moisture levels in the home, not just the conditions at that point in time.

See more on this topic: Moisture & Wood Floors

Bill Zoetvelt is president/owner at Mokena, Ill.-based All American Flooring Solutions.

Bill. Thank you for your article which explains acclimation cupping very well. I have seen this condition several times. What time of year was the floor installed and when did the cupping become evident?
When he says the "flooring had no where to go but up" wouldn't proper expansion space left around the perimeter of the room have prevented this?
Hi Toby,
Regarding whether expansion space at a perimeter prevents cupping, please see the second Q&A at this link:
Thanks David, The floor was installed, sanded and finished in late March 2014 and within six weeks the cupping with the new flooring only occurred.
George Hurman Rufurous Jones Wednesday, 07 June 2017
You Sure About This?
Am i sure about what George? The date, the causation for the cupping?
This conclusion matches what i have seen in several homes in the area im in, we are a high desert area.
normally we install florring around 6 percent, but the homes I inspected were in the river basin area within walking distance to the river. Another florring company had laced in and added 500 sq . A year later you could see where the old flooring was flat but all the new flooring was cupped. It was obviuos that the installers had either installed right from the warehouse or didnt acclimate long enough. the region around the river was a higher average humidiy then most of our high desert valley.
Hi Steve,

Your consumer should know how long the flooring was acclimated in their home prior to installation, and what time of year the flooring was installed. This will provide you with much need information to make your analysis. Good Luck
Good article why it's so important to have the right humidity before, during and after installation of hardwood... we've had contractor shut the dampers on the HVAC units thinking that will help the wood acclimate quicker but all in all the HVAC units need to be run as if it was being occupied I am also talking commercial wood flooring. Also the wood we get these days do not go through all the seasons they did 30 - 40 years ago they were old growth trees now what is manufactured is Young growth trees and installed so therefore they're much more sensitive to humidity that's why the MFMA and the NWFA have humidity charts for different parts of the country. Also the time of year this particular floor was installed was really the dry season so as a contractor you should already be aware that this floor is going to tighten up....
Have a great day.
If the floor is expanding after install and finish, the moisture attacking woiuld be coming from beneath? Was a vapor barrier, felt or rosin paper used. I love a mystery
Hi Jeff,

Yes, Vapor barrier was in place. The reason for this article is so the mystery is removed and to educate installers/retailers and other inspectors to causation of some forms of cupping. Hope it helped.
I need to find the company to start to produce PRESHRUNK SOLID HARDWOOD FLOORS. If You know any - please let me know.NO cupping, shrinking, buckling and so on problems. Thanks.
This guy has a point here. I don't think the moisture content differential was that far off. Moisture meters do not read with any accuracy below 6%. Wood comes from the mill between 5 and 9% and installed into an environment that supports 6 to 9%. This cupping issue is consistent with the way trees are farmed today. Grown, cut, dried, milled, sold, installed, sanded and finished. I always let two weeks go by between the install and sand and finish, I can't control what happens before I get it. I'll sell engineered sawn veneer strip and plank when I can to minimize this issue. Let's not shoot the installer just yet. BTW I'm also an inspector.
as a matter of interest were the expansions checked and were they open, this could also be pressure cupping if the expansions are closed.