I recently had twelve year old hard wood floors refinished. We paid an experienced guy (30 years) to sand and refinish over the course of a week. We switched from traditional poly to a water-based finish for the lighter color.

The guy doing the work put on five coats of finish (Bona Traffic), adding an extra 1-2 coats because he thought it looked like it needed it.

Within about a month, the floor began to cup and now 3-4 months later it remains cupped. We've never had any moisture issues in the house, and up until this point the floors have always been perfect.

In researching this it seems possible that too much finish might make the surface impervious, not letting moisture out of the wood, and causing it to build up on the back of the board - causing the convex cupping.

Is this a known issue? If not, what else might it be? If it is this, is there a way to remove finish to allow moisture to exit? It seems like sanding now might be a problem once the board returns to its original form.

thanks in advance for any help on this.

I have never heard of poly ever being the source of this. That doesn't make any sense to me.

But, I do think you probably have some sort of moisture or water issue, and most likely it is under the floor - maybe in the plywood, esp based on the way you are describing the cupping being convex.

(Also, BTW, 5 coats of Bona traffic is not unheard of at all...it's prob. the equiv or so of 3 coats oil based poly which is normal).

While it's important to look for a water source, I would also look into a dehumidifier as that might help. Remember, it's August - usually the most moisture in the air. Not sure where you live, but here in NY, it's been very humid here.

Debbie Gartner aka The Flooring Girl
http://TheFlooringGirl.com
Westchester County NY

Thanks for the response. It's very hard for me to believe I have a moisture problem because we've never had one before this. I've lived in the house for a dozen years and nothing has changed in the house other than the floors just being refinished.

The space below this floor is a finished space and dry. We haven't had any water events (in fact it just rained for the first time in 23 days) The plywood is the same plywood that has been under this floor for 12 years. The floors have never cupped in the past, even in August.

Any other thoughts? explanations? places to contact?

thanks again.

Your floor may be dry cupping. It is tecni8cally cupping but the moisture content is low instead of high. Ive seen old hvac units that run and run and run cause it and ive seen it caused in other desert like conditions. You need to check the r/h in the house both upstairs and down. From what you have described the lower level should have a elevated number (not necessarily high but higher than above) Also you have to keep in mind the most stable form of a piece of wood is a 1" square block, the thinner and wider you go the less stable the piece becomes. So when you sanded the floor you effectively lowered the range necessary to make your floor move by making it thinner. All it takes to make a floor cup is an imbalance throughout the piece, a board that has a higher moisture content on the bottom than the top will cup thinner floors do it faster. Some of you might have seen Bruce's 5/16 solid natural reflections, that stuff would cup if you left the front door open for too long.

Find an installer with a moisture meter with insulated pins. Measure top 1/4" of wood and then every 1/4" until you hit the plywood measure the mc in the plywood. If the mc in the plywood is more than 10 probably coming from underneath. Dry cupping will always happen with gaps so if no gaps not dry cupping. Ambient relative humidity can also cause cupping if more than mfg recommendations usually over 55% for a prolonged period. How much is it cupped? .01 is considered normal at .02 some remediation may need to be accomplished. Only testing and comparing to industry standards can definitively prove the cause. Any thing else is speculation!