Crowing or rasing wood floors


My new wood floor has a problem. It is crowning too much. The problem seems to be worse in the 4 to 6 winter months, and is actually the worst on the coldest days. I have 4-in. wide planks. There is a crawl space under the floor. I have gas heat. I have learned that the floor was sanded after it had cupped during the installation. My builder and I are trying to determine what would be considered too much crowning, or if any crowning is acceptable. Do have an answer? Is there some industry standard? What would it take to correct the problem?


Dear John

While in ideal conditions a 4″ board should not have any crowning, when the moisture content on one side of the board is different that the other this will happen. And cupping or barreling is certainly a indication of this. The side of the board that has the concave shape is most likely the side that has the excess moisture in the air, and wood being hygroscopic, will absorb this excess. The crawl space is the most probable culprit.

As far as I know there is no industry standard, and it’s not talked about in the industry in any case.

That said, you must ventilate the crawl space in the summer and possibly heat it in the winter to equalize the moisture content of both sides of the wood. The space should be at least 2 foot high under the floor, and the soil should be covered with sand, then a heavy poly and then more Dry sand.

And a hardwood floor over a crawl space needs a very heavy exterior plywood, so that the 2″ floor nails ( never use flooring staples ) don’t penetrate the plywood and allow moisture to get in to the hardwood. So because the hardwood floor nails in a 3/4″ floor penetrate the subfloor 7/8″ you will have needed at least a 1″ CDX plywood subfloor on the joists to provide any kind of moisture barrier. Adding simply a flooring paper to the subfloor, will not help anything, as it’s punched full of holes during the installation. Or of course you can add a layer of thinner plywood to the existing subfloor to accomplish this.

And I hate to say this but a 4″ board almost qualifies it for the plank floor nailing and pegging schedule, as outlined in the Hardwood Authority section. A narrower 2 or 3″ wide strip floor is a better choice. And in some damp climates or very dry climates they apply a finish to the underside of the hardwood before it’s installed.

So you may have to do some or all of these things if you are going to have a stable flat hardwood floor. A wet winter provides the excess moisture under the floor from the exposed soil, and you crank up the heat and make the living side of the wood dry as a bone. Try to keep the air humidity at about 40-60% all year round on both sides of the floors.

Oh, and don’t just sand this floor flat, you will be removing 1/4 of the floor’s life, and it’s better to sort out the moisture difference, and the problem may diminish on it’s own in 6 months.

As always your Most humble servant, Joseph, the Wood Floor Doctor.-