Painting Advice and Premium Products from Experienced Professionals.
Stephen Smulski, Ph.D., President of Wood Science Specialists Inc.
I know of no better advice about wood flooring than that given long ago by an anonymous author at the USDA Forest Products Laboratory wrote: "The cure for cracks in a floor lies wholly in preventing them." Except when indoor relative humidity is mechanically controlled, narrow cracks (1/32 to 1/16 inch with 2 1/4 inch-wide flatsawn oak) should be expected to open between some courses in wood strip floors during the heating season. But wider-than-expected cracks can develop when flooring is allowed to adsorb excess moisture before or after being laid. Flooring acclimated on-site before concrete, masonry, drywall, or plaster is thoroughly dry, or before the heating plant is operating, will likely pick up moisture and swell. Edges butted at installation will shrink apart as flooring moisture content drops during the first few months of occupancy. By the middle of the first heating season, cracks become chasms.
Even if at the proper moisture content when laid, flooring that picks up excess moisture before or after finishing in a meagerly heated, unoccupied home, can later develop wide cracks due to a phenomenon known as compression set. As moisture is adsorbed, tightly butted edges prevent strips from widening, so no apparent swelling takes place. In reality, swelling is accommodated by partial crushing of the strips' edges. Though crushed, a compressed strip will still shrink by the same percentage as an uncompressed strip.
But because its swollen width is narrower than that of an uncompressed strip, its shrunken width will be narrower too, making cracks between compressed strips wider. Subsequent swelling pressure during later periods of high relative humidity can increase the amount of compression set and the width of cracks. Compression set explains why old wood floors that were mopped with water often have gaping cracks. It's also why wooden tool handles continue to loosen after soaking them in water to tighten them. The tightening is temporary; subsequent drying produces even greater looseness. The solution is to keep flooring (and tool handles) dry so compression set can't develop.
Though it can't be eliminated, wood movement can be minimized, masked, and otherwise managed through attention to detail during design, installation, and finishing.
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Stephen Smulski, Ph.D., is President of Wood Science Specialists Inc., a consulting firm in Shutesbury, Massachusetts that specializes in solving performance problems with wood products in all types of wood-frame construction.
The importance of this cannot be overemphasized. Very good results can be obtained by using a vacuum sweeper for this purpose. First, it should be run with the grain of the wood; then, a second pass should be made across the grain.