Cracks in Hardwood Floors
Cracks Between Boards -- Causes
Cracks are the most common cause of complaints on wood floors, and this problem in recent years has been exacerbated by pastel nd white (or pickled/bleached) finish colors, which tend to make normal cracks appear much larger than when earth-tone or natural finishes are used.

It is normal for the interior of homes to become dry during heating seasons, for obvious reasons. Under this circumstance wood floors also dry out and shrink slightly. Properly made and properly installed wood floors should be expected to have "hairline cracks" between boards in dry months in most areas of North America. Depending on the width of the boards (or parquet members) used, the size of the room and the severity and duration of low outside temperatures (and hence the intensity of heating), the term "hairline cracks" can have various interpretations.

Generally, "hairline cracks" can be considered to be normal if, in strips 2-1/4" wide or less:

	They close up during non-heating months, and
	They are not wider than the thickness of a dime in some locations, and vary from the thickness of a piece of stationery in most areas to scattered larger cracks up to the thickness of a dime.

 

Foundation settlement. When outside walls settle -- or the center supports under the house's center beam -- the area of the floor actually stretches, causing cracks over joints in plywood subfloors. This can be detected in foundation walls (See Inspection Procedures) or by checking the levelness of the floors.

Over-drying above forced air heating ducts. If the cracks are restricted to hallways or other areas above heating plants, etc., check for insulation (See NOFMA "Installing Hardwood Flooring" for correct insulation techniques).

Improper subfloor materials. Nail-holding capability is an imperative consideration in floor installation. If the subfloor does not hold nails, cracks can occur from less-than-abnormal moisture absorption or heavy traffic.

Either kiln-dried boards of NO. 1 or NO. 2 Common Pine or other dense, Group 1 Softwoods or exterior plywood are suitable subfloor materials for nail down strip or plank floors. If plywood, 5/8" (19/32") or 3/4" (23/32") performance rated products are preferred. Also, 3/4" (23/32") OSB is a comparable substrate.

When parquet is the chosen floor the subfloor should be either SE plywood, preferably 5/8" thick or heavier, or boards. Either should have an underlayment of 1/4" or heavier plywood (offsetting seams or on a diagonal) nailed in 4" grids to the subfloor.

Wood-fiber composition panels, commonly referred to as fiberboard, OWB (Oriented Wafer Board), particle board, or others, are widely considered to be unsatisfactory subfloors or underlayment for any type of hardwood flooring.

According to test results currently available, these types of composition panels expand from moisture, like most any wood product, but do not shrink appreciably when the moisture dries out. In this process, according to one maker's tests, they lose nail retention power. Hardwood floors depend on lasting nail retention in the subfloor in order to perform well over the life of a home -- perhaps 100 years or more. Inadequate nail-holding characteristics should be avoided at all costs.

 

Heavy vehicular movement, such as fork lifts or truck in public buildings, factories and commercial buildings. Flooring installed for normal foot traffic often cannot support heavy loadings without shifting.

ASSESSING CAUSES FOR CRACKS - PARQUET

Patterned floors made from short pieces of 3/4" flooring are usually glued to the subfloor with a mastic. If the subfloor is concrete, the inspection procedure should determine whether a vapor retarder was installed under the flooring, by removal of a section of the floor if necessary.

A phenomenon peculiar to mastic applied parquet, due to the fact no piece is permanently anchored to the subfloor, is that the floor units move in no particular pattern when they expand. Somewhere near the center of a room, however, one or two sections of the parquet pattern act as the anchor, and all other pieces will move away from anchor points toward the walls. It's for this reason cork expansion joint filler is needed to support the flooring at the walls. This is a special cork with a very resilient binding resin, which acts as a compression spring. It should be cut in small pieces to fit snugly between walls and each unit of parquet. If it is not used, cracks near the walls of the room will be larger than in the center, because there was nothing to push the parquet back as it dried.

Good floor mastics allow slight movement of parquet units without breaking the bond, and in fact stretch somewhat under pressure. Some types will re-tack even after breaking loose. If the parquet units appear (or sound) loose, expansion has probably moved the units too far for the mastic to retain its bond, or perhaps an inappropriate mastic was used.

Parquet with abnormally large cracks during the heating season has no doubt gone through the post-installation high-moisture cycle described earlier for nail-down products, and the proof is established by moisture checks and measurements, much as described for strip or plank floors. (Since many parquet patterns alternate grain direction, movement will occur in all directions, but only 50% as much in any direction as plank or strip.)

Regardless of what moisture environment a floor has been exposed to, or exists when inspected, removal and replacement of a wood floor to alleviate cracks is usually both unnecessary and self-defeating. New flooring material which has not acclimated to the home's environment is likely to have a different moisture content than is required to remain stable in the home. Handling, storage and shipment of hardwood flooring often exposes it to periods of high humidity, higher, at least, than floors already in place during heating months when the question of replacement is most likely to arise.

All parties involved are usually well advised to leave the offending floor in place and effect repairs if possible. Once an existing floor has been acclimated to a home's environment, it is far more likely to remain stable and, with professional repairs, can regain the appearance it had when new with no loss of service.

NORMAL CRACKS: If truly normal, in the sense the cracks close up in summer months, no repairs are practical. Any filler used to fill up cracks when they appear -- i.e., when the floor is dry -- will be pushed out as the wood expands when it picks up moisture. In the process fillers, some of which are as hard as the wood, can crush and damage edges of boards. Thus, fillers may cause uglier cracks than those Mother Nature forced on the floors, and the process of filling solves nothing.

ABNORMAL CRACKS: Even floors which have gone through a very high period of moisture absorption, then dried to leave abnormal cracks, can be repaired by a professional so that the cracks very nearly disappear. If the floor has a surface finish (i.e., Polyurethane), matching filler should be troweled into all cracks. When dry, the floor can be screened and a new coat of Polyurethane applied.

If the polyurethane or other surface finish has been waxed, however, a new coat of finish probably will not adhere. A better solution is to steel wool with No. 00 or finer wool, clean and wax.

In the process of expanding and re-shrinking, grain direction in boards may play a role in the evenness of the resulting surface. Boards with vertical (quartered) grain may become higher than adjacent flat-grain boards, thus requiring re-sanding.

NOTE: If the floor is cupped or crowned, these solutions are not effective. See Cupping & Crowning.

The National Oak Flooring Manufacturers Association supports sustainable forestry and the responsible stewardship of all natural resources.