Solid hardwood can develop gaps, especially after being flooded. Gapping can be such a problem that removing and replacing the flooring is the only choice. By contrast, engineered hardwood flooring is designed to minimize many of solid hardwood's problems—such as gaps due to the expansion and contraction of the wood.
But gapping in engineered hardwood still can occur. Factors include humidity, installation practices, and flaws in the material. During installation of hardwood flooring, leave 3/8" to 3/4" around the perimeter for expansion of the wood. Cover the gap with baseboards and shoe molding.
Gaps Due to Humidity
Wood expands and contracts with changes in humidity. It swells in high humidity and shrinks when the air is drier.
The amount of expansion and contraction largely depends on the orientation of the wood grain. Wood moves most across the grain rather than with the grain. Traditional hardwood flooring boards (and the top layer of engineered floors) are cut parallel to the grain.
As a result, the boards tend to get wider and narrower (across the grain) more than they get longer and shorter (with the grain). Most of solid hardwood flooring's moisture-related problems such as cupping or warping are across the grain.
Engineered flooring essentially consists of a hardwood top layer laminated to a plywood core, or base. Plywood is assembled in layers using a technique called cross-graining. The grain of each layer is perpendicular to the grain of the layers above and below. As one layer wants to expand or contract side to side, the neighboring layers want to move front to back.
Thus, the layers work against one another, with the result of minimal overall expansion and contraction. This is why engineered flooring is sometimes called dimensionally stable.
The plywood core of engineered flooring helps reduce gapping due to natural expansion and contraction. But it doesn't always eliminate it. That explains one common cause of gapping in engineered flooring. It's also why installers should acclimate the flooring before installing it.
One of the common solutions for this problem is to carefully condition your home year-round to maintain ideal humidity levels. The idea is to run an air conditioner to lower humidity in summer and run a humidifier to raise it in winter.
Any water spills on engineered wood flooring should be mopped up immediately before the water can seep into the joints between the floorboards.
Gaps Due to Installation
Engineered hardwood flooring can be nailed down, glued down, or floated (attached from one floorboard to the next, with no attachment points on the subfloor).
In all cases, the floorboards must be fitted tightly together during the initial installation. If not, and there are gaps between boards, the gaps are there for good, although they may shrink slightly during periods of high humidity.
Glue-down floors present some special challenges that can vex inexperienced installers. If the boards aren't fit together properly before the adhesive begins to set, any resulting gaps can be hard to correct.
Even if the installer comes back and closes the gaps, the adhesive has a memory that can pull the boards back to their original position, opening up the gaps again. For this and other reasons, glue-down installation is best left to experienced installers.
A floating floor, the natural choice of most DIYers, involves click-together flooring that is simply laid over a foam pad underlayment. The best way to prevent gapping with this installation is to cut the boards to the right size and to click them together properly, making sure there is no gapping with each board before moving to the next one.
Gaps Due to Material Flaws
The construction of engineered flooring helps minimize the effect of natural flaws in wood, but it's still wood, and wood does have some flaws. It's impossible to machine flooring with 100-percent accuracy on every board.
As a result, some boards won't fit together perfectly. If a gap shows up when you're installing the flooring, try to find a board that fits better. Otherwise, if the gap is left on the floor, the only solution is to replace the board or to fill the gap with a color-matched wood filler.