When gaps develop in a hardwood floor, the result can be unsightly and the repair difficult. Gaps tend to develop when the floorboards shrink in dry conditions and will be more pronounced if moisture from the subfloor has caused them to swell first. Spot repairs with floor filler are feasible if the gapping isn't too extensive, but when it has overtaken the whole floor, you usually have to refinish the floor.
Causes of Gaps
Wood floorboards absorb moisture from the air and the subfloor and expand when it is available, and lose this moisture and shrink in dry conditions. When floorboards expand, they can push against each other; when they shrink again, gaps can form. In many cases, these gaps are normal and don't need to be filled. When they are extreme, the cause may be moisture in the subfloor or extreme humidity or dryness in the room. Gaps can also form when floorboards work themselves loose from the subfloor and either move against one another or pop up.
Types of Fillers
While any type of wood filler will seal gaps between floorboards temporarily, those that last are either epoxy-, lacquer- or latex-based. Epoxy-based resins dry harder than wood and have strong enough adhesion properties to bind to the floorboards and prevent gaps from widening. Epoxy and lacquer fillers have a plastic appearance and look best with prefinished floorboards. Latex floor filler remains flexible enough to move with the flooring boards and accepts stain, but it isn't strong enough to prevent the boards from moving. It will chip and fall out of a gap if the wood moves and the size of the gap changes.
Sometimes a series of localised gaps will develop because of one or two defective boards or because water has been sitting on one part of the floor. As long as the boards are dry, you can fill these gaps with a putty knife or paint scraper in the same way you would repair a crack in a wood cabinet. Completely fill the gaps with epoxy or lacquer filler, and wipe away as much excess as you can before it dries. Sand off the dried filler with medium-grit sandpaper, and then touch up the finish with a paintbrush.
Filling a Whole Floor
If an entire floor has developed gaps, properly filling them usually involves sanding and refinishing the floor. Because most of the excess will be sanded off in the process, a more efficient way than spreading it with a putty knife is to float it into the gaps with a grout float. Latex floor filler, thinned with water to a pourable consistency, works best for this. Spread it over the entire floor with the float, working it into gaps as you go. When the filler dries, sand the floor and stain it. The filler will blend best if its colour matches that of the unfinished wood.