A previously filled floor with numerous permanent gaps. Note the cracked filler. Since many gaps are 3/32” wide, re-installation is a likely repair option for this floor.

The normal gaps in this floor were filled during the winter. Fortunately an improper filler was used and resulted in squeeze out and did not buckle the floor as the floor expanded seasonally.

Gaps between boards” is one of the most often registered complaints with a wood floor. Customer expectations should include the fact that normal gaps will occur during seasons that are dry, most often associated with the heating season. Also, customers should be advised that occasional permanent gaps that are not prominent, when normal seasonal gaps close, are considered acceptable.

How gaps are addressed and possibly repaired depends on many factors. First, determine which gaps are normal, in that they close seasonally. If the gaps in question are normal, then read no further since normal gaps require no repair. If the customer insists on repairing normal gaps, then the customer should be advised that cupping or buckling is likely to occur as normal seasonal expansion over-tightens the flooring.

The factors that affect how you repair gaps are:

•How tight or secure is the flooring?

•How wide are the gaps? 

•How frequent are the gaps?

How secure is the flooring?

A typical scenario is a 30 – 40 year-old floor with gaps, creaks, and movement throughout. Filling this kind of floor with wood floor filler will likely result in the filler popping out and the flooring will look worse than it did to begin with. Another consequence of movement among boards is that the new finish will likely be stressed and show separation and white lines along those board edges that move. Stopping the movement is essential for good performance of a repair that involves filling and applying a new surface finish.

To stop the movement, first determine which edge of the board moves most and face nail along that edge. If both edges move, alternately nail along each edge. Use a flooring cleat, trim screw, or 7d-galvanized casing nail- dimpled head flooring nail- threaded flooring screw nail. Pre-drill pilot holes when using the larger nails /screws. Pick the open grain of the boards to better hide the nail when filled. An air-operated finish stick nailer won’t hold the flooring as securely as the others. Face nailing every 10” to 12” may be necessary to secure the flooring.

This 1/8” gap in maple flooring is a candidate for a shim or a board repair. Any filler would be obvious in this floor and likely break away from this wide a gap.

If the flooring system is accessible from below, such as an unfinished basement, you can secure the flooring by driving screws from below. The screw should be long enough to extend only 2/3 of the way into the flooring. With ¾” flooring and ¾” subflooring, that is 1¼”.  To locate where to position the screws, try drilling a small hole into the gaps from above and inserting a broom straw or colored monofilament. Pilot holes need to be drilled into the subfloor and slightly into the flooring. Use a drill stop or pilot hole drill to control the depth. If the pilot hole is not made, the flooring is likely to be pushed up and away from the subflooring when the screw is inserted. When driving the screw, use a washer to back up the screw so the screw head does not bury into the subfloor. Use wood screws, not drywall screws, which tend to break as they tighten.

How wide are the gaps?

Now, since most of the movement has been stopped, we can address the gaps. Gaps that are 3/64” and less can generally be filled. Contact the supplier of the filler for their specific recommendations. Old gaps that have old filler or debris in them will have to be cleaned. For areas with frequent gaps trowel filling is the procedure. A necessary technique forces the filler to completely fill the gap from the flooring tongue to the face. An incompletely filled gap will result in the filler breaking away, leaving a jagged unsightly edge. An occasional wider gap to near 3/32” can be filled if all conditions are correct. The wider gaps may have to be filled twice because of shrinkage of the filler. Remember, not all gaps will necessarily be filled. Do not fill the very small hairline gaps and normal gaps.

These numerous gaps in an unfinished floor can only be filled if there is no board movement. They also must be filled completely from the tongue up.

For new floors with occasional gaps that are objectionable, filling can also be an option. Of course, this is best done during original sanding and finishing. If filling is done correctly; it qualifies as a proper repair, will not be noticeable, and will perform for the long term. These gaps can also form after finishing. Filling can be done with regular filler that is appropriately colored to match the stain color and then the flooring can be screened and recoated. Spot filling without a complete recoat may also be accepted by the customer. In this case the filler must be sealed with finish or it will show prominently from soiling over time. There are factory finish fillers available that have a sealer incorporated in them which may be a good choice for this situation.

Another option for repair is to not fill gaps. Older floors, in particular, where numerous gaps and movement is present are candidates. These floors may have been previously waxed and are not good choices for polyurethane finish. Refinishing this type of floor with a sealer/stain and wax system can be a good choice (See FCI September 2005 Refinishing a Wood Floor). Typically, in this case only the individual wider gaps the customer objects to will have to be addressed.

Filling, board replacement, or shimming, are all options for addressing wider gaps. Again, the flooring must be secured, the gap less than 3/32” and properly cleaned, if filling is the option. For repairs to gaps using a wider board, choosing boards that blend with the color and grain patterns within the flooring is important. The boards should also be properly acclimated to the space. The look of the repair should also blend with the overall look. Even though the gap is the target, if the replacement is the lone board(s) in the floor with no gaps, it will stand out. In this case, minor gaps around the repair may be preferred.

Shimming is another method for filling gaps. The shim is a thin wedge shaped sliver ripped from a length of flooring. The piece should match the color and basic grain of one of the flooring pieces along the gap. The shim is glued into place, then the excess above the flooring is cut off, scraped and sanded. This method is generally used for gaps that are wider than 1/16”.

Another way to repair gaps is to take up, clean, and reinstall the existing flooring. This requires careful removal of flooring in order to not damage the tongue and groove. A scenario is an older floor with numerous gaps resulting from an event such as a roof leak where expansion and subsequent drying has occurred. The situation may be the customer wants to retain the look of the existing floor or new flooring cannot be obtained to blend sufficiently with the old.

The consumer objected to the extensive filler required for gaps in this select floor, and rightly so. This resulted in the customer being very critical at final inspection. The finisher paid attention to the details and the floor was accepted.

The procedure is to determine the direction the flooring was installed and remove the last run of boards laid. This creates access along the tongue edge. Use two claw hammers and slide the claws under the tongue edge, the flooring can be lifted up and removed with minimal damage. Clip the nails with end nippers or side cutters. If the nails are too large it may be necessary to back them out or pull them through, use the procedure that creates the least amount of damage. Use a flooring scrapper to clean the edges of debris. You can then reinstall the boards together without gaps.

Gaps between ends are very difficult to repair. Wood filler does not stick to end grain well and there is some inherent movement between ends as the boards react differently. Small gaps, less than 1/32”, will generally hold filler but larger gaps probably will not. In an old floor, it may be possible to jack or pry a run together from the ends enough to close end gaps. Otherwise, board replacement is the normal repair for larger end gaps.

What about numerous

permanent gaps in a new floor?

The previous conditions and guidelines for repair options, still apply. However, with a new floor customer expectations are more demanding and rightly so. As a rule frequent, noticeable permanent gaps are not acceptable in a new wood floor. First, the chosen repair will have to pass a critical “Is it noticeable?” inspection. If the gaps are NOT noticeable after repair, the repair qualifies as acceptable. Next the repair will have to pass multiple seasons without deterioration. These repair options can still work with critical attention to ALL the details.

In any case, specify and ask for a certified wood flooring product, such as NOFMA or MFMA products with specific tolerances of manufacture. These products assure they are properly manufactured for moisture content and configuration which are essential in the first step toward preventing objectionable gaps. Second, proper site conditions and following proper installation technique will result in normal seasonal gap and the occasional minor gap.