Remember, with any wood floor, the standard of acceptance is: "Inspection should be done from a standing position with normal lighting. Glare, particularly from large windows, magnifies any irregularity in the flooring should not determine acceptance."*
When a customer calls and complains, "My floors are cupped!" the first order is to determine that cupping is actually what is being described.
What is cupping? Cupping is where the long edges of the wood units are higher than the center of the pieces (Illustration 1). Cupping can be caused by any of three events:
1. Extra moisture affecting the underside of flooring
This is the most frequent cause of cupping, and is often a direct result of adverse jobsite moisture conditions. This causes a difference in moisture content between the face and the back with related expansion of the back. The back or bottom of the flooring expands and is wider than the top; cupping is the result. The flooring affected by this condition will be tight together (Illustration 2).
2. Acclimation of the flooring to a higher moisture condition than at installation
An example: a dry flooring product is installed tight together, and the area environment is on average higher in moisture than the installed average moisture content of the flooring. The flooring acclimates to the higher moisture condition and expands. If no field expansion gaps are provided, the expansion causes stress along the edges of pieces and results in a slightly cupped floor. The flooring affected by this condition will also be tight together, as in Example 1 (Illustration 3).
In this case, the face of the flooring loses moisture and shrinks, and is smaller than the back, so the edges show cupping. The telltale sign for this event is that the flooring is drying. When wood loses moisture, it shrinks, and shrinkage results in gaps. A cupped floor caused by shrinkage will show gaps between the pieces.
How to assess the different conditions
First, take moisture readings of the flooring. In Example 1, moisture readings of the face of the flooring will probably be higher than the area’s normal moisture content. The moisture content of the back of the flooring will be elevated higher than the face generally by more than 1/2 percent. The subflooring moisture content will be even higher than the back of the flooring. This situation indicates a source of moisture is present under the flooring. The most often identified causes include: high or excessive moisture in the subfloor from the building process (Illustration 4), moisture in the slab and or lack of a proper vapor retarder over the slab, a wet or overly humid basement, or a wet muddy crawl space with no complete ground cover (Illustration 5). A remedy for the cupping is to identify the source(s) of moisture and eliminate them.
The flooring should then be allowed to re-acclimate to the new conditions, which may take a heating season, before any remedial repair.
Sometimes no repair will be called for as the flooring can flatten to the original condition after the moisture source has been eliminated.
In Example 2, this is an area environmental issue, and moisture readings will be close to normal for the area and high moisture readings are not found. The subfloor moisture may be higher than the flooring, but will be less than a 2-4 percent difference on average.
This type of cupping can occur when remodeling or installing a new floor in an existing home.
This cupping is normally permanent. The cupping is permanent when the flooring goes through a complete set of seasons and doesn't significantly change. For a permanently cupped floor, the remedies are to live with the cup or to sand the floor flat. A very minor cup may well flatten over time as the seasonal change helps relax the flooring.
In Example 3, two different conditions can be observed. First, the flooring will generally have an elevated moisture content at installation. As the flooring acclimates to the environment, the face dries more quickly than the back resulting in the cupping. As the flooring fully acclimates and is no longer drying, it will generally flatten and associated gaps will be noted because of shrinkage. A remedy is to live with the gaps if small and not prominent or fill those gaps that are prominent and apply finish as necessary.
Second, a very dry environment can over-dry the flooring. In this case, moisture readings will be very low or the moisture meter will not even register. Again gaps will result from the shrinkage. This is generally a temporary condition, and as soon as the over-drying is discontinued, the flooring should flatten and many of the gaps, if not all, will close. This condition can occur in specific areas of the flooring such as around heating sources or near refrigerators with nearly constant heated air movement.
For cupping, always define the condition, take appropriate moisture readings, and make correct observations. For remedial activity, never sand a cupped floor without first determining the cause. After fixing the cause, allow the flooring to re-acclimate to the new conditions, probably through a heating season. It should be noted that slight cupping of plank flooring can be normal and occur as seasonal change affects the product. This is because of the different grain orientation across the width of planks.
•Finishing Hardwood Flooring:
NOFMA Technical Service