In Moisture

One of the most common questions we are asked is why a floor cupped. The questions mostly come twice a year–in the summer, as humidity rises causing floors to expand, and in the winter, as humidity falls and floors shrink. Floors cup for one reason– as a response to moisture. There are multiple sources of moisture that can be the culprit in a cupped floor.

One of the most common causes of a cupped floor in our market is when a floor is installed in the summer before the air conditioner is working. When building a house, there are many contractors who will install the floors early in the building process because it is easy. Flooring should be installed after the heat and air conditioning system has been working for at least two weeks. Installing floors later in the building process also helps decrease the chance of the flooring being damaged by other trades on the site.

Another common cause of cupped floors is failure to check the moisture contents of the subfloor and wood flooring before installation. The two readings should be within 3-4% for solid flooring less than 3″ wide and within 1-2% for solid flooring wider than 3″ wide. These readings can be taken with a moisture meter.

Causes of Cupped Floors

The NWFA lists the following causes for cupped floors:

  • Wood is an anisotropic material, meaning it will shrink/swell in relation to its grain orientation. The extent to which solid wood shrinks or swells is also directly proportional to its width. With an installed solid wide plank plainsawn material, it is not uncommon to see the top of a board shrink or swell more or less than the underside of the board when seasonal conditions change, which may result in a slight crowned or cupped appearance.
  • A moisture imbalance through the thickness of the wood, a gradient between the top and the bottom of the board; the wood usually has a higher moisture content on the underside of the flooring than on the top.
  • An increase in moisture conditions. Potential sources of moisture may include:
    • Inadequate or inoperable HVAC systems resulting in elevated humidity levels. (See Greenhouse Effect.)
    • Wet subflooring (wood or concrete slab).
    • Inadequate moisture control systems installed beneath the wood floor.
    • Poor building drainage resulting in rainwater runoff or subsurface groundwater.
    • Moisture from inadequately ventilated or unconditioned crawl spaces or basements.
    • Portable water sources/appliances, building leaks, or plumbing leaks.
    • Maintenance-related:
      • Wet-mopping resulting in standing/topical liquid moisture left on the floor.
      • Use of steam mops/cleaning devices.
      • Use of non-breathable rugs on the floor.
      • Not maintaining relative humidity levels year-round to support the installed flooring.
  • Wood flooring installed at a lower moisture content than the in-use conditions allow. This gain in moisture may result in the installed boards swelling and cupping.
  • Wood flooring installed in an environment drier than the in-use conditions.
  • Slight cupping/edge-lift can occur when floors have been damp-mopped in dry-climate conditions. This cupping will usually subside when conditions return to normal.

Fixing a Cupped Floor

The NWFA lists the following remedies for cupped floors:

  • Never attempt to repair a cupped floor until all of the sources of moisture have been located and eliminated. The moisture content of the solid wood floor must be at equilibrium with the expected in-use (normal living) conditions prior to resanding a cupped floor. As long as the wood is not permanently deformed or damaged, the flooring should return to its original shape and size when it returns to its original moisture content. This process may take weeks, months or even an entire heating season. Sanding should not take place until all moisture levels have returned to normal. As a general rule of thumb, if there is a difference of 2% moisture content or more between the top-side and the underside of the flooring, it is not likely done drying.
  • When excessive moisture compromises the ability of the flooring fastener or the adhesive to adequately hold the flooring in place, or results in movement, audible squeaks, crackles, or pops, the affected flooring and/or wood subflooring material may need to be replaced, fastened, or injected with an approved adhesive.
  • Maintain the environment within the flooring manufacturer’s requirements. When conditions are not specified by the manufacturer, maintain the temperature between 60°-80°F (15.60-26.7oC), and relative humidity between 30%-50%.
  • With slight cupping caused by maintenance, from the finishing processes, or as seen with wide plank flooring, no remedy may be necessary.
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