Wood floor cupping is a common problem that occurs with both solid and engineered wood floors. If you think you can escape cupping by installing a bamboo or laminate floor you can’t for they can also cup.
What is Cupping and How Does it Occur?
A cupped floor has a concave shape across the width of the boards. The sides of the boards will be curled upward so that the edges are higher than the center of the board. Solid wood floor cupping occurs as a result of an elevated moisture content in the bottom of the flooring as compared to the moisture content at the face. Unlike solid wood, engineered wood floor cupping often occurs when the moisture content of the flooring is lowered. This is referred to as “Dry Cupping.”
Wood Floor Cupping and Product Quality
Wood floor cupping can occur with any width board though the cupped appearance will often be greater with a wide plank due to its wider width. While width is not a major factor the quality of the wood is for a number of reasons.
- High quality wood flooring is manufactured from mature trees. Since the best wood is cut from the center of a tree a mature tree has more heartwood and the boards will have more vertical grain.
- The best quality wood is milled with the help of portable sawmills from trees that are cut from locals where the species grows and matures best.
- Better wood flooring is both air dried and kiln dried resulting in a floor that is less prone to cupping.
- Better wood flooring is usually manufactured with quarter sawn boards which are less prone to cupping than flat sawn boards. Cupping occurs as a board expands in width. A flat sawn board primarily expands in width. A quarter sawn board expands in thickness.
Wood floor covering usually but not always develops slowly and it a result of a moisture differential within the flooring boards. When moisture testing is performed, excessive moisture will normally be found on the underside of the boards.
Manufacturing Causes of Wood Floor Cupping
Flooring manufacturer from wet lumber is possible though not likely when manufactured by a flooring mill. At the mill, lumber is checked prior to milling and wet lumber will rarely be milled into flooring. Different wood species do have different properties and even within the same species drying rates can vary and the wood can experience different drying defects. Cupping occurs after the flooring is manufactured. While wet lumber, improper air or kiln drying can result in cupping this should be seen at the time the flooring is received and prior to installation. If wood flooring is cupped at the time it is received, it should be returned and not installed.
Installation Causes of Wood Floor Cupping
- Underlayment and/or subfloor that is too wet at the time of installation will cause wood floor cupping. A cupped floor develops as moisture from the underlayment or subfloor moves upward through the hardwood as the back of the wood becomes wetter than the face of the wood.
- A wood floor that is placed in a house for acclimation during the time wet work such as painting and plastering is being performed may pick up moisture resulting in wood floor cupping following installation.
- The failure to properly acclimate a wood floor can result in cupping that is often permanent. This cupping is generally less subtle than site related cupping from excess moisture.
- Wood flooring that is installed prior to the HVAC system being installed may result in wood floor cupping.
- Flooring that is installed before the structure is fully enclosed may cup.
- Failure to properly performing testing of concrete or wood substrate prior to installation of the hardwood.
Site Related Causes of Wood Floor Cupping
- Leaks in pluming systems and fixtures, sink, dishwasher, icemaker and toilet overflows.
- Water vapor emissions from concrete.
- Wet crawlspaces and basements will cup a floor as moisture moves upward.
- Wet maintenance can cause cupping. When an excess amount of water is introduced to the top of a floor it may cup as the water goes between the boards entering the wood from the bottom. While a floor finish will slow water movement it does not stop the passage of moisture into or out of the flooring. Wet floors with finish on them will dry out slower than unfinished wood.
- The lack of ventilation or adequate ventilation in a crawlspace to the failure to use a proper vapor retarder over soil can result in cupping.
- While moisture from below is the usual cause of cupping, wood floor cupping can also occur from rapid surface drying. With this type of cupping use will see gaps that develop as the floor shrinks.
- High humidity. When the relative humidity is above zero the air contains water vapor. Over time the moisture content of wood reaches an equilibrium value that is based on the RH of the air. When MC increases the wood is taking in a greater amount of moisture and starts to swell as it would with liquid water. While wood floor cupping is a slower process from water vapor, taking weeks or months, the results are the same.
- In today’s well insulated and sealed buildings it is not unusual to have seasonal wood floor cupping.
Engineered Wood Floor Dry Cupping
While solid wood flooring normally cups with an increase in moisture content the opposite is often true with engineered wood flooring. Dry cupping in engineered wood flooring occurs when the MC is lowered instead of increased. Not all engineered flooring is the same though the common construction is the face of the desired wood species applied to a plywood backer. This backer provides structural integrity and stability during changing moisture conditions. The face of an engineered floor is relatively thin as compared to the backer. During manufacturing the moisture content of both the face and the back should be the same and within an expected range that products experience during use. Often the face and the backer are of different species and the two layer may act differently during changes in MC after the flooring is manufactured. When the moisture in the face experiences a significant decrease, it can shrink and as the face pulls across the width of the backer it starts to curl up or cup.
Dry cupping can be a seasonal occurrence, especially in climates that are cold and have a long heating season. Unlike solid wood flooring, extended acclimation times will not reduce the possibility of the engineered flooring cupping during periods it is experiencing a low moisture content.
Correction of Wood Floor Cupping
If the cupping is slight to moderate, eliminating the source of the excessive moisture and drying out the flooring may resolve the problem. Since moisture movement can be slow, this may take weeks, months or even a year or longer. While running a dehumidifier or fan may speed up the process it will still take an extended period of time. Commercial drying services may speed up the process but one must not be in a hurry. Sanding a cupped floor flat before it has been brought to a normal moisture content may result in crowning. A crowned floor will have a convex surface once the floor has dried.
NWFA Problems, Causes, Cures states, “Never attempt to repair a cupped floor until all of the sources of excessive moisture have been located and eliminated. This can be verified only with a moisture meter that takes readings of the underlying subfloor. As long as the wood is not permanently deformed or damaged, the flooring will return to its original shape and size when the excessive moisture is removed. This may take weeks, months, or even an entire heating season.”
“Attempting to sand a cupped floor while it is still too wet may cause subsequent crowning when the floor dries. Flooring that does not return to its original shape, even after completing an entire heating season, probably is permanently deformed. (Taking moisture readings at different levels in the wood flooring also can help determine this—if there is a gradient of 1 percent or more between the top and bottom of the boards, they probably are not done drying.) If the boards are permanently deformed, the cupped edges may be sanded off.”
This floor can be corrected by sanding and refinishing once there is no more than a 1 percent moisture variation between the hardwood flooring and the subfloor. This may take several more weeks, months or even a year and can best be accomplished through controlling room temperature and relative humidity.
If the floor is sanded prior to reaching the 1% variation between the top and bottom of the hardwood, there is a risk of crowning after the sanded and newly refinished floor reaches that point. If crowning occurred the floor might need to be replaced. To avoid this risk, it would be advisable to wait until spring of 2014 and perform the work between the heating and cooling months. At that time, the work should only be performed if the sand and finish contractor can show through moisture testing the the floor is ready for sanding.
Moisture readings taken at 1/4″ and 3/4″ indicate that the floor is near a point that it can be sanded and refinished but not quite there. A number of readings are still greater than the 1% safe variation recommended by the National Wood Flooring Association for correction. The second option would be full replacement.
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