Cupping and crowning have been seen in various hardwood and bamboo installations and can be caused by various sources of moisture intrusion. However, the one factor which is common to all cupping and crowning complaints is moisture. Most cupping/crowning issues have been seen in solid wood floors, which are essentially 75% less stable than any engineered hardwood floor. Another product which has a propensity to cup and crown is bamboo. Bamboo is a grass and not a wood product, and thus has differing cell structures and reacts differently than hardwoods.
Hardwoods and bamboo will cup for one reason and one only – they gain or lose moisture from one side faster than the other. (See Figure 1) Typically, when cupping occurs, the surface of the boards will be concave (the edges will be higher the center of the boards). This indicates the back of the boards are absorbing water vapor (moisture) to a point where expansion occurs. If sufficient expansion areas are not maintained during the installation, buckling of the flooring will occur.
If the cupped boards are dried soon after their initial exposure, they will usually return to a flat position. However, if they remain cupped long enough, the stresses within the boards may be sufficient enough that even after drying they will remain cupped.
Another factor which can contribute to the propensity of a product to cup is the way the hardwood is sawn. Plain sawn boards have grain (growth rings flat with the board). This is also known as flat sawn wood. Slight curves in the growth rings will make the boards cup slightly, especially when manufactured in wider width. Quarter sawn boards have grain (growth rings) that are vertical to the board wide dimension. Expansion will follow along the grain pattern on both of these. Therefore, when the quarter sawn product is exposed to moisture, it will actually gain in thickness of the product. (See Figure 2)
- Flat sawn boards have growth rings that are flat with the board
- Quarter Sawn have growth rings that are vertical to the boards width dimension
Some cupping should be considered normal especially in wide planks, those that are 5”, 6” 7” and wider, particularly in flat sawn boards.
Crowning of the floor surface may occur because of the above conditions; however, the moisture source is topical. Another reason crowning can occur is with a ¾” unfinished hardwood floor. After the floor was initially installed it showed signs of cupping prior to sanding and finishing. The floor was then sanded and finished – the customer moves in starts the HVAC system thus removing the moisture source and the wood contracts creating a crowning problem. (See Figure 3)
Crowned Floor Result When Moisture Source Is Removed
The first step that should be taken in curing a cupped/crowned floor is to determine and remove the moisture source. Check the grade; make sure that a hydrostatic condition does not exist. If there is a crawl space under the house; make sure that it is dry. Look at the foundation in the crawl space to see if any water may have been flowing underneath it during heavy rainfalls. Check the exterior drainage of the building lot. Make sure the flow of rain water is away from the house. Has the homeowner installed any French drains if the flow is toward the foundation? Are there any weep holes in the foundation that can be a channel for moisture intrusion? Is there an intact vapor barrier beneath the on grade concrete subfloor?
Other questions that need to be addressed when looking at a cupping complaint is were the hardwood flooring/adhesive acclimated to the job site prior to the installation? Was the HVAC system operational at the time of the installation? Were sufficient (specific quantity for the size of the installation) moisture tests performed prior to the installation? (Either CaCl2 or Relative Humidity tests) Did the consumer use any topical cleaner on the floor that may have imparted moisture into the flooring? Have there been any significant humidity changes since the installation? Are there any plumbing issues that may have lead to the cupping of the hardwood flooring
Moisture cured adhesives such as MS (modified silane polymers) and moisture cured urethanes will not and cannot contribute to the cupping or crowning of wood. These products do not have any water in them. Cupping is a direct result of moisture (water intrusion), and nothing else. These adhesives depend upon moisture for the adhesive to cure. If any moisture comes in contact with the adhesive, it will begin to set up in the pail, and create a film on top of the adhesive. Therefore, other avenues of moisture intrusion need to be investigated. Once the moisture issue has been corrected, cupping may be alleviated by using a dehumidifier in the affected area.