FCITS Defect Library: Curling

Also Known As

  • Cupping (sometimes used informally)
  • Edge Curl
  • End Curl
  • Plank Curling
  • Tile Curling
  • Edge Distortion
Field photograph using low-angle lighting to make raised plank edges and ends more visible across the installed flooring.

Primary Classification

Dimensional / Surface Deformation Condition

Definition

Curling is an observable condition in which the edge(s) and/or end(s) of a resilient flooring plank or tile become elevated relative to the center portion of the material. The condition may affect isolated planks, specific areas, or broad sections of an installation.

The term cupping is sometimes used to describe a similar visual appearance; however, cupping is more commonly associated with wood and wood-based flooring. For resilient flooring products, curl or curling is generally the more appropriate terminology when describing this change in profile.

Although Curling may be associated with moisture influences, dimensional change, internal product stress, installation characteristics, support conditions, thermal exposure, or other contributing factors, the presence of Curling alone does not establish the underlying cause.

What Does It Look Like?

  • Raised plank edges
  • Raised plank ends
  • Shadowing along joints
  • Rippled or uneven floor appearance
  • Increased visibility under low-angle lighting
  • Localized or widespread surface distortion
  • Profile appearance may resemble cupping observed in wood flooring products; however, the materials and mechanisms involved are different.
Low-angle field photograph showing a straightedge spanning the flooring and documenting visible elevation beneath the straightedge.
Field video documenting widespread raised plank edges and ends as the camera moves across the installed flooring under reflected light.

Degree of Severity

  • Minor: Slight edge or end elevation visible primarily under certain lighting conditions with minimal impact on appearance or performance.
  • Moderate: Clearly visible Curling that may be noticeable underfoot or associated with additional flooring movement or distress.
  • Severe: Significant or widespread Curling that may create trip concerns, affect serviceability, or be associated with locking mechanism distress, debonding, peaking, buckling, or floor instability.

What Causes It?

Curling develops when stresses acting upon the flooring system become uneven, causing the edges or ends of the flooring material to distort. Multiple contributing factors are common.

Moisture-Related Curling

Common Contributing Factors

  • Elevated concrete moisture conditions
  • Moisture vapor transmission
  • Flooding events
  • Appliance leaks
  • Excessive wet maintenance
  • Moisture absorption from below the flooring
  • Elevated alkalinity

Mechanism:

Moisture exposure may create dimensional changes within flooring materials, adhesives, or supporting components. Uneven moisture influence may contribute to stresses that result in edge or end elevation.

Product Stability / Internal Stress-Related Curling

Common Contributing Factors

  • Dimensional instability
  • Product imbalance
  • Internal manufacturing stresses
  • Layer-to-layer movement differences
  • Material formulation concerns

Mechanism:

Multi-layer resilient flooring products may respond differently to temperature, moisture, humidity, or environmental exposure. Differences between layers may create internal stresses that contribute to Curling.

Uninstalled attic-stock plank displaying upward curling at both edges while the center remains relatively flat, providing comparative evidence of the material profile prior to installation.
Field video documenting an uninstalled plank under a straightedge while pressure is applied at the plank edge, demonstrating the visible upward curl and movement of the edge relative to the center portion of the plank.

Installation-Related Curling

Common Contributing Factors

  • Inadequate expansion space
  • Restrictive perimeter conditions
  • Acrylic caulk bonding flooring to fixed objects
  • Improper adhesive selection or application
  • Improper substrate preparation
  • Excessive vertical deflection

Mechanism:

Installation conditions that restrict movement or create stress concentrations may contribute to distortion.

Thermal-Related Curling

Common Contributing Factors

  • Direct sunlight exposure
  • Elevated surface temperatures
  • Large windows or sliding glass doors
  • Skylights
  • Heat-producing appliances

Mechanism:

Localized heating may create uneven dimensional changes resulting in internal stresses and visible distortion.

What This Condition Does Not Tell You

Curling is an observable flooring condition. The presence of Curling alone does not confirm:

  • A manufacturing-related condition
  • An installation-related condition
  • A moisture-related condition
  • Product failure
  • Warranty responsibility
  • The need for replacement without further evaluation

Inspection Considerations

  • Evaluate distribution pattern and severity.
  • Evaluate moisture, environmental history, and thermal exposure.
  • Inspect expansion space and restrictive perimeter conditions.
  • Evaluate substrate flatness, support conditions, and vertical movement.
  • Evaluate adhesive performance and bond integrity when applicable.
  • Compare installed material with available attic stock.
  • Consider laboratory evaluation when material dimensional stability concerns are suspected.
  • Base conclusions on the totality of available evidence.

Curling should be evaluated as part of the complete flooring system. Field observations, installation conditions, environmental exposure, moisture evaluation, product characteristics, and other site-specific factors should be considered before determining the contributing cause(s).

When attic-stock or uninstalled rigid-core material is available, dimensional observations should be compared with the applicable ASTM F3261 requirements. Slight inherent concave bow may be permitted by the standard and should be distinguished from abnormal curling or dimensional change.

Attic Stock Evaluation

Uninstalled attic stock may provide valuable comparative information because installation-related influences such as substrate conditions, adhesive interaction, restricted movement, rolling loads, maintenance practices, and in-service environmental exposure have not acted upon the material.

When available, attic stock should be visually evaluated for similar dimensional changes, including edge curl, end curl, bowing, twisting, or other changes in product profile. The presence of similar conditions in properly stored attic stock may indicate the need for additional evaluation of product dimensional stability or internal material stresses.                                                   

The absence of curling in attic stock does not independently eliminate product-related concerns, as storage conditions, exposure history, production variables, and installed environmental conditions may differ.

Storage method, temperature exposure, humidity exposure, packaging condition, and storage duration should be considered when evaluating attic stock comparisons.

Applicable Test Methods / Industry References

Laboratory testing evaluates material behavior under specific test conditions and does not independently establish the cause of Curling observed in an installed flooring system. Testing results should be considered together with field observations, installation conditions, substrate conditions, moisture exposure, environmental history, thermal influences, and other site-specific evidence.

ASTM F2199 – Standard Test Method for Determining Dimensional Stability of Resilient Floor Tile After Exposure to Heat

ASTM F2199 evaluates dimensional stability characteristics of resilient flooring materials after controlled laboratory exposure to elevated temperature conditions. Testing may assist in determining whether a material demonstrates measurable dimensional change or curling tendencies under the conditions outlined within the test method.

ISO 23999 – Resilient Floor Coverings — Determination of Dimensional Stability and Curling After Exposure to Heat

ISO 23999 evaluates dimensional stability and curling behavior of resilient floor coverings after exposure to heat under controlled laboratory conditions. Results may provide information regarding material response; however, laboratory findings should be evaluated together with field conditions and other inspection evidence.

ASTM F3261 – Standard Specification for Resilient Flooring in Modular Format with Rigid Polymeric Core

ASTM F3261 provides product specification requirements for rigid polymeric core resilient flooring products. When applicable, product performance requirements and manufacturer specifications should be considered during evaluation.

For rigid polymeric core resilient flooring within the scope of ASTM F3261, the standard recognizes that an inherent concave bow across the width of a plank or tile may be present. For products 9 inches wide or less, the permitted concave bow may be up to 0.008″. For perspective, 0.008″ is approximately 1/128″ or roughly the thickness of two sheets of standard copier paper.

Because even a small amount of permitted concave bow can allow visible light to be seen beneath a straightedge placed across the width of the plank, the presence of a visible light gap alone does not establish that the plank is defective. The amount of bow should be measured with an appropriate measurement gauge and evaluated against the applicable product specification.

This dimensional provision should be considered when evaluating slight curvature in uninstalled or attic-stock material. A measured curvature within the applicable allowance, by itself, should not be characterized as a manufacturing defect.

Common Misdiagnoses

  • All Curling is caused by moisture.
  • Curling automatically means the product is defective.
  • Curling and wood flooring cupping are the same condition.
  • Replacing curled flooring permanently corrects the concern.

Related Conditions

  • Edge Lift
  • Buckling
  • Peaking and Raised Joints
  • Gapping and Joint Separation
  • Locking Mechanism Failure
  • Excessive Vertical Deflection
  • Adhesive Failure
  • Telegraphing

Typical Findings

  • Raised plank edges or ends
  • Visible surface distortion
  • Evidence of dimensional movement
  • Associated locking mechanism distress
  • Moisture-related indicators
  • Thermal exposure patterns
  • Similar distortion in attic stock

Effect on Performance

  • May affect appearance
  • May create trip concerns
  • May increase edge wear
  • May contribute to locking mechanism damage
  • May reduce long-term serviceability

Recommended Documentation

  • Overall room photographs
  • Low-angle lighting photographs
  • Edge profile photographs
  • Moisture testing results
  • Environmental conditions
  • Flatness measurements
  • Deflection measurements
  • Attic stock evaluation
  • Field photograph showing multiple installed planks with raised edges and ends visible across the flooring surface.
  • Low-angle field photograph using a level as a straightedge to document visible elevation beneath the level at the affected flooring.
  • Field photograph documenting the raised flooring profile with a straightedge and graduated measuring tool positioned at the affected area.
  • Comparative photograph showing an uninstalled attic-stock plank lying flat beneath a straightedge.
  • Field photograph documenting the span of the affected installed flooring using a laser measuring device and tape measure.
  • Low-angle photograph of uninstalled attic-stock material showing both plank edges elevated above the supporting surface while the center remains relatively flat.

Curling vs. Similar Conditions

Curling should be differentiated from Edge Lift, Peaking, Buckling, and Vertical Displacement by evaluating the flooring profile, movement pattern, and associated conditions.

Reoccurring Conditions Following Repairs

If replacement flooring develops similar Curling in the same location, evaluate whether the same underlying conditions remain present.

Additional Forensic Consideration

Associated conditions frequently provide more useful information than the Curling itself. Conclusions should be based on the totality of evidence.

Is It a Manufacturing Defect?

Not necessarily.

Curling may result from manufacturing-related dimensional instability, moisture influences, installation conditions, thermal exposure, support conditions, or multiple interacting factors.

  • Close-up field photograph showing a plank edge elevated above the adjoining flooring, exposing the edge profile along the joint.
  • Field photograph showing repeated raised plank edges and ends across multiple rows of the installed flooring.
  • Low-angle field photograph showing multiple plank edges and ends visibly elevated under reflected light.
  • Field photograph showing multiple marked locations where raised plank edges and ends are visible within the installed flooring.
  • Close-up field photograph showing a localized raised plank edge and visible separation from the adjoining plank.

Summary

Curling is an observable dimensional distortion condition involving elevation of flooring edges or ends. Proper diagnosis requires evaluation of the complete flooring system and supporting evidence.