By Greg Mowat, Forensic Tile Consultants
When natural or artificial lighting shines on walls and floors at an angle almost parallel to the tile surfaces, the shadow-producing effect is often referred to as critical lighting. Normal and acceptable inconsistencies in tile work are highlighted by these shadows, which can exaggerate natural variations in the tile and magnify their effect.
The critical lighting may be a desired effect as chosen by the architect, interior designer, owner or general contractor, but most often the resulting exaggerated appearance of the tile was not considered and creates a concern of workmanship by the installing tile contractor. When the authority inspecting the critical lighting is unfamiliar with these results, they will often inappropriately blame the tile contractor/installer and request removal and replacement of the tile instead of relocating the source of the lighting or accepting existing conditions.
The following are causes and concerns related to critical lighting:
- Use of wall wash, graze (touching) or cove lighting, where the lights are located at the wall/ceiling interface or mounted directly on the wall, may produce shadows with undesirable effects in installed tile assemblies.
- Allowable excessive lippage and warpage can be accentuated by critical lighting.
- Critical lighting can worsen the appearance of tilework that does not meet the finish flatness and lippage tolerances considered acceptable by the tile contractor’s workmanship.
- Wall wash lighting can accentuate installation and preparations by other trades (such as drywall, plaster, CMU, concrete, backer board, etc.)
- Direct bonding of tiles to substrates that have not been properly prepared can increase shadowing effects.
- Lippage is affected by substrate flatness and facial dimension irregularities of the tiles and pattern layout, including the width of grout joints between tiles.
- Tile installation patterns, especially with large format tiles installed with 50% offset (called running bond/brick joint pattern), accentuate the effects of manufactured warpage and result in lippage at the T-grout joint intersection. Because of this, large format tiles with 50% offset is not a recommended tile pattern.
- Critical lighting can also occur with exterior walls and floors caused by exterior lighting or sunshine exposure.
The tile contractor should notify the owner, interior designer, architect, general contractor or person making the decisions that lighting fixtures not properly coordinated may produce shadows and undesirable effects in ceramic tile installations. Recommend that the lighting should be placed at least 24” away from the tile wall.
When the lighting is not installed on tile surfaces, request the lighting be installed prior to tiling. The National Tile Contractors Association (NTCA) Reference Manual offers a sample letter for discussing critical lighting.
Allowable lippage is guided by ANSI A108.02 and included in the Tile Council of North America Handbook for Ceramic, Glass and Stone Tile Installation (referred to as the TCNA Handbook).
Allowable warpage, wedging and facial dimension is identified in ANSI A137.1 and ANSI A137.2. Special-purpose tile may not be required to meet the standard. Rectified tiles may assist in reducing the amount of warpage in the tile selected to be installed.
When visually inspecting finished ceramic, glass and stone tile installations impacted by critical lighting, turn off the critical lighting and use lighting perpendicular to the tile surface to ensure the assembly is well installed. Viewing distance should be 36” from walls and 60” or normal standing height from floors. Additional lighting may include flashlights, spotlights or other temporary lighting.
Always work toward successful tile installations!
