What is Glancing Light?

Plaster & Painting defects are common industry norms during the final PCI (Practical Completion Inspection) or otherwise referred to as the Handover Defects Inspection for new domestic construction works.

 Some surface variation is inevitable due to the following factors:

• Natural variations in the framing.

• The hand-finished nature of a plasterboard wall or ceiling.

• Subtle differences between the textures of plasterboard and the jointing compounds.

 Under the majority of lighting conditions, a plasterboard surface finished to a Level 4 standard, as defined in AS/NZS 2589:2007 ‘Gypsum Linings - Application and finishing’, will appear flat. Unless otherwise specified, a level 4 finish shall be taken as the default standard level of finish for plasterboard.

 However, in critical lighting conditions, an effect referred to as ‘glancing light’, will highlight any surface variations.

 What is Glancing Light?

CSR Gyprock, ‘The Red Book’, defines glancing light (or critical light) as a condition which exists when light hits the plasterboard surface at an acute angle and casts shadows that highlight any surface irregularities.

On plasterboard walls and ceilings this can make the surface look uneven and highlight the non-uniform appearance where stopping-up compound has been used. This is normally at sheet plaster joins, screw fixings, and wall & ceiling junctions.

 Plaster imperfection from glancing light is commonly found in situations where there are:

• Floor to ceiling windows.

• Windows directly adjacent to walls.

• Unshaded batten holder ceiling lights or ceiling mounted fluorescent lights

• Wall lights and downlights close to walls.

• Windows at the end of long corridors (ie front door sidelight)

• Lights installed just below skillion/raked ceilings.

• Reflections of light from water features.

According to the Guide to Standards & Tolerances 2015, variations in the surface colour, texture and finish of walls & ceilings are to be viewed where possible from a normal viewing position under non-critical lighting conditions. A normal viewing position is looking at a surface from a distance of min. 1.5 m. Non-critical light means the light that strikes the surface is diffused and is not glancing or parallel to that surface.

It is important that designers, builders, supervisors, plastering & painting contractors understand the impact that light plays on the finished level of the plaster/painted surface. I find it a poor excuse when builders and contractors simply point to glancing light as the reason for poor workmanship. If glancing light is anticipated to be a problem, especially from a window positioning point of view, then additional care should be performed by the trades, or the builder should recommend a Level 5 finish to the area.

 Furthermore, to achieve the best possible results, a three (3) coat system (one coat of prep-coat followed by two coats of topcoat) is necessary in accordance with the guidelines contained within AS/NZS 2311 (2009) “Guide to the Painting of Buildings”.