GLANCING LIGHT
When you have designed your dream home or office you don’t want the overall effect ruined by
glancing light… We have put together a guide to help you understand the effects of lighting on
decorated plasterboard.
Decoration
ALL PLASTERING SOLUTIONS … GLANCING LIGHT
Our intention in this guide is to point out some of the problems which arise
when installing plasterboard, and how thoughtful lighting set out and good
painting practice can assist in minimising flaws and imperfections being
visible.
These problems may occur even when a skilled professional tradesman has
completed the installation and followed all installation recommendations.
When consumers think of plasterboard, they generally expect an absolutely
flat, blemish free surface. Generally this is also what your builder and
plasterboard fixer would like to produce – but in reality this is not achievable.
No building material is perfectly flat and defect free – including plasterboard
used in interior lining systems. Plasterboard is made from gypsum
compressed between two layers of paper. It is machine made, and not
perfectly flat.
What your professional tradesman will produce is the appearance of
absolute flatness. This illusion of flatness is assisted by how you use lighting
in and around your home.
What your professional tradesman will produce is
the appearance of absolute flatness
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Lighting
Planning your lighting in your new project is both exciting and important.
Your carefully selected finishing touches can enhance the room or highlight
the flaws.
Larger more open rooms with greater unbroken ceiling areas and large
windows or skylights are the most susceptible to problems with glancing light.
Planning lighting features in advance this to combat effect and achieve a
more satisfactory result.
Don’t forget lighting accessories which also help to illuminate your home such
as lamps.
If you are planning on using fluorescent bulbs never run the lights in the
same direction as the plasterboard joints. Make sure non-recessed
fluorescents have caps which will help minimising illumination of surface
defects.
If you are building a multi story dwelling and you are planning on having
down lights check with your builder there is enough room in the floor cavity
before you start your project.
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What can cause lighting problems?
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A bare light globe without a cover / shade
Fluorescent lights installed on the ceiling
Using gloss or semi gloss paint haphazardly
Positioning spotlights incorrectly
Large uncurtained windows
Reflective surfaces both inside and outside
Glancing light
What is glancing light?
Glancing light is the light which shines obliquely across the surface of a wall
or ceiling.
What does it do?
When the light shines at a particular angle it will illuminate one side of the
undulation and create a shadow on the other side. Even the smallest
imperfections will be visible under some lighting circumstances.
Surface imperfection
Plasterboard Ceiling
Shadow
Critical Light
Figure 1.a. Light shining at a particular angle will highlight imperfection and cast shadow
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Images taken from the CSIRO Illumination and Decoration of Flat Surfaces, M.J. Ridge, 5th
edition, 1990
Natural Glancing Light
Natural light which enters your room will vary in intensity and tone at
different times throughout the day.
Natural Light Conditions
Dawn/Early morning: warmer, orange tint, deep shadows
Direct/Overhead: intense light, washes out detail, minimises imperfections
Afternoon/Dusk: warmer, deep shadows (effects reversed from dawn)
Reflected Light: harsh sunlight reflected back from external source, light
strikes surface from many angles
Some common conditions which occur when natural light enters and
highlights imperfections are outlined below.
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Windows positioned close to walls
When a window is positioned close to the corner junction of a wall light will
be able to shine onto the side wall at a shallow angle and highlight surface
imperfections. The effect will vary at different times of the day.
Surface imperfection
Wall
Shadow
Critical Light
Window
Wall
Figure 1.b. Light at a shallow angle will highlight defects
When light falls parallel to a surface the imperfections will be highlighted by
minute shadows.
Direct light will also affect wall and ceiling surfaces, the more direct the
glancing light and the more severe the angle the worse the effect will be.
If you are placing lighting in a wall recess you may find that there is excess
glare. This may reflect onto the ceiling and cause imperfections to be
highlighted.
“Large, flat unbroken surfaces give us an
appearance of spaciousness, which is
attractive in modern design. While these
design trends create this impression, they also
make it more difficult for even a highly skilled
professional plasterboard installer to produce a
finish the home owner finds desirable”
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Full height windows
When windows extend close to the ceiling height they allow light to enter and
shine onto the surface of the ceiling. These openings will also allow reflected
light to enter the room and shine onto the ceiling.
The time of day and the strength and angle of the light will determine the
severity of the effect.
When the sun is angling up or down, for example dawn or dusk you may find
the problem most pronounced.
Surface imperfection
Ceili
Ceiling
Shadow
Critical Light
Full height
window
Figure 1.c. light entering windows close to ceiling height will highlight imperfections
Planning
When you are planning your project, work with your architect and builder and
consider the overall affect glancing light will have on your home. Understand
how light affects a plasterboard finish and consider what you are willing to
accept while still in the planning stage of your project.
Plasterboard surfaces are subject to many different lighting conditions –
sunlight, artificial light, spotlight and fluorescent light to name a few. Light
changes in direction, intensity and source. Glancing light exists, and is a
serious consideration
Your plasterboard installer and builder want to give you the best possible job
they can. If you are aware of the issues of glancing light and work together
you can avoid disappointment.
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Favourable and Unfavourable Lighting
Favourable
Opaque light shade – prevents the ceiling from being illuminated
while still lighting the room.
Recessed down lights – light is aimed downwards providing
illumination for the room without shining light directly across.
Lamps used as décor – illuminate a room but diffuse and soften
the light minimising the impact on imperfections
Unfavourable
A bare light globe – emits strong severe light and highlights ceiling
flaws
Unguarded fluorescent lights – these will cast shadows and
highlight ceiling imperfections because of the harsh light they
emit.
Lamp shades which are translucent or direct light back up to the
ceiling – they will highlight minor imperfections in your finish
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Paint
Preparation and Application
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All surfaces should be clean and free of
plaster dust
The first coat should be a good quality
sealer which is rolled on
The work should be given a light
sanding between coats
Plasterboard defects will be visible after
the first coat and the fixing contractor
should be advised and given time to
repair
(Painters are capable of attending to
minor imperfections)
Roller application is preferable to spray
application
Spraying vs. Roller Application
Contrary to popular belief application with a roller gives a better finish than
the spray method.
Plasterboard is paper covered gypsum and jointing compound is porous
cement. To marry these two very different textures and provide a smooth
finish is difficult. Applying paint and sealer with a roller will help to equalise
the surface giving a better finish.
Spraying paint onto plasterboard will emphasise the joint lines and highlight
the nap on the paper covered plasterboard. Back rolling straight after
applying paint with a spray gun will give a better finish.
Choice of paint – to gloss or not?
Full gloss paint is not recommended for wall or ceiling
application. It reflects light which is distorted by the
surface imperfections, highlighting the minor blemishes;
it reflects an image like a blurred mirror.
Semi-gloss paint will have the same effect as gloss paint,
but to a lesser degree.
A flat paint will tend to diffuse the light it reflects back
and as such will help to mask slight imperfections.
Colour choice – a darker colour will highlight flaws; white
or light colours reflect light more efficiently and hide
imperfections.
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My Project is Complete and Joints are Visible –
What Can I Do?
Once your project has been completed if you are still having problems
with visible joints there are a number of things you can do to help
minimise the imperfections.
Remember the joints generally show because of light, damp or
incorrect paint application.
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Check the wattage of your globes – consider replacing with softer
light
Look at your light shades – are they opaque? Do they direct light
downwards?
Down lights – can they be angled to face away from shining directly
down a wall?
Fluorescents – the enemy of plasterboard. If you must have ceiling
mounted fluorescent lights joints will invariably be visible.
Do you have shiny floor tiles? Prevent the direct light glancing off
tiles with a rug.
Look at the accessories you have in the room – are they reflective?
If you reside near a body of water light will enter windows and
glance off anything reflective. This encompasses swimming pools as
well as ocean fronts.
Feature windows – particularly east and west facing – consider
installing a blind to completely block the light or hanging soft mesh
curtains to diffuse the direct sunlight.
Paint – is it a dark colour? Consider re-painting a pale colour.
Paint type – is the finish a gloss or semi-gloss? Can it be repainted
in a flat finish?
Is there a water leak?
Are the joints visible in an area subject to damp or steam? Check
you have adequate ventilation.
Can you draw attention away from the affected walls? Make a
feature of other areas.
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Levels of finish
The minimum specification used will be as per AS/NZS 2589:2007 Gypsum
linings in Residential and Light Commercial Construction- Application and
Finishing. Part 1: Gypsum Plasterboard.
The level of finish required on a project is determined at the design stage. A
level 4 finish is the accepted level of finish in domestic construction.
This standard has established six levels of finish to determine the quality of
finish desired. Each level has specified minimum requirements which must be
met.
Levels of Finish Defined
Level 0
Plasterboard shall be fixed in accordance with the manufacturers instruction –
no stopping, taping, finishing or accessories shall be installed.
Level 1
Plasterboard shall be fixed in accordance with the manufacturers instructions.
Joints shall be taped in setting compound. Excess jointing compound shall be
removed. No further finishing is required.
Level 2
Plasterboard shall be fixed in accordance with the manufacturers instructions.
Joints shall be taped in setting compound. When dry, one further coat of
setting compound shall be smooth trowelled over joints and fastener heads.
No further finishing is required.
Level 3
Plasterboard shall be fixed in accordance with the manufacturers instructions.
Joints shall be taped in setting compound. When dry, one further coat of
setting compound shall be smooth trowelled over joints and fastener heads.
All jointing shall be finished smooth by scraping or sanding.
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Level 4
Plasterboard shall be fixed in accordance with the manufacturers instructions.
Joints shall be taped in setting compound. When dry, apply two further coats
of setting compound over all joints, angles, fastener heads and accessories,
allowing each to dry before further coating is applied. All jointing shall be
brought to a smooth finish by sanding, leaving the surface free of tool marks
and ridges.
This is the accepted level of finish for domestic construction.
Level 5
Plasterboard shall be fixed in accordance with the manufacturers instructions.
Joints shall be taped in setting compound. When dry, apply two further coats
of setting compound over all joints, angles, fastener heads and accessories,
allowing each to dry before further coating is applied. All jointing shall be
brought to a smooth finish by sanding, leaving the surface free of tool marks
and ridges. Skim coat to be applied to surface to remove porosity and
different surface texture.
Used where gloss or semi-gloss paint is specified or where critical lighting
conditions occur.
Levels of finish defined as per AS/NZS 2589:2007 Gypsum linings in
Residential and Light Commercial Construction- Application and Finishing. Part
1: Gypsum Plasterboard.
This booklet has been created as a guide only in an effort to improve
preparation and painting of plasterboard surfaces. It has been compiled by
the author’s own research and personal experiences. The content is not
intended to constitute structural advice, or to include all information about the
installation decoration of plasterboard.
Reference documents:
1. CSIRO Notes on the science of Building No 61: Condensation in Houses
1991.
2. Some Condensation Problems No 78 1994.
3. Guide to Standards and Tolerances For Home Builders and Renovators
– Office of Fair Trading, Department of Commerce Home Building
Services
4. Illumination and Decoration of Flat Surfaces, M.J. Ridge, 5th edition
revised 1990
5. Lighting and Decoration – The Facts, Boral Plasterboard
6. CSIRO Division of Building, Research Report TR 90/1 Illumination and
Decoration of Flat Surfaces and Glancing Light leaflet
7. http://www.awci.org.au – Welcome to the Association of Wall and
Ceiling Industries Australia and New Zealand
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