What Skylight Suits My Roof

Understanding Your Roof Before Choosing a Skylight 


Before selecting a skylight, it is worth taking a few minutes to understand how your roof is constructed. The spacing of roof trusses, ceiling type, roof pitch and available roof space will often influence which skylight sizes can be installed and whether structural modifications may be required.

Understanding a few basic roofing terms will also make it easier to discuss your project with a skylight supplier, roof plumber, builder or installer. It can help you understand quotations, compare different installation options and decide which parts of the project may be suitable for DIY installation and which are best left to the professionals.

The purpose of this guide is to help you understand the key components commonly found in Australian roofs and how they can influence skylight size, placement, installation complexity and overall project cost.

By understanding a few basic roofing terms, you'll be better equipped to discuss your project with a skylight supplier, roof plumber, builder or installer and make informed decisions about the most suitable solution for your home.

Let's start by looking at what is hidden above your ceiling.

[BASIC ROOF STRUCTURE DIAGRAM]


Roof Trusses
 
Most modern Australian homes are constructed using prefabricated roof trusses.

These triangular timber structures support the roof and are commonly spaced at either 600mm or 900mm centres. This spacing is often one of the first measurements checked when selecting a skylight.

In many cases, the spacing between trusses determines the maximum skylight width that can be installed without modifying the primary roof structure.

As a general guide: 

  • 600mm truss spacing often suits skylights up to approximately 550mm wide. 

  • 900mm truss spacing often suits skylights up to approximately 800mm wide. 

These dimensions generally refer to the external width of the skylight base frame (sometimes called the sash frame), rather than the visible glass area.
Where possible, it is preferable to install a skylight between existing trusses rather than altering structural roof members.

[600mm AND 900mm TRUSS SPACING] 

 

Rafters 


Older homes are often constructed using rafters rather than manufactured roof trusses.

Rafters run from the ridge of the roof down toward the external walls and support the roof structure above.

Because rafter spacing varies considerably between homes, older properties often require individual assessment before selecting a skylight size.

As with roof trusses, fitting the skylight between existing structural members is generally the preferred approach wherever practical. 

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Purlins, Battens and Ceiling Joists 


Purlins and battens are secondary structural members used to support the roofing material above. They provide the fixing points for roof tiles, corrugated iron, Colorbond® and other roofing systems.

In residential construction, tiled roofs commonly use timber battens, while metal roofs often use steel purlins. Although the terminology and materials may differ, they perform a similar role within the roof structure.

Ceiling joists are the horizontal framing members that support the plasterboard ceiling below. In homes with flat ceilings, these members are often encountered during the construction of a plaster shaft.

Purlins, battens and ceiling joists are routinely modified during a skylight installation, unlike roof trusses and rafters, which are generally best left unaltered wherever possible.

Sections are often removed to create the roof and ceiling openings and are then incorporated into the framing around the skylight and plaster shaft.

For this reason, purlins, battens and ceiling joists rarely determine the final skylight size, whereas roof truss or rafter spacing is often one of the primary sizing constraints. 

 

Sarking and Insulation 


Many Australian roofs contain reflective foil sarking, insulation batts or both.

These materials improve thermal performance and help reduce heat transfer into the home.

During a skylight installation, sections of sarking and insulation may need to be cut, repositioned or reinstated around the skylight opening and shaft construction. Good detailing around these materials can significantly influence the thermal performance of the finished installation.

[SARKING AND INSULATION DETAIL] 


Ceiling Type
Once the roof structure has been assessed, the next consideration is the ceiling below.

The ceiling type has a major influence on installation complexity and often determines whether a plaster shaft will be required.

The two most common ceiling types are cathedral ceilings and flat ceilings. 


Cathedral Ceilings

A cathedral ceiling follows the roofline, allowing the skylight or roof window to sit directly above the room below.

Benefits include: 

  • Maximum daylight transmission 

  • Direct views of the sky 

  • No plaster shaft required 

  • Excellent natural ventilation when using opening roof windows 

Because there is little or no roof cavity between the ceiling and roof, cathedral ceilings often deliver the most dramatic skylight results.

Opening roof windows are particularly effective in cathedral ceilings because warm air naturally rises and can be vented from the highest point of the room. 

[CATHEDRAL CEILING SKYLIGHT] 

 

Flat Ceilings 

Most Australian homes have a flat plasterboard ceiling with a roof cavity above.

In these situations, a plaster shaft is required to connect the skylight in the roof to the room below.

The plaster shaft often becomes a project within the project and is frequently overlooked when budgeting and planning a skylight installation.

Constructing a quality shaft requires framing, plastering, finishing and painting. The design of the shaft can have a significant impact on both the appearance and performance of the finished skylight.

[FLAT CEILING WITH PLASTER SHAFT] 

 

Understanding Plaster Shafts 


The plaster shaft directs daylight from the skylight opening down into the room below.

A well-designed shaft can dramatically improve the spread of natural daylight and enhance the visual impact of the skylight.

For the purpose of this guide: 

  • Vertical means a wall is normal (90°) to the ceiling. 

  • Splayed means a wall is normal (90°) to the roof pitch.

 

Standard Plaster Shafts

The most common plaster shaft design used in Australian homes keeps the side walls and lower wall vertical, while the upper wall is splayed.

This arrangement follows the roof geometry, improves daylight penetration and provides a better view of the sky while remaining relatively straightforward to frame and plaster.

For most residential installations, this provides an excellent balance between daylight performance, appearance and construction cost.

[STANDARD PLASTER SHAFT] 

 

Architectural Plaster Shafts 


More elaborate shaft designs may incorporate additional splayed surfaces, exposed roof trusses, feature bulkheads or custom plaster detailing.

These designs can create striking architectural features but generally require additional framing, plastering and finishing work by experienced tradesmen.

For most homeowners, a well-constructed standard plaster shaft will deliver excellent daylight performance without the additional complexity and cost of a custom architectural feature.

[ARCHITECTURAL PLASTER SHAFT]

 

Where Should a Skylight Be Positioned?

Once the shaft geometry has been established, the next consideration is where the skylight should be positioned on the roof.

Many homeowners assume the best location is as close as possible to the ridge of the roof. While this may maximise sky exposure, it does not always provide the best daylight distribution within the room below.

The goal is not simply to get light into the room, but to get the light to land where it is most useful.

For example, in a kitchen the objective may be to direct daylight onto benchtops, islands and preparation areas. In living areas, the goal may be to spread light evenly throughout the room or reflect daylight from nearby walls and ceilings.

In many homes, skylights are positioned within the upper half of the roof slope rather than directly adjacent to the ridge. The ideal location will depend on room dimensions, ceiling height, roof pitch and the location of internal walls.

The path of the sun should also be considered. The daylight pattern entering a room during summer can be very different to winter. In many situations it is beneficial to favour the lower winter sun, particularly in living areas where natural daylight is most valued during the cooler months.

Rather than relying on a single large skylight, multiple well-positioned skylights will often provide a more balanced and usable distribution of daylight throughout the room.

[DAYLIGHT PATH AND SKYLIGHT POSITION]


Roof Pitch 


Roof pitch has a significant influence on shaft design, skylight placement and flashing requirements.

Steeper roof pitches naturally create a greater splay in the upper shaft wall, allowing daylight to spread further into the room. Lower-pitch roofs generally produce a shallower shaft geometry.

As a general guide, roofs with a pitch below approximately 10 degrees require additional consideration of flashing design and weatherproofing details. These roofs are commonly constructed using deck roofing profiles and often require specialised flashing systems.

In many Australian states, roof penetrations through deck roofs are commonly carried out by a licensed roof plumber.

Flat roofs require additional consideration again. Whether constructed from deck roofing, membrane roofing or concrete, a skylight will often need to be mounted on a raised curb or hob. This ensures the skylight sits sufficiently above the finished roof surface and allows water to drain freely around the installation.

The required curb or hob height will depend on the roof design, roofing material and applicable building requirements.

[CURB / HOB MOUNTED SKYLIGHT]


Roof Orientation 


In Australia, the sun travels through the northern sky.

As a general rule: 

  • North-facing roofs receive the greatest amount of direct sunlight. 

  • South-facing roofs receive softer and more consistent daylight. 

  • East-facing roofs receive morning sun. 

  • West-facing roofs receive afternoon sun. 

Orientation can influence both skylight sizing and placement. A north-facing skylight may provide significantly more daylight than an identical skylight installed on a south-facing roof.

When planning a skylight location, it is worth considering both the path of the sun and how the light will be used within the room throughout the day and throughout the seasons.

[AUSTRALIAN SUN PATH DIAGRAM]


Combining Daylight and Ventilation 

Daylighting and ventilation are often related, but they are not always the same design problem.

A skylight positioned for optimal daylight may not necessarily be in the ideal position for ventilation.

Warm air naturally rises and accumulates near the highest point of a room. For this reason, opening roof windows are often most effective when positioned high within the roof structure.

For larger rooms, longer skylights can often provide a more even distribution of daylight than wider skylights. By extending further down the roof slope, they can capture daylight from a larger portion of the roof and help spread it more effectively throughout the room below.

Longer opening roof windows can also provide a practical balance between daylight and ventilation. The lower portion of the skylight can introduce daylight deeper into the room, while the upper portion remains closer to the highest point of the roof where warm air naturally accumulates.

In some situations, however, a combination of two roof windows may provide an even better result. A lower fixed roof window can be positioned primarily for daylighting, while a second opening roof window positioned higher on the roof can be used for natural ventilation.

This approach is particularly effective in living rooms, stairwells and cathedral ceiling spaces where both daylight and airflow are important considerations.

[COMBINED DAYLIGHT AND VENTILATION EXAMPLE]


Roofing Materials and Flashings

The roofing material itself does not normally determine skylight size or placement, however it does determine the flashing system required for a weatherproof installation.

Common Australian roof types include: 

  • Concrete and Terracotta Roof Tiles 

  • Corrugated Iron Roofing (ZINCALUME® or Colorbond®) 

  • Deck Roofing (Trimdek®, Custom Orb® and similar profiles) 

  • Concealed-Fix Roofing (Klip-Lok®, Standing Seam and similar systems) 

Each roof type requires a flashing system specifically designed for that roof profile and installation method.

Selecting the correct flashing is one of the most important factors in achieving a long-term weatherproof installation.

Roof colour is primarily an aesthetic consideration. Many homeowners choose flashings and external trims that closely match the roof colour to create a cleaner and more integrated appearance.

[FLASHING TYPES BY ROOF PROFILE] 

 

Next Steps

Understanding your roof structure is the first step toward choosing the right skylight.

Once you understand how your roof is constructed, the available space between structural members, the ceiling type and the ideal skylight position, selecting the most suitable skylight becomes much easier.

The next step is to compare the available skylight options and determine which solution best suits your roof, room layout and project objectives.