Which Type Of Dome Is Best for a Tubular Daylight System?

That is understandable. It is the most visible part of the system on the roof and must withstand sunlight, rain, hail, wind, temperature changes and airborne pollution for many years.

However, the type of material alone does not tell you enough about its quality or safety. The grade and origin of the raw material, UV protection, manufacturing method, fire classification and quality control are all equally important.

The shape of the dome also does not automatically determine how much daylight reaches the room below.

Round, shallow, tall, faceted or ribbed: manufacturers sometimes attribute special light-capturing properties to the shape of a roof dome. In reality, the shape alone does not guarantee that more usable daylight will enter the tubular daylight system.

A transparent dome allows available daylight to pass through it. To capture low-angle sunlight and redirect it into the tube, an effective reflector or optical light-directing system is required. Once the light enters the tube, the reflectance of the tube becomes one of the main factors determining how much daylight reaches the ceiling diffuser.

The following factors all influence the final performance:

• The diameter of the tube
• The reflectance of its internal surface
• The total tube length
• The quantity and angle of bends
• The orientation of the roof dome
• Obstructions and permanent shading on the roof
• The design of the ceiling diffuser

Every bend and every additional metre of tube introduces some light loss. A visually impressive dome shape may suggest advanced technology, but without effective light redirection and a highly reflective tube, its practical benefit can be limited.

You can read more about this in our article: Why a Dome Reflector Can Significantly Improve Sun Tunnel Performance

Polycarbonate is a strong, lightweight thermoplastic. Its exceptional impact resistance makes it particularly suitable for exposed roof applications where hail, falling branches or accidental mechanical impact may occur.

Advantages of polycarbonate

• Very high impact resistance
• Low weight
• Excellent forming capabilities
• Less prone to breakage than glass or acrylic
• Suitable for a wide range of roof constructions
• Good resistance to hail and mechanical impact
• Available in flame-retardant grades

Points to consider

Polycarbonate requires protection against long-term ultraviolet exposure. Without sufficient UV stabilisers, a co-extruded protective layer or a suitable coating, the material may eventually yellow, become cloudy or lose some of its mechanical properties.

This does not mean that every polycarbonate dome will discolour quickly. There are significant differences between material grades. A dome manufactured from high-quality, UV-stabilised polycarbonate can remain clear for many years. Lower-grade material, insufficient UV protection or an unknown material formulation can age much faster.

Polycarbonate is also more sensitive to scratching than glass. Aggressive or unsuitable cleaning products can damage its surface.

Acrylic, also known as PMMA, is valued for its high light transmission and optical clarity. High-quality acrylic generally has better inherent resistance to UV exposure than standard unprotected polycarbonate.

Advantages of acrylic

• Very high optical clarity
• High light transmission
• Good weather and UV resistance
• Relatively scratch resistant for a plastic
• Lightweight
• Can be formed into a wide variety of shapes

Points to consider

Acrylic is less impact resistant than polycarbonate. It is more likely to crack or break under severe impact, making the grade and thickness of the sheet particularly important in roof applications.

There are also considerable differences between acrylic raw materials. High-quality PMMA can retain its clarity for a very long time, but acrylic domes that have yellowed or become cloudy can still be found in practice. This may be related to the raw material quality, additives, manufacturing process, exposure conditions or maintenance.

The word “acrylic” alone is therefore not a complete guarantee of long-term quality.

One of the main advantages of glass is that it does not yellow as a result of UV exposure in the way that insufficiently protected plastics can. Its hard surface offers excellent resistance to scratching and long-term weathering.

Glass is also non-combustible as a base material, which can be an important consideration in projects where reaction-to-fire performance is a priority.

Advantages of glass

• Does not yellow due to UV exposure
• Retains its optical clarity
• Highly scratch resistant
• Resistant to long-term weather exposure
• Compatible with many commonly used cleaning products
• Non-combustible as a base material
• Stable material properties
• Durable, high-quality appearance

Points to consider

Glass is heavier than plastic and places different demands on the support structure, fixing system and installation method.

Although suitable safety glass can be very strong, glass remains more vulnerable to breakage under extreme impact than polycarbonate. Its weight and more limited forming freedom can also increase manufacturing and installation costs.

The precise type of glass, its thickness, treatment and method of retention must therefore match the intended application.

Crystal glass is used in some tubular daylight systems because of its clear appearance and long-term optical stability. It is important to understand that crystal glass is not a type of plastic. It belongs to the glass family, although its exact composition and technical properties may differ between manufacturers.

Advantages of crystal glass

• Clear, high-quality appearance
• Does not yellow as a result of UV exposure
• Hard, scratch-resistant surface
• Good resistance to long-term weathering
• Smooth surface that is relatively easy to clean
• Stable optical properties
• Non-combustible as a base material

Points to consider

Like other types of glass, crystal glass is heavier than a plastic dome. It requires a properly engineered fixing system and can break under exceptionally severe impact. It will also generally cost more than a standard plastic collector dome.

Crystal glass can therefore be a durable and fire-safe material choice, but the term “crystal” does not automatically mean that the dome captures more daylight. Effective light redirection, tube diameter, reflectance and system design remain decisive

Fire safety is a central consideration in the specification of building products in England. It is therefore important to distinguish between reaction to fire and fire resistance.

Reaction to fire describes how a material or product behaves when exposed to fire, including:

• Whether and how easily it ignites
• How much it contributes to fire growth
• The amount of smoke produced
• Whether flaming droplets or particles occur

Fire resistance describes how long a complete building element can resist the passage of fire, heat and smoke.

A dome may have a particular reaction-to-fire classification, but this does not automatically establish the fire resistance of the complete roof penetration. The roof flashing, reflective tube, seals, insulation, ceiling termination and method of installation can all affect the performance of the complete system.

Polycarbonate and fire safety

Polycarbonate is a combustible thermoplastic, so it should not be described as non-combustible. However, this does not mean that it is unsuitable where safety is important.

Polycarbonate offers exceptionally high impact safety and is available in specially formulated flame-retardant grades. Depending on the formulation, thickness and product construction, these grades can achieve significantly better reaction-to-fire performance than standard untreated plastics.

This makes correctly specified polycarbonate an attractive option where both impact resistance and controlled fire behaviour are required.

Its precise performance depends on:

• The particular polycarbonate grade
• Flame-retardant additives
• Material thickness
• The geometry of the finished dome
• The supporting components
• The complete product construction
• The method of installation

Not every polycarbonate dome has the same fire performance. The classification and test evidence for the actual finished product must therefore be checked rather than relying on the material name alone.

Acrylic and fire safety

Acrylic is also a combustible thermoplastic. PMMA can ignite, soften, melt and continue to burn when exposed to sufficient heat.

Some transparent acrylic products produce relatively little smoke compared with other plastics, but low smoke production does not make a material non-combustible or automatically suitable for every application.

The reaction-to-fire classification can vary according to the grade, thickness, colour, additives and product design. A general statement covering all acrylic domes would therefore be misleading.

Glass and crystal glass in fire-conscious projects

Glass and crystal glass provide an important advantage in fire-conscious design: the glass itself is non-combustible and does not provide fuel to a fire.

This makes glass a strong material option where non-combustibility, optical stability and long service life are key priorities.

However, the complete product must still be assessed. Laminating films, coatings, gaskets, seals, fixings, insulation and the surrounding roof construction may behave differently from the glass itself. Ordinary glass can also crack when exposed to severe and uneven heating.

The glass specification and complete system construction must therefore be appropriate for the project.

Two different forms of safety

Polycarbonate and glass are both strong safety choices, but for different reasons.

Polycarbonate provides outstanding impact safety. Its high resistance to hail and mechanical damage reduces the likelihood of sudden breakage. When an appropriate flame-retardant grade is used, it can also provide controlled and documented reaction-to-fire performance.

Glass provides non-combustibility and long-term optical stability. It does not yellow under UV exposure, has a hard surface and does not add fuel to a fire.

For this reason, we do not believe that one material is automatically right for every project. The appropriate choice depends on whether impact resistance, non-combustibility, weight, optical performance or another project requirement has the highest priority.

A material name is not a fire certificate

The terms “polycarbonate”, “acrylic” and “glass” are not substitutes for product certification.

Two polycarbonate domes can achieve different reaction-to-fire results because they use different raw materials, additives, thicknesses or constructions. The same principle applies to acrylic products. Even where glass is used, the classification of the complete assembly may differ from that of the glass alone.

For projects in England, fire safety should be considered in relation to the applicable Building Regulations and the current guidance in Approved Document B. Reaction-to-fire performance is increasingly specified using the BS EN 13501 classification system.

Where roof performance is relevant, additional classifications and requirements may apply to the complete roof assembly. Requirements can also vary according to the building type, height, use, location of the rooflight and the surrounding construction.

Always request the documented classification for the actual product being supplied. Where the project has specific fire-safety requirements, the complete tubular daylight system and roof penetration should be reviewed by the project’s competent fire-safety or building-control professional.

During vacuum forming, a pre-manufactured plastic sheet is heated and formed over or into a mould using vacuum pressure. The properties of the original sheet can be inspected and documented before forming.

This makes it possible to verify:

• The sheet manufacturer
• The type and grade of polymer
• The original sheet thickness
• The specified UV protection
• Any fire-performance formulation
• The technical material specification
• The source and production batch

The quality of the starting material is therefore clear and traceable.

The sheet can become thinner in certain areas during forming, so proper tool design and controlled processing remain essential. Nevertheless, vacuum forming provides a clearly defined starting point: a sheet with known and verifiable properties.

Injection moulding

During injection moulding, plastic granules are melted and injected into a closed mould under pressure. The process is particularly suitable for high production volumes, complex shapes and products with integrated details.

Injection moulding can produce technically excellent and highly consistent parts. However, the quality of the finished dome depends heavily on the formulation and control of the manufacturing process.

Relevant factors include:

• The source and grade of the polymer granules
• The proportion of virgin material and any recycled content
• The type and quantity of UV stabilisers
• Any flame-retardant additives
• Correct drying of the raw material
• Processing temperature
• Residence time in the machine
• Quality control for each production batch

It is not normally possible to identify the exact raw-material blend simply by examining the finished dome. Without full traceability, the buyer must rely on the manufacturer’s formulation, declarations and batch-control procedures.

Why we prefer vacuum forming

For plastic collector domes, we deliberately favour vacuum forming. It enables us to define and verify the sheet quality before production. We know where the material comes from, which technical specification applies and what UV or fire-performance properties have been specified.

This gives us greater control over the origin, consistency and expected service life of the finished product.

This does not mean that an injection-moulded dome is automatically inferior. A manufacturer using certified raw materials, a controlled formulation and strict batch testing can produce an excellent injection-moulded component.

Our preference is based on traceability: with a vacuum-formed dome, the quality of the original sheet can be directly identified and documented.

Polycarbonate and acrylic domes that have become yellow, cloudy or opaque can still be found on existing buildings. This is not determined solely by the name of the plastic.

Long-term clarity depends on:

• The purity and quality of the raw material
• The type and quantity of UV stabilisers
• Any co-extruded UV layer or protective coating
• Material thickness and consistency
• The level of sunlight and heat exposure
• Manufacturing quality
• Air pollution and environmental conditions
• The use of suitable cleaning products
• Ongoing maintenance

Two domes both described as “polycarbonate” or “acrylic” may therefore age very differently.

A low-cost material containing minimal UV protection cannot be fairly compared with a high-quality, fully traceable grade specifically d

Our choice: polycarbonate and glass

We do not consider one material to be the best solution for every application. That is why we choose to work with both polycarbonate and glass.

Polycarbonate provides a strong combination of low weight, design freedom and exceptional impact resistance. When manufactured from a high-quality UV-stabilised and, where required, flame-retardant grade, it is a safe and durable option for many roof applications.

Glass offers permanent optical clarity, a hard surface, high scratch resistance and natural protection against UV yellowing. As a base material, it is also non-combustible. This makes it particularly suitable where long-term optical stability and fire performance are priorities.

The right choice depends on the building, the roof construction, the external environment and the technical requirements of the project.

Look beyond the name and shape of the dome

Whatever roof dome is selected, it is important to look beyond the material name, an unusual dome shape or a general marketing description.

A high-quality dome begins with a carefully selected and verifiable raw material. UV protection, material thickness, fire behaviour, manufacturing method and quality control all influence long-term performance.

A high-performance tubular daylight system is created only when the roof dome is combined with effective optical light redirection, a highly reflective tube and a carefully designed route.

The dome admits the daylight. The quality and safety of the complete system determine how effectively that daylight reaches the room.