A Guide to Multiwall Roofing Sheets: Benefits, Installation and Maintenance

15th Apr 2024 -

Multiwall roofing sheets present a range of benefits that make them an appealing choice. These roofing panels offer a practical and visually appealing solution for various applications.

In this comprehensive guide, we explore these benefits in detail. Also, we provide insights into their installation process and maintenance tips to ensure optimal performance.

What is a multiwall roof sheet?

Also known as polycarbonate roofing sheets or plastic roof sheets, multiwall roofing sheets are a translucent roofing material. They are composed of 2 or more layers of polycarbonate with an air gap in between. 

The multi-layered design provides exceptional thermal insulation. It blocks out excessive heat from the sun while still allowing natural light to pass through. 

9 benefits of multiwall roofing sheets

Multiwall polycarbonate roof sheets have emerged as a superior glazing solution, offering an array of benefits. Their unique properties make them an exceptional choice for various residential and commercial applications. 

1. Excellent thermal insulation

The multi-layered design with air pockets between polycarbonate layers creates an effective thermal barrier. Multiwall sheets can achieve U-values (thermal transmittance) as low as 0.8 watts per square metre and Kelvin (W/m2K). This is comparable to double-glazed windows. The high insulation value helps reduce heat transfer by up to 75% compared to single-layer roofing.

2. Natural light transmission

Depending on the thickness and number of layers, multiwall sheets can transmit between 35-80% of natural light. This diffused light creates an even, glare-free illumination without hot spots. It reduces the need for artificial lighting during the daytime.

3. Durability and impact resistance

Polycarbonate is 200 times more impact-resistant than glass and can withstand hail stones up to 25mm in diameter falling at terminal velocity. The structured sheets are also resistant to high winds up to 130 mph and can span distances up to 7 metres unsupported.

4. Low maintenance

The UV-protected outer layer of multiwall sheets prevents yellowing and brittleness caused by UV exposure. They are also resistant to most corrosive substances and will not degrade from exposure to moisture or condensation.

5. Strong and lightweight

Multiwall sheets weigh around 1.5kg/m2 for 10mm thick 4-wall sheets, about one-quarter the weight of glass. Yet they can support distributed loads up to 1.2 kilonewtons per square metre (kN/m2). This makes them structurally robust despite their lightweight nature.

6. Design versatility

Coloured tints, patterns and speciality finishes like prismatic, opaque or embossed textures allow for customisation. Sheets can be cold-curved to tight radiuses down to 900mm depending on thickness.

7. Energy efficiency

The insulating air chambers combined with light diffusing properties reduce reliance on artificial lighting and HVAC. This lowers energy usage by up to 40% compared to traditional glazing materials.

8. Fire resistance

Polycarbonate multiwall roof sheet materials have a Class B fire rating, resisting the spread of flames and producing very little smoke if ignited. Some fire-rated sheet options are even Class A-rated.

9. Installation and lifespan

Pre-formed lengths up to 12m and simple tongue-and-groove joinery allow fast modular installation with minimal equipment or labour. With proper installation and cleaning, multiwall polycarbonate roofing is expected to last 10-20 years before replacement is needed.

What are multiwall roof sheets used for?

Multiwall roofing sheets are commonly used in the following applications. Different colour options are suitable for various purposes. 

Residential and commercial roofing

• Commercial and industrial buildings: Clear or opaque multiwall sheets are used as roofing materials for warehouses, factories, retail spaces and other commercial structures. 
• Greenhouses, agricultural and shed structures: Translucent or tinted sheets in shades like green or red can help filter specific wavelengths of light. This promotes plant growth in greenhouses and agricultural structures.

Skylights and daylighting solutions

• Residential spaces: Translucent or tinted multiwall sheets are often used as skylight or daylighting systems in home settings such as a conservatory. Options like bronze or grey tints help to reduce glare and heat gain.
• Atria and covered walkways: Lightly tinted or clear polycarbonate roofing sheet options can be used to create translucent roofs or canopies. This allows natural light while offering some shade.

Outdoor living spaces

• Patios, decks and outdoor living areas: Tinted or coloured multiwall sheets in shades like bronze, grey or blue can provide shade and protection. These colours allow diffused natural light and enhance the aesthetic appeal.
• Storefronts and entryways: Translucent or lightly tinted canopies made from multiwall sheets create an inviting and well-lit environment for customers.

Partition walls and dividers

• Office spaces and commercial interiors: Multiwall translucent or clear plastic roofing sheets are often used as partition walls or dividers. They allow natural light to penetrate while offering privacy.
• Retail displays and kiosks: Coloured or patterned sheets add visual interest and boost branding for retail displays and kiosks.

Shelters and enclosures

• Bus stops and outdoor shelters: Tinted or opaque multiwall sheets protect from the elements. They reduce glare and heat gain in outdoor shelters and waiting areas.
• Sports facilities and outdoor structures: Clear or translucent sheets can be used to create naturally lit indoor courts or outdoor covered areas.

Multiwall vs twinwall roofing sheets: which is best?

Multiwall and twinwall roofing sheets are both types of polycarbonate panels. However, they differ in their construction and properties. Here are the 5 key differences between the 2 sheet types to consider.

1. Number of walls

The number of walls is a key difference. Multiwall sheets typically have 3 to 16 parallel walls or layers. Common configurations are 3-wall, 4-wall, 6-wall, and 10-wall. In contrast, twinwall sheets have only two parallel walls separated by a single air cavity.

2. Insulation properties

Insulation properties vary significantly. Multiwall sheets have superior insulation due to the multiple air cavities acting as thermal breaks. For example, a 10-wall sheet can provide an insulation value (U-value) as low as 0.8 W/m²K. However, twinwall sheets have a single air cavity, resulting in lower insulation performance, typically with U-values around 2.0-2.8 W/m²K.

3. Light transmission

Light transmission capabilities also differ between the two types. Multiwall sheets achieve light transmission levels of up to 80% for clear panels, allowing ample natural light into a building. Twinwall sheets have lower light transmission around 60-70% for clear panels due to their simpler two-wall construction.

4. Strength and load-bearing capacity

In terms of strength and load-bearing capacity, multiwall sheets can span longer distances and support higher loads because of their multi-layered structure. A 10-wall sheet can span up to 7 metres. Twinwall sheets have lower load-bearing capacity, typically limited to spans of 2-3 metres for most applications.

5. Thickness and weight

The thickness and weight of the sheets also vary. Multiwall sheets are thicker, ranging from 10mm to 40mm or more. Twinwall sheets are thinner, typically around 6-10mm. This makes them lighter and easier to handle for certain installations.

How does multiwall differ from corrugated roofing sheets?

While multiwall and corrugated roofing sheets serve similar purposes, they have distinct characteristics that make them better suited for different applications. The following table provides a comprehensive comparison of these 2 roofing systems.

Characteristic	Multiwall	Corrugated
Material composition	Multiple layers of polycarbonate sheets for roofing.	Fibre cement, bitumen, polycarbonate and metal (steel or aluminium roofing sheet).
Light transmission	35-80% natural light transmission.	Opaque with no light transmission.
Thermal insulation	Insulated roof sheets with excellent U-values as low as 0.8 W/m²K.	Low insulation properties.  May require additional insulation.
Impact resistance	Strong and can withstand hail and high wind speeds.	Metal roofing sheets are prone to denting; fibre cement can crack.
Weight	Lightweight at around 1.5 kg/m² for 10mm 4-wall.	Heavier at 4-6 kg/m² for steel; 2-3 kg/m² for aluminium.
Durability and maintenance	Highly durable and resistant to UV, corrosion and chemicals. Low maintenance.	Metal and steel roof sheets can corrode; fibre cement may crack. Regular maintenance is required.
Design versatility	Wide range of colours, tints and patterns. Can be cold-curved.	Limited design options but can be coated or painted.
Fire resistance	Multiwall can achieve a Class A fire rating and low smoke emission.	They can be made fire-resistant, but generally lower performance.

How to install multiwall roofing sheets

Installing multiwall roofing sheets requires careful planning and execution to ensure optimal performance. This section will guide you through the installation process, covering essential steps, best practices and important factors to think about.

1. Surface preparation

Start by ensuring the roof structure is level, clean and free from debris or protrusions that could damage the sheets. Apply a compatible sealing tape or gasket along the purlins or rafters to provide a waterproof seal.

2. Layout and overlapping

Lay the multiwall sheets with an overlap of at least 200mm between adjacent sheets to create weathertight joints. Plan the layout carefully, considering the direction of the primary rain exposure to ensure proper water runoff.

3. Cutting and trimming

Use specialised saws or shears designed for cutting polycarbonate to trim the sheets to size as needed. Avoid using traditional woodworking tools, which can cause cracking or chipping. Always cut the sheets oversize and trim them once in place.

4. Fastening and fixings

Multiwall sheets should be secured using compatible fasteners and fixings specifically designed for polycarbonate roofing. These include rubber-grommeted self-drilling screws, aluminium load-bearing bars and sliding aluminium glazing bars. 

Fasteners should be installed following the manufacturer's recommended spacing and patterns to ensure proper load distribution and prevent leaks.

5. Sealants and closures

Apply compatible sealants and gaskets around the perimeter of the sheets, joints and around any penetrations. These sealants should be flexible, UV-resistant and designed for use with polycarbonate to ensure long-term weather tightness.

6. Thermal expansion and contraction

Polycarbonate sheets are subject to thermal expansion and contraction due to temperature changes. It is important to leave adequate clearance between the sheets and any fixed structures or frames to accommodate this movement. Failure to do so will likely result in buckling, cracking or damage to the sheets.

7. Roof pitch and drainage 

Multiwall sheets should be installed at a minimum pitch of 5 degrees to facilitate proper water runoff and prevent pooling or standing water on the surface. Ensure that the roof design includes drainage paths and channels to direct water away from the structure.

8. Ventilation and condensation management

Proper ventilation is essential to prevent condensation buildup between the layers of multiwall sheets. Follow the manufacturer's guidelines for installing vents, ridge caps or other ventilation systems. This promotes air circulation and minimises condensation issues.

How much maintenance does a multiwall roofing sheet need?

Multiwall roofing sheets are known for their low-maintenance properties compared to other roofing materials. However, proper maintenance is still necessary. Below are some key maintenance tips to follow.

Cleaning

• Regular cleaning prevents dirt, debris and contaminant buildup.
• Clean at least once a year and more often in high pollution and dust areas.
• Use a soft-bristle brush or low-pressure water hose for gentle cleaning.
• Avoid abrasive scrubbing or high-pressure cleaning to prevent surface damage.
• Mild, pH-neutral detergents or plastic-safe solutions can be used for stains.
• Thoroughly rinse sheets after cleaning to remove any residual cleaning agents.

Inspection and sealant maintenance

• Periodic inspections (at least yearly and after severe weather) for cracks, scratches, as well as sealant or gasket deterioration.
• Promptly repair any damage and reseal using manufacturer-recommended sealants or gaskets.
• Proper sealing prevents water infiltration, leaks, moisture buildup and structural damage.

Surface protection

• Apply compatible surface protectant or polish every 1-2 years.
• This prevents scratches, minimises UV radiation effects and enhances durability.
• Be sure to follow the manufacturer's instructions for product application.

Snow and ice removal

• Remove accumulated snow and ice to prevent excessive weight loads.
• Use soft-bristle brushes or plastic shovels for snow removal.
• Deicing solutions or heat cables may be needed for melting ice buildup.
• Avoid sharp tools or excessive force to prevent sheet damage.