Interior and finishes · Glass & Glazing
Polycarbonate Glazing Sheet (Multiwall and Solid)
Explains that this is a plastic rather than a glass, describes the difference between solid and multiwall forms, and sets out the movement, scratching and ultraviolet questions it raises.
Educational reference entry. Suitability depends on the complete assembly, substrate, exposure, installation method, manufacturer documentation, local requirements and qualified professional review.
Overview
What polycarbonate sheet is
Polycarbonate glazing sheet is not glass at all but a thermoplastic, supplied either as solid flat sheet or as multiwall sheet in which two or more skins are connected by internal ribs forming continuous flutes along the length. Those flutes are what give multiwall sheet its stiffness and its light weight relative to a glazed panel covering the same area.
Its distinguishing properties are toughness and lightness. Polycarbonate absorbs impact by deforming rather than by fracturing, and it is much lighter than glass covering the same opening, which changes what the supporting structure and the fixings have to do. Against that, the surface is far softer than glass: it scratches from grit, brushes and unsuitable cleaning products, and those scratches are permanent in normal use.
Two behaviours dominate the detailing. Polycarbonate expands and contracts with temperature considerably more than glass, so glazing bars, fixing holes and end closures have to let the sheet move rather than pinch it. And the polymer degrades under ultraviolet light unless the weather face carries a protective co-extruded layer, which means the sheet has a correct way up and a correct face outwards; even so, gradual discolouration over long exposure is a known characteristic to discuss rather than a defect to be surprised by.
Terminology
Common names and aliases
These names describe the same thing. The library keeps one entry per material so that an acronym, a trade name or a regional spelling never becomes a second, thinner page.
- Polycarbonate sheet
- Polycarbonate glazing
- Multiwall polycarbonate
- Twinwall sheet
Applications
Common applications
Where this material is typically encountered. A typical application is not a statement that it suits your project.
- Roofing to canopies, carports, covered walkways and lean-to structures where light weight matters.
- Glazed roofs of conservatories and garden rooms, particularly where an existing structure was designed for it.
- Greenhouse and cold frame glazing, where impact tolerance and weight are the driving considerations.
- Secondary glazing panels and internal screens where a lightweight sheet is preferred to a glass pane.
- Machine guards, screens and workshop partitions where knocks are expected as part of normal use.
- Infill panels in balustrade and screen frames where a designer has specified a sheet material.
Consideration
Where it is commonly considered
- Where weight is the governing constraint and an existing structure was built around a lightweight covering.
- Where impact from hail, branches, balls or equipment is a realistic part of everyday use.
- Where a diffused rather than clear light is acceptable, as multiwall sheet is not optically clear.
- Where a large covered area is being formed and the sheet can run in long lengths between purlins.
- Where an existing polycarbonate roof is being replaced and the framing already suits the system.
Professional review
Where additional professional review is especially important
These are the situations where the answer depends on the specific building, and where a qualified professional should be involved before anything is decided.
- Where an optically clear, distortion-free view is wanted, since multiwall flutes are always visible.
- Where the sheet will be walked on or leaned against, since it is not a walkable surface without a designed system.
- Where surfaces are cleaned with solvents or ammonia-based products, which can attack the polymer.
- Where sealants and gaskets have not been checked for compatibility, since some materials attack polycarbonate.
- Where local requirements may govern what can be used in the position, which a qualified professional must determine.
Properties
Key properties to discuss
Qualitative topics worth raising. Measured values, classes and grades come from the manufacturer's technical documentation for the specific product, not from a reference page.
- Surface wear
- The surface is much softer than glass and marks from grit, stiff brushes and abrasive cleaners. Scratches accumulate over time on any face that is cleaned regularly, and they cannot be polished out in normal use.
- Weathering and exposure
- Ultraviolet light degrades the polymer unless the exposed face carries a co-extruded protective layer, so the sheet has a correct orientation. Gradual discolouration over long exposure is a known characteristic rather than a fault.
- Installation dependency
- Thermal movement is much greater than in glass, so fixing holes are oversized to let the sheet move and glazing bars are chosen to accommodate it. Fixings tightened hard against the sheet are the usual cause of distortion and cracking.
- Moisture behaviour
- In multiwall sheet the open ends of the flutes have to be closed and treated so that dust and water do not enter and sit inside, since internal condensation and debris within the flutes cannot be cleaned out afterwards.
- Appearance
- Multiwall sheet diffuses light and its ribs are visible from both sides, so it never reads as clear glazing. Solid sheet is far closer to clear but still lacks the optical flatness of float glass.
- Repairability
- Damaged sheets are replaced rather than repaired, and because sheets run in long lengths a local failure often means renewing a whole run rather than a small area.
- Maintenance
- Cleaning is the main ongoing question, and the safe materials are narrower than for glass: several common solvents and ammonia-based cleaners are associated with crazing or attack on the surface.
Exposure
Exposure and environmental conditions
- How much direct sun does the covering receive over the course of a year, given how the polymer ages?
- Is the sheet fixed so that it can expand and contract freely through the full range of temperatures it will see?
- Are overhanging trees likely to drop debris, sap or branches onto the surface?
- Is the covering in a position where snow or ponding water could load it unevenly?
- Will the sheet be cleaned regularly, and by whom, given how easily the surface scratches?
Assembly
Substrate and system dependencies
What this material sits on, is fixed to or depends on. This is usually where performance is won or lost.
- The supporting frame has to suit the sheet system, since bars, gaskets and closures are designed to work together.
- Fixings need to allow movement, so hole sizes and washer types are part of the system rather than incidental.
- End closures and tapes on multiwall flutes are essential parts of the assembly, not optional extras.
- Sealants and gaskets in contact with the sheet must be confirmed compatible with polycarbonate.
- Rainwater from the covering has to be collected and taken away by a designed drainage arrangement.
Appearance
Appearance and finish considerations
- Multiwall flutes are visible from both sides and cast their own pattern in the light beneath the covering.
- Tinted, opal and heat-reflective variants change the light quality under the covering quite noticeably.
- Solid sheet is much closer to clear but still shows more surface distortion than a glass pane would.
- Long-term discolouration is gradual and is most obvious where a new sheet is set beside an older one.
- Fixing lines, bars and end closures are a strong part of the finished appearance and are worth setting out deliberately.
Durability
Durability questions
Questions to raise rather than a service life to expect. No material has a universal lifespan — it depends on the assembly, the exposure and the upkeep.
- Is the protected face being installed the right way up, and how will that be checked on site?
- Do the fixings and bars allow the sheet to move without being clamped tight?
- Are the flute ends properly closed so debris and moisture cannot enter and stay there?
- Have all sealants and gaskets in contact with the sheet been confirmed compatible with the polymer?
- Is anyone likely to walk on the covering during maintenance, and what arrangements prevent that?
Upkeep
Maintenance and repair considerations
- Which cleaning materials are safe for polycarbonate, and does everyone maintaining the building know that?
- How will the surface be reached for cleaning without abrasive contact or foot traffic on the sheet?
- If one sheet is damaged, does the system allow a single run to be replaced without disturbing the rest?
- Are the flute closures being checked, since a failed closure fills the sheet with dust and water over time?
- How will a replacement sheet be matched to older sheets that have already discoloured?
Installation
Installation dependencies
What the installation depends on — not how to do it. Method belongs to the manufacturer's instructions and the trades carrying out the work.
- The protective face has to go outwards, so the labelling and orientation of every sheet matters during fitting.
- Fixing holes are oversized to allow movement, and over-tightening is the classic installation mistake.
- Flute ends need their closures and tapes fitted as the sheet is installed rather than left for later.
- Sheets must be supported at the spacings the system was designed for, which is a design matter for a professional.
- Handling long sheets in wind is awkward and needs enough people and suitable access arrangements.
Documentation
Documentation to request
Ask for these in writing, for the specific product being proposed, and keep them with the project record.
- The manufacturer's installation guidance for the specific sheet and bar system being supplied.
- Confirmation of which face carries the ultraviolet protective layer and how it is identified.
- A list of sealants, tapes and cleaning products the manufacturer states are compatible.
- Guidance on support spacing and fixing detail for the sheet being used.
- Warranty terms, particularly what is said about discolouration and about installation conditions.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- Is polycarbonate being chosen for weight, for impact tolerance, or simply because it is what was there before?
- How does the framing allow for the thermal movement of these sheets?
- Which face carries the protective layer, and how will that be verified before fixing?
- Are the sealants, tapes and gaskets confirmed compatible with the polymer by the manufacturer?
- How will the flute ends be closed, and what stops water and dust entering them?
- What does the design assume about access for cleaning, given that the sheet is not walkable?
- How will this covering look once the sheets have aged, and is that acceptable to the client?
- Has a qualified professional confirmed what local requirements mean for glazing in this position?
Blind spots
Commonly overlooked points
- Polycarbonate is a plastic and behaves like one, so carrying over expectations from glass is where most problems start.
- Over-tightened fixings are the single most common installation fault, because they stop the sheet moving as it must.
- Once dust or water gets inside multiwall flutes there is no practical way to clean it out again.
- Common household and workshop cleaners are associated with crazing on this surface, which surprises people who treat it as glass.
- The sheet has a right way up, and a run installed with the protected face inwards will age far more quickly.
What this page does not do
- Polycarbonate is not glass, and this entry makes no claim about impact behaviour, weather resistance or fire behaviour for any product.
- Support spacing, loading and fixing detail are engineering questions for a qualified professional and depend on the specific system.
- Whether glazing of any kind may be used in a given position depends on local requirements, which must be confirmed for that opening.
- Build Design Hub does not test, certify or recommend polycarbonate sheet products or glazing bar systems.
Related entries
Related materials
Materials from the same family, an adjacent role in the assembly, or a shared application.
Inspiration
Related Ideas Library pages
Design directions where this material commonly appears.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this material comes up.
Go deeper
Related Build Design Hub guides
Planning guidance and neutral comparisons behind the decisions on this page.
Glass & Glazing Materials
Reference entries for glass and glazing products — float, toughened and laminated glass, insulated glass units, low-emissivity coatings, obscured glass, glass block and polycarbonate.
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