Sky Light Cover: Materials, Sizes, and Replacement Options

A damaged or yellowed sky light cover is one of those problems that's easy to ignore until it isn't. Water starts getting in, the diffused light turns patchy, or the plastic cracks along an edge and suddenly you're looking at a repair job that can't wait. The good news is that replacing a sky light cover is straightforward when you know what material to specify and how to get it cut to the right size.

This guide covers the most common materials used for sky light covers, how to measure and order a replacement, and what to expect from the fabrication process.

Why Plastic Is the Standard Choice for Sky Light Covers

Glass was once common in skylights, but plastic has largely replaced it in residential and commercial applications. The reasons are practical: plastic is lighter, easier to handle at height, less likely to shatter, and far simpler to cut to a custom size. It also performs well under UV exposure when you choose the right grade.

The two plastics you'll encounter most often in sky light applications are acrylic and polycarbonate. Both are clear, lightweight, and available in a range of thicknesses — but they behave quite differently in fabrication and in service.

Acrylic vs Polycarbonate: Which Is Right for Your Skylight?

Acrylic is the most popular material for flat sky light covers. It's optically clear, holds its colour well over time, and is available in clear, opal (white diffusing), and tinted versions. UV-stabilised acrylic won't yellow the way cheaper plastics do, and it's easy to cut cleanly with a CNC router or laser. The main limitation is impact resistance — acrylic is stiffer and more brittle than polycarbonate, so it can crack if struck hard or if it's installed under significant mechanical stress.

Our acrylic and Perspex sheet range includes both clear and opal grades that are well suited to skylight replacement. Opal acrylic is particularly useful where you want to diffuse direct sunlight rather than transmit it as a hard beam.

Polycarbonate is the better choice where impact resistance matters — think industrial buildings, workshops, or any location where debris could land on the skylight. It's virtually unbreakable at equivalent thicknesses and handles thermal expansion better than acrylic. The trade-off is that polycarbonate scratches more easily and can yellow over time without a proper UV coating. For most residential sky light cover replacements, acrylic is the more practical and cost-effective option.

If you're unsure which to specify, our polycarbonate sheet page has a detailed breakdown of grades and thicknesses.

Thickness: What to Specify for a Sky Light Cover

Thickness affects both rigidity and load-bearing capacity. For a flat sky light cover sitting in a rebated frame, the span of the opening is the key variable. Here's a general guide:

Spans up to 400mm: 3mm acrylic is usually adequate for a lightly loaded flat panel.

Spans 400–800mm: 4–5mm acrylic or 3–4mm polycarbonate gives better stiffness and reduces the risk of deflection under load.

Spans over 800mm: Step up to 6mm or thicker. Large unsupported panels need the extra rigidity, particularly in areas with wind load or where people might lean on the frame.

If you're replacing a domed or curved sky light cover, the geometry itself adds structural rigidity, so you may be able to use a thinner sheet than a flat panel of the same span would require. Curved covers are typically vacuum-formed or heat-formed — a process we can handle in-house.

How to Measure for a Replacement Sky Light Cover

Accurate measurement is the most important step. A panel that's 5mm too wide won't fit; one that's 5mm too narrow will rattle or let water in. Here's how to measure correctly:

1. Measure the rebate, not the opening. The rebate is the recessed ledge the panel sits in. Measure its internal dimensions — that's the size your replacement panel needs to be. If there's no rebate and the panel simply lays over the frame, measure the frame's outer edge and subtract the overlap you want on each side.

2. Check for square. Older buildings especially can have frames that are out of square. Measure both diagonals — if they differ by more than a few millimetres, you'll need to supply a parallelogram cut rather than a true rectangle.

3. Note any cutouts or holes. Some sky light covers have ventilation holes or fixing points. These can be machined in during fabrication — just include the dimensions and positions in your brief.

Once you have your measurements, our cut to size plastics service can produce a replacement panel to your exact dimensions — no minimum order, no need to buy a full sheet.

Clear, Opal, or Tinted: Choosing the Right Finish

The finish you choose affects both light transmission and the character of the light entering the space below.

Clear acrylic transmits around 92% of visible light — more than glass. It's the right choice when you want maximum brightness and don't mind direct sunlight entering the space.

Opal acrylic diffuses light evenly across the panel, eliminating hot spots and harsh shadows. It's the standard choice for commercial ceilings and is often used to replace prismatic diffuser panels in fluorescent light fittings — though the two serve slightly different purposes.

Tinted acrylic reduces glare and solar heat gain. Bronze and grey tints are common in warmer climates where cutting down on heat transmission is a priority. If you're on the Gold Coast or in South East Queensland, a tinted sky light cover can make a real difference to comfort in summer.

Fabrication: What Happens After You Order

For flat sky light covers, the fabrication process is simple. Sheet material is cut to your specified dimensions using a CNC router or saw, edges are finished, and any holes or cutouts are machined in. Turnaround is typically fast — most standard flat panels can be ready within a few business days.

For domed or curved sky light covers, the process involves heat-forming the sheet over a mould. This is more involved but still well within standard fabrication capability. Our acrylic bending and forming service handles both simple curves and more complex dome profiles. If you're replacing a domed cover, bring the old panel in if you can — it makes matching the profile much easier.

Protective masking is left on the sheet until installation to prevent scratching during handling and transport. Peel it off once the panel is in place.

Installation Tips for a Long-Lasting Result

Plastic expands and contracts with temperature changes more than metal frames do. When fixing a sky light cover into a rigid frame, leave a small gap — typically 2–3mm per metre of panel length — to allow for thermal movement. Use flexible sealant rather than rigid adhesive where the panel meets the frame, and avoid over-tightening any mechanical fixings.

Clean acrylic with mild soapy water and a soft cloth. Avoid ammonia-based cleaners (including many glass cleaners) — they can craze the surface over time. A plastic-specific polish can restore clarity to a lightly scratched panel without needing full replacement.

Need a sky light cover cut to size? P&M Plastics fabricates replacement panels in acrylic and polycarbonate from our Gold Coast workshop. Send us your dimensions and we'll get a quote back to you quickly — contact us today to get started.