What is Perspex?

Perspex is a brand name for sheet material made of polymethyl methacrylate (PMMA), commonly called acrylic — or, under a different brand name, Plexiglas. It is a crystal-clear, rigid thermoplastic used in construction wherever you want to let light through without the weight and brittleness of glass.

Properties

• Light transmission up to about 92 per cent — comparable to clear float glass or better.
• Light. Density is around 1.2 g/cm³, less than half that of glass. That matters for large rooflights and for anyone installing single-handed.
• Impact resistant. An acrylic sheet takes far more impact than glass of the same thickness and does not shatter into shards.
• Weather resistant. PMMA is inherently UV stable and does not yellow; polycarbonate needs a coating to achieve this.
• Scratch prone. This is the weak point: acrylic scratches more easily than glass. Fine scratches can be polished out.
• High thermal movement. The coefficient of expansion is roughly 0.07 mm per metre per degree Celsius, some seven to eight times that of glass.
• Limited heat resistance. Depending on the grade, the service temperature sits around 65 to 80 °C; above that the sheet deforms.

Cast or extruded

• Cast (GS) — better to machine, more chemically resistant, fewer stress cracks when routing and drilling. More expensive, with a wider thickness tolerance.
• Extruded (XT) — cheaper and more accurate in thickness, but more prone to stress cracking when bonded or machined.

Applications

Rooflights and light domes, small roof windows in outbuildings, wind screens, balustrade infills, machine guards and glazing in greenhouses and animal housing.

Working tips

• Leave the protective film on until the job is finished; most scratches happen during installation.
• Cut with a fine, unset tooth at high speed, or use a circular saw blade made for plastics. Clamp the sheet firmly so it cannot chatter.
• Always drill oversized holes. Allow roughly 3 mm of clearance per metre of sheet: a 40-degree temperature swing already moves a metre of acrylic by nearly 3 mm. Never screw down tight — use a washer and let the sheet move.
• No sharp internal corners in cut-outs; drill the corners round first, or the sheet will crack from the corner.
• Clean with soap and water, never with methylated spirit, acetone or alcohol-based glass cleaner: these cause stress cracking.

Point to watch

Unlike glass, acrylic is combustible. Where reaction-to-fire requirements apply — escape routes, fire compartments, facades of taller buildings — Perspex is not an automatic choice. Always check the product’s declared fire classification.

Related terms

• Acrylate
• Polycarbonate multiwall sheet
• Light dome
• Rooflight
• Sheet material

Want to learn more about construction terms? Visit our knowledge base at fredsdiyplans.com.