Hail Took Out the Glass Roof and Polycarbonate Was the Answer

When hail took out the glass roof and polycarbonate was the answer, the replacement basis was not a single laboratory value but a combination of impact energy absorption, thermal movement compatibility, weathering retention, and code compliance. Bisphenol A polycarbonate (PC) is an amorphous thermoplastic with a glass transition temperature near 147 °C and a tensile response that yields under localized overload rather than propagating a fracture. Clear 6 mm solid PC sheet transmits 88% to 90% total luminous transmittance per ISO 13468-1:2019, comparable to glass in a roof aperture. Density is 1.20 g/cm³ under ISO 1183-1:2019, versus 2.5 g/cm³ for soda-lime glass, reducing mass on existing purlins and mullions. The coefficient of linear thermal expansion is 65 × 10⁻⁶ m/m/°C per ISO 11359-2:2021, approximately 7.6 times the thermal movement of glass. That single property forces the retrofit to be treated as a moving system rather than a fixed lite.

Impact qualification for hail exposure is kinetic-energy driven. A 50 mm ice sphere at 30 m/s carries approximately 27 J; a 25 mm sphere at 20 m/s carries approximately 1.5 J. There is no single ISO standard that assigns a universal hail rating to polycarbonate glazing in every jurisdiction. Project specifications may therefore combine FM 4473 freezer-ice-ball testing, ASTM E822 for collector covers, and projectile-resistance testing under UL 752:2015 when security glazing is also required. Polycarbonate absorbs this input through yielding. Notched Izod impact strength under ISO 180:2023 Method A for general-purpose sheet is typically 12 kJ/m² to 35 kJ/m² at 23 °C. The brittle-to-ductile transition of standard bisphenol A PC lies below -20 °C, so cold-climate hail impacts do not automatically convert the sheet to a brittle failure mode. Published data for a specific polycarbonate grade against a specific hail-size distribution is limited; instrumented puncture testing per ISO 6603-2:2023 at 23 °C and -30 °C is therefore used to compare deformation energy, maximum force, and failure mode instead of relying on a single Izod number.

Why Does a 9.4 mm Thermal Growth on a 2.4 m Panel Govern Fastener Placement?

A 2.4 m polycarbonate roof panel subjected to a 60 °C temperature differential expands by 9.4 mm. Direct substitution of glass dimensions creates a fixed-hole bearing problem; field inspections after roof replacement have found radial cracks starting at fastener holes before any center-of-pane puncture occurred. A fully restrained panel would develop a compressive stress of approximately 9 MPa, based on an elastic modulus of 2.3 GPa, and repeated cycles can initiate hole-edge grazing even when the stress is below the yield point. The installed detail must permit movement. Hole diameters should be at least 1.5 times the fastener shank diameter, and stainless steel screws with EPDM or neutral-cure silicone washers of 18 mm to 25 mm diameter are used. Edge engagement should be 12 mm to 20 mm for 6 mm solid sheet, with expansion joints at panel ends and sliding clips. The ISO 11359-2:2021 coefficient is not an installation code, but the movement equation derived from it is the controlling arithmetic. A perimeter gap below 5 mm per meter of panel length can produce mid-span buckling in dark-colored frames during summer service.

Exposed polycarbonate roof sheets require a coextruded UV-absorber cap layer on the weather side. Unmodified polycarbonate yellows and microcracks under solar ultraviolet below 400 nm. Accelerated xenon-arc weathering per ISO 4892-2:2013 cycle 1 at 0.51 W/m² at 340 nm for 4,000 h typically yields a yellowness index increase of 1.5 to 3.0 for sheets with a 50 µm cap layer, compared with an increase above 10 YI units for unprotected sheet under the same exposure. The cap layer is not an applied coating; it is a polycarbonate melt stream coextruded into the sheet at the die lip. This prevents delamination because cap and core share the same polymer backbone. ISO 11963:2019 defines minimum requirements for solid polycarbonate sheet in building applications and should be cited in procurement specifications. Test reports for the specific thickness and cap configuration remain necessary because weathering retention is grade-dependent.

When the roof aperture is also part of the building thermal envelope, the replacement must specify multiwall or solid sheet based on U-value and solar transmittance. Solid 6 mm clear PC sheet has a thermal transmittance in the range of 3.5 W/m²K to 3.8 W/m²K under EN 673:2011, while a 16 mm three-wall or five-wall PC sheet is published in the range of 2.0 W/m²K to 2.5 W/m²K. Solar transmittance for clear PC is approximately 85% to 88% per ISO 9050:2003, but tinted or opal multiwall products reduce solar heat gain coefficient to 0.40 to 0.65. The hail-resistant cover therefore cannot be selected only for impact; the same sheet must satisfy the building's solar gain and condensation-resistance boundary conditions. Framing intersections must include thermally broken or insulated glazing bars where roof condensation is controlled by occupancy and ventilation rates.

Euroclass B-s1,d0 Is Not an Hourly Fire Rating

Some solid PC sheets achieve a reaction-to-fire classification of B-s1,d0 under EN 13501-1:2018, but this does not confer fire resistance or an hourly rating. Polycarbonate is thermoplastic. Vicat B50 softening temperature under ISO 306:2022 is typically 145 °C to 150 °C, and heat deflection temperature at 1.82 MPa per ISO 75-2:2013 is 125 °C to 135 °C. In a developing fire, the sheet can sag, pull out of edge pockets, and create openings. Molten droplets from ordinary grades are a hazard in overhead installations. Building authorities may limit the use of thermoplastic roof glazing, require sprinkler protection, or mandate drip-control details. ASTM E84 Class A results for thicker multiwall PC are surface flame-spread and smoke-developed values, not fire endurance. UL 94 V-2 or V-0 classifications are common for electrical and lighting enclosures but are not substitutes for roof assembly performance. The fire-code path must be resolved before hail resistance is treated as sufficient.

When Plasticized PVC Flashings Introduce Stress Cracking, Not Support

Field installations on metal roofs have recorded environmental stress cracking at bearing points where flexible PVC edge trims contacted polycarbonate. Plasticizer migration into the sheet lowers local ductility; a subsequent hail strike at an ambient temperature below 10 °C can initiate radial cracks from the contact line. Chemical resistance testing per ASTM D543-20 documents incompatibility with aromatic hydrocarbons, ketones, esters, and amine-bearing sealants. Installation specifications must replace PVC flashings with EPDM, TPE, or neutral-cure silicone. Cleaners containing glycol ethers, ammonia, benzyl alcohol, or abrasive particles are incompatible. Maintenance should use mild soap solution or aliphatic hydrocarbon–based products, followed by rinsing with clean water. The operational boundary is chemical as well as thermal: polycarbonate's impact toughness can be erased by environmental stress cracking before the first hail event.

Where the original glass roof was curved, replacement also requires forming. Thermoforming of solid PC is performed at sheet surface temperatures of 150 °C to 170 °C; below this window, part webbing increases, and above 190 °C, degradation may occur. Oven temperature uniformity of ±3 °C is required to prevent localized thinning. Cold bending for very large radii is possible, but published minimum-radius data for a specific sheet grade is limited and should be obtained from the supplier.

To obtain the required sheet performance, extrusion lines use a 32:1 L/D or 36:1 L/D vacuum-vented single-screw extruder with a precision melt pump, coat-hanger die with restrictor bar, and three-roll polishing stack. Polycarbonate must be dried at 120 °C for 4 h to a residual moisture content below 0.02%. A desiccant-bed hopper dryer with an outlet dew point below -40 °C is standard. Melt temperature is maintained at 280 °C to 310 °C for sheet extrusion, with roll stack temperatures at 80 °C to 120 °C. At higher melt temperatures, degradation reduces melt strength and increases sheet haze. At lower temperatures, residual stress in the sheet increases susceptibility to solvent stress cracking. Melt flow rate measured at 300 °C/1.2 kg per ISO 1133-1:2022 is typically 6 g/10 min to 10 g/10 min for sheet grades. Injection-molded glazing frames and connector blocks operate in a separate window: 300 °C to 320 °C melt temperature and 80 °C to 120 °C mold temperature. Production records show that melt-flow drift outside the sheet-grade specification correlates with inconsistent thermoformed edge radii and higher in-service cracking around mechanical fasteners.