Greenhouses built from salvaged glass, including recycled window panes, face a persistent structural threat during the summer hail season. Flat glass offers good light transmission but fractures easily under repeated high-energy impacts, exposing crops and driving up repair costs.

Growers using such structures are advised to combine physical shielding with routine inspection of frames and seals before storm activity intensifies.

Glass is favoured in greenhouse construction for its clarity and durability under normal conditions, but it performs poorly against concentrated impact loads. A single significant hailstorm can crack or shatter multiple panes, leaving the crop canopy unprotected and creating a secondary hazard from falling fragments.

The risk is compounded in structures assembled from older window units, where pane thickness and frame condition may already be inconsistent. Once a pane fails, wind entering the enclosure can destabilise adjacent panels.

The most effective countermeasure is a physical shield installed above the glazing before the hail season begins. High-impact polycarbonate sheeting or purpose-made greenhouse netting absorbs and disperses the kinetic energy of hailstones, reducing the load transmitted to the glass beneath.

Such coverings also retain broken glass, limiting the risk of injury to people working inside and of contamination in the growing area. Where glazing has already been damaged, the covering layer should be treated as a temporary measure pending replacement.

Loose or distorted window frames are more likely to fail when strong wind accompanies hail, because movement concentrates stress at the edges of each pane. Installers should leave an adequate expansion gap between glass and frame and use a high-quality sealant to improve the assembly's resistance to shock and vibration.

Regular inspection of the roof assembly is as important as any added covering. Growers should look for hairline cracks, corrosion in metal frames and sealant that has hardened, detached or weathered away, since each of these conditions points to a loss of structural reliability.

Where damage is widespread, replacing the roof material ahead of the hail season is generally more economical than repairing the structure after a failure, which can also involve crop losses and disruption to planting schedules.