GRP (glass-reinforced polyester / fibreglass) has long been a default choice for domestic flat roofing. Silicone is a newer chemistry competing for the same jobs. This is an honest, side-by-side look at when each wins, aimed at specifiers and contractors working across Ireland and Northern Ireland.
GRP in one paragraph
GRP is a cold-applied composite: glass-fibre matting laid onto a timber deck, saturated with polyester resin, topped with a pigmented topcoat. It cures to a rigid, inert, walkable surface and carries 20-year warranties across most major suppliers (Permaroof, Cromar, Hambleside Danelaw). It’s a thoroughly proven chemistry.
Silicone in one paragraph
Silicone roof coating is a liquid elastomer applied over almost any existing substrate — including over existing GRP — and cures to a seamless, flexible, hydrophobic membrane. It’s routinely used for refurbishing failing GRP, overcoating EPDM, and sealing profiled metal and asbestos cement.
Side-by-side
| Metric | GRP | Silicone (GacoFlex S42) |
|---|---|---|
| Substrate required | New timber deck only | Almost any existing substrate |
| Flexibility | Rigid — vulnerable to deck movement | Elastomeric — flexes with substrate |
| UV stability | Topcoat can chalk over 10-15 yrs | Stable across full service life |
| Ponding | Tolerates | Rated unlimited |
| Repair | Patch possible but visible | Simple recoat, invisible |
| Recoat at end of life | Full strip often needed | Direct recoat, no strip |
| Walkability | Excellent — hard surface | Limited — for maintenance access only |
| Installation conditions | Dry, >5°C, no frost | Tolerates wider window |
| Typical warranty | 20 yrs (system) | 20 yrs (premium tier) |
| Best for | New-build domestic flat roofs, walkable decks | Commercial/industrial refurb, profiled roofs, large areas |
When GRP is right
- New-build or full strip-and-replace on a timber deck
- Walkable deck required (balconies, roof terraces)
- Small domestic flat roofs under 100 m²
- When a hard, chip-resistant finish is critical
When silicone is right
- Refurbishment of any existing roof without strip-and-replace
- Large commercial or industrial roofs (warehouses, factories, schools)
- Profiled metal, asbestos cement, EPDM, single-ply substrates
- Where ponding water is a known issue
- When programme, cost and disruption matter (single-coat application) — and GRP’s dry, frost-free installation window is hard to guarantee in a wet Irish winter
The overlap: refurbishing an existing GRP roof
One of the most common silicone applications is overcoating weathered GRP. After 15–20 years, GRP topcoats typically chalk and become porous. Rather than strip and relay the full system, a silicone overcoat restores weatherproofing for another 15–20 years. No strip, no disposal, no disruption. GacoFlex S42 bonds directly to cleaned, sound GRP with no primer.
Got a failing GRP roof? Silicone overcoat details here or email info@duratite.ie.