Background and Approach
In fibreglass-reinforced polyester (GRP) flat roof construction, durability and adaptability are essential for long-term performance. Environmental extremes from the UK’s increasingly wet weather, material aging, and structural anomalies can, however, undermine established systems.
This application note describes a sophisticated remediation initiative in Kingstown, Bristol, for a commercial property with a comprehensive GRP flat roof, installed 18 years earlier.
The structure displayed localized water penetration, necessitating a thorough evaluation and customized reinstatement. Drawing on specialized knowledge of Strandek fibreglass solutions, we achieved a reliable, impermeable overlay, while resolving foundational weaknesses with limited operational interruption.
Project Overview and Preliminary Evaluation
The client engaged our services to address ongoing water ingress across a GRP roof encompassing the full building envelope. Our engagement commenced with a comprehensive condition report and targeted recommendations to reinstate functionality efficiently and economically.
The initial site examination, which included core sampling of the laminate and underlying substrate, suggested suitability for a Strandek ‘Overdeck’ application. This established method involves applying a robust GRP laminate atop the current deck, delivering exceptional waterproofing, structural reinforcement, and insulation. It reduces the scope of disruptive interventions by sealing superficial imperfections, in accordance with guidelines for GRP roofing. These observations supported a streamlined, economical resolution that maintained the roof’s visual and operational characteristics.
Identification of Substrate Deficiencies
A detailed follow-up assessment by qualified surveyors uncovered a significant departure from preliminary conclusions: severe wood wasp infestation had degraded the substructure extensively. This led to destabilized and decayed joists (as documented in core samples and survey visuals). In Bristol’s moist, temperate climate, such organic deterioration is prevalent in aging timber frameworks, demanding a shift to a complete refurbishment to uphold structural integrity and adherence to Building Regulations Part A (Structure).
This finding highlights the importance of sequential evaluations in GRP roof endeavors, as superficial indicators frequently conceal profound concerns. An overlay at this stage would have invited subsequent breakdowns, intensifying potential harm to interior elements.
Adapted Procedure and Site Preparation
The modified protocol emphasized substrate restoration to establish a secure base for the Strandek installation:
- Dismantling and Clearance: Degraded components—decayed timbers, the current roof deck, aggregate chippings, and the original GRP laminate—were methodically dismantled. This encompassed the dual 50 mm rigid insulation within joist spaces, guaranteeing total removal of contaminated elements.
- Structural Enhancement: Viable timbers were preserved, with substitutes crafted from preserved softwood. Both retained and new timbers underwent a dual gel application of fungicide and insecticide, aligned with BS 8417 wood protection standards. This preventive strategy counters future occurrences in Bristol’s damp conditions.
To shield the building’s interior from adverse weather and promote seamless progress, a custom temporary enclosure was assembled over the work area. Fabricated from UV-resistant polyethylene on a sectional steel framework, this shelter ensured arid conditions throughout dismantling and assembly, shortening the schedule by about 20%.
Incorporation of Insulation and Decking
Following substrate stabilization, the reconstruction advanced systematically:
- Insulation Integration: Replacement 150 mm Kingspan Thermaroof TR27 PIR boards were tailored to precise dimensions, supplanting the excised originals. Positioned between joists with offset seams and sealed via low-expansion foam, this configuration yielded a U-value of 0.13 W/m²K, improving energy efficiency as required by Building Regulations Part L.
- Firrings and Boarding: Conical firring strips were positioned to achieve accurate gradients (minimum 1:80) for effective drainage, per BS 6229. The surface was then covered with 200 × 50 mm (8 × 2 inch) 18 mm OSB3 Sterling interlocking panels, fastened with at least 21 hot-dip galvanized screws per panel (75 mm length, 8-gauge). This formed a continuous, weight-bearing platform suitable for foot traffic and upkeep.
- Skylight Renewal: Outdated skylights were substituted with high-performance GRP-framed assemblies incorporating double-glazed polycarbonate (U-value 1.4 W/m²K) and built-in curbs, facilitating fluid incorporation into the refreshed deck.
Visual harmony was upheld through retention of the simulated lead roll-top features. Factory-molded lead-mimic profiles were secured to the OSB via corrosion-resistant stainless-steel fixings, blending heritage aesthetics with contemporary sealing.
Strandek GRP Deployment and Completion
Factory-formed GRP edge trims (150 mm vertical rise, aluminum substrate with powder coating) were affixed along the roof boundaries, featuring overhangs and aggregate barriers for added resilience.
The primary sealing membrane employed the Strandek GRP formulation: a 600 g/m² chopped-strand mat laminate (resin-to-glass proportion 1.5:1), applied manually in successive layers for seamless adhesion. Catalyst concentration was calibrated at 1.5–2% MEKP, yielding a 20–30 minute gelation period and flawless hardening in Bristol’s typical ambient range (10–20°C). The surface received a durable dark grey flowcoat (600 g/m²) at 0.5–0.7 mm depth, promoting sheen durability and microbial deterrence.
Outcomes and Assurance
The finalized roof exhibits outstanding impermeability, with zero penetration during simulated deluge trials. The Strandek formulation’s pliability manages subtle expansions, while the fortified base supports a service life exceeding 25 years.
This endeavor illustrates our dedication to versatile, client-oriented fibreglass strategies. The installation is protected by an extensive firm warranty, reinforced by FCA-supervised insurance for independent validation.
For additional insights into Strandek implementations or Bristol-area assessments, please reach our technical specialists.
For more information, please contact us. For other examples of our work, such as our recently completed work on a historic roof, please view our blog.