A substantial kitchen extension on a lakeside property in Windermere was suffering repeated leaks from its deteriorating felt flat roof. Our GRP fibreglass roofing team installed a premium fibreglass system with integrated insulation, creating a warm roof construction that eliminated both the leaks and the condensation problems that had plagued this high-moisture kitchen environment.
This modern lakeside property in Windermere had a large single-storey kitchen extension (approximately 35 square metres) with a felt flat roof that was only 12 years old but had already failed. The kitchen generates significant moisture from cooking, and the original roof construction was a cold roof design with inadequate ventilation. Warm, moist air from the kitchen was condensing on the cold underside of the roof deck, causing the chipboard decking to swell and the felt adhesion to fail from beneath. Several large blisters had formed where trapped moisture expanded in warm weather, and two blisters had split open, creating direct leak paths. The kitchen ceiling below showed extensive water staining, and the homeowner was concerned about the potential for mould growth in the ceiling void. The existing flat roof also had very poor falls — water pooled across the centre of the roof for days after rain, creating additional loading and accelerating the felt's degradation. The homeowner wanted a premium, long-lasting solution that would address both the waterproofing failure and the condensation issue simultaneously.
We designed a warm roof construction that places the insulation above the roof deck rather than below it — this keeps the deck warm and eliminates the condensation that destroyed the original roof. After stripping the failed felt, we found the chipboard beneath was too damaged to reuse. We replaced it with new 18mm OSB3 boarding, then laid 80mm Kingspan Therma TR26 PIR insulation boards on top with staggered joints and taped seams, achieving a U-value of 0.18 W/m²K that complies with current Building Regulations. The insulation boards were laid with a built-in taper to create a 1:60 fall from back to front. A second layer of 11mm OSB3 was installed over the insulation as the GRP substrate. The fibreglass system was then applied in the standard sequence: primer, pre-formed trims, 450g/m² CSM laminate, consolidation coat, and a light grey topcoat chosen by the homeowner to match the property's contemporary styling. The GRP was dressed up the abutment wall 150mm and under a new Code 4 lead flashing chase.
The new warm roof construction has completely transformed the extension's performance. The condensation problem has been eliminated entirely — the deck stays warm even in winter, so moisture from the kitchen never condenses on the roof structure. The GRP membrane is performing flawlessly, with the improved falls draining water within minutes. The light grey finish reflects more solar heat than the original dark felt, keeping the kitchen noticeably cooler in summer. The homeowner has also reported reduced heating costs thanks to the new insulation layer above the kitchen. We provided a 25-year guarantee on the complete GRP fibreglass roof system.
The GRP system applied to this Windermere extension consists of a pre-accelerated polyester resin laminate reinforced with 450g/m² E-glass chopped strand mat. The cured laminate creates a 2.5mm thick, completely seamless waterproof membrane that bonds permanently to the OSB substrate. The light grey topcoat was selected by the homeowner for its contemporary appearance and heat-reflective properties — lighter colours reduce solar heat gain through the roof, keeping the kitchen below noticeably cooler in summer.
This premium rigid insulation board has one of the lowest thermal conductivities of any commercially available insulation (0.022 W/mK), meaning less thickness is needed to achieve the target U-value. The 80mm boards specified for this project achieve a U-value of 0.18 W/m²K, comfortably meeting current Building Regulation requirements. The foil facings on both sides of the board provide an additional vapour barrier, and the boards were laid with staggered joints and taped seams to eliminate thermal bridging.
Two layers of OSB3 were used in this warm roof construction: 18mm structural decking on the joists and 11mm overlay on top of the insulation. OSB3's moisture-resistant phenolic binder provides long-term dimensional stability, and the oriented strand construction gives excellent in-plane strength and stiffness.
The wall abutment flashing was formed from Code 4 milled lead, chased into a mortar joint 150mm above the roof surface and dressed down over the GRP upstand. Code 4 is specified at wall abutments where thermal movement is less than at exposed ridges or valleys, providing a neat, slim profile.
Our GRP fibreglass roofing team specialises in extension roofs across Windermere, Ambleside, Kendal and all surrounding areas. Whether you need a simple re-cover or a full warm roof upgrade, we deliver premium results with a 25-year guarantee. Contact us for your free survey.
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