A single-storey rear extension on a sandstone property in Penrith needed its leaking felt roof replaced with a long-lasting, high-performance system. Our EPDM rubber roofing team installed a fully insulated warm roof with Firestone EPDM membrane, solving both the waterproofing failure and the thermal underperformance in a single project.
This attractive sandstone-built property on the outskirts of Penrith has a single-storey rear extension housing the kitchen and utility room, built in the 1990s with a cold roof design — three-layer built-up felt over chipboard decking with mineral wool insulation between the joists below. The felt had failed across multiple areas after 30 years of service, with cracks at the overlaps and complete debonding at the back wall abutment where the extension meets the main house. Water was entering the kitchen ceiling in three locations during heavy rain. The cold roof design had always been problematic — condensation formed on the cold underside of the felt during winter, dripping onto the insulation below and reducing its effectiveness. The homeowner reported the kitchen was noticeably colder than the rest of the house and required constant heating even in mild weather. A surveyor had recommended a complete roof replacement with warm roof construction to address both the waterproofing failure and the thermal performance issues.
We stripped the existing felt and chipboard, revealing sound joists that only needed minor treatment. Rather than simply replacing like-for-like, we upgraded to a warm roof construction: new 18mm OSB3 structural deck on the joists, 90mm Celotex GA4000 PIR insulation boards laid on top with staggered joints and taped seams to eliminate thermal bridging, and 11mm OSB3 overlay board providing the membrane substrate. The insulation boards were laid with factory-cut tapers to create a 1:60 fall towards the new rear gutter. A Firestone RubberGard EPDM membrane was then fully bonded to the overlay in a single sheet, dressed over aluminium edge trims at the sides and front, and carried 200mm up the main house wall at the rear abutment. A new Code 4 stepped lead flashing was installed at the rear wall, chased into raked mortar joints to follow the stepped coursing of the sandstone wall. New mineral wool insulation was fitted between the ceiling joists below, creating a sandwich construction with insulation above and below the structural deck for maximum thermal performance.
The transformation in the kitchen's comfort has been remarkable. The warm roof construction has eliminated both the water ingress and the condensation problem that plagued the cold roof design. The kitchen is now noticeably warmer — the homeowner reported they no longer need to run the radiator during mild weather, and the overall heating bill for the property dropped by approximately 15%. The EPDM membrane presents a clean, uniform surface, and the stepped lead flashing at the sandstone abutment provides a traditional detail that complements the property's character. We provided a 20-year system guarantee on the complete EPDM installation.
The 1.14mm Firestone RubberGard EPDM membrane was factory-cut to cover the entire 30m² extension roof in a single sheet. Firestone's EPDM is manufactured from pure ethylene propylene diene monomer rubber with no fillers or plasticisers that could leach out over time, ensuring the membrane maintains its properties indefinitely. The material's natural elasticity allows it to accommodate structural movement in the timber extension frame without stress cracking — an important consideration for the temperature extremes experienced in Penrith.
This premium rigid insulation board achieves excellent thermal performance with a lambda value of 0.022 W/mK. At 90mm thickness, the boards provide a U-value of 0.17 W/m²K, exceeding current Building Regulation requirements. The closed-cell polyisocyanurate foam prevents moisture absorption, and the foil facings provide vapour resistance. The warm roof placement — above the structural deck — keeps the entire roof structure warm and condensation-free.
The rear abutment flashing was formed from Code 4 milled lead, stepped to follow the coursing of the sandstone wall. Each step was chased 25mm into a raked mortar joint, secured with lead wedges, and pointed with lime mortar to match the existing pointing. Stepped flashings are the traditional and most durable method for sealing the junction between a flat roof and a masonry wall.
Powder-coated aluminium trims provide clean terminations at the front and side edges of the EPDM roof. The trims incorporate a drip detail that ensures water falls cleanly into the gutter or away from the wall rather than tracking back under the edge by capillary action.
Our EPDM rubber roofing specialists deliver premium flat roof installations throughout Penrith and the Eden Valley. From warm roof conversions to simple re-covers, we provide permanent waterproofing solutions with 20-year guarantees. Contact us today for your free survey.
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