A pair of flat-roofed dormer windows on a semi-detached property in Cockermouth had been leaking persistently through their aging felt coverings, causing damage to the bedrooms below. Our GRP fibreglass roofing team replaced both dormer roofs and cheek cladding with seamless fibreglass, creating a fully waterproof enclosure that will perform for decades.
This 1970s semi-detached property in Cockermouth has two flat-roofed dormer windows projecting from the front roof slope — a common design feature of that era. Both dormers had their original felt covering on the flat top surfaces and the vertical cheek walls. After 50+ years, the felt had cracked, lifted, and split in numerous locations. The most severe damage was at the junctions where the flat dormer tops met the cheek walls — these internal corners are notoriously difficult to detail in felt, and the folds had opened up allowing rain to run down inside the dormer structure. Both bedrooms below showed significant water damage: stained plaster, bubbling paint, and damp timber window surrounds. The cheek walls were particularly problematic — the vertical felt had detached from the plywood substrate in large sections, flapping in the wind and looking extremely unsightly from the street. One cheek had been partially repaired with roofing mastic that had itself failed and was dripping black streaks down the rendering below. The homeowner was planning to sell and knew the dormers needed proper restoration.
We erected scaffold to both dormers and stripped all existing felt from the flat tops and cheek walls. The plywood substrate on the cheeks needed replacing on three of the four faces where delamination had occurred. The flat dormer decks were in reasonable condition — only minor localised soft spots needed cutting out and patching with new OSB3. With all substrates prepared, we applied the GRP fibreglass system to both dormer tops and all four cheek walls as a continuous, seamless membrane. The key advantage of GRP for dormers is that the laminate flows continuously from the flat top surface down over the curved angle and onto the vertical cheek — there are no joints at these critical corners where felt systems invariably fail. We used 600g/m² chopped strand mat on the vertical cheek surfaces for additional impact resistance, and standard 450g/m² on the flat tops. Pre-formed GRP trims were installed at all edges, and the membrane was dressed under the slate courses above and into the window frame reveals. A smooth grey topcoat was applied to match the main roof slates.
Both dormers now present clean, uniform surfaces that look like part of the original roof design rather than the patched, flapping mess they had become. The seamless GRP construction means there are zero joints at the vulnerable top-to-cheek junctions — the exact locations where the old felt failed. The bedrooms below have been dried out and redecorated, and there has been no recurrence of damp since completion. The homeowner successfully sold the property shortly after, with the estate agent noting that the dormer renovation had significantly improved the kerb appeal. We provided a 25-year guarantee on all GRP work.
We specified two different weights of chopped strand mat for this dormer project. The flat roof surfaces received standard 450g/m² CSM, providing a 2.5mm thick cured laminate with excellent waterproofing properties. The vertical cheek walls received heavier 600g/m² CSM, producing a 3mm thick laminate with significantly greater impact resistance — important for vertical surfaces that could be struck by wind-blown debris, ladders, or maintenance equipment. Both weights use the same polyester resin system, and the continuous lamination between them creates a monolithic GRP structure with no joints.
The replacement cheek panels were cut from 12mm WBP marine-grade plywood, which resists delamination even in persistently damp conditions. Marine-grade plywood uses phenolic resin adhesive between all layers, providing far superior moisture resistance compared to the standard exterior plywood that was originally used on these Cockermouth dormers.
Factory-manufactured trims were used at all dormer edges to provide clean, straight terminations. The trims incorporate a drip detail that prevents water from running back under the edge — critical on dormer cheeks where capillary action can draw water behind inadequate edge details into the wall structure.
The grey topcoat was specified to match the existing slate roof colour, creating visual continuity between the dormers and the main roof. The UV stabilisers in the topcoat protect the underlying resin from solar degradation on the south-facing cheek surfaces, which receive intense UV exposure throughout the day.
Don't let leaking dormers cause further damage to your bedrooms. Our GRP fibreglass dormer specialists serve Cockermouth and all of west Cumbria with expert seamless dormer restoration. We also handle flat roof repairs, felt roofing, and all types of roof repairs. Contact us for a free inspection and quotation.
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