Key roofing terms explained in plain English for East London homeowners.
A condition where the iron nails fixing roof slates corrode and expand over time, cracking the nail holes and causing slates to slip. Extremely common on Victorian terraces across East London where original 150-year-old iron fixings are failing. The cure is re-fixing with copper nails.
Strips of lead, zinc, or synthetic material used to waterproof the junctions between the roof covering and walls, chimneys, dormers, and other penetrations. Flashing failure is one of the most common causes of roof leaks in East London.
Glass Reinforced Polyester — a seamless, rigid flat roof system created by laying fibreglass matting saturated with catalysed resin. Cures to form a waterproof shell stronger than the deck beneath. Carries a 25-year manufacturer guarantee.
Ethylene Propylene Diene Monomer — a synthetic rubber membrane laid in a single piece for flat roofs. Flexible, UV-resistant, and unaffected by temperature extremes. Carries a 20-year guarantee and typically lasts 30+ years.
A modern roofing underlay that replaces traditional felt beneath slates or tiles. Allows moisture vapour to escape from the roof space while blocking wind and rain, reducing condensation risk and keeping insulation dry.
A mechanical fixing system for ridge tiles that uses clips and a ventilated roll instead of mortar bedding. Cannot crack, erode, or be blown off by wind. Now the recommended standard for all new roofs and ridge replacements.
The mortar joints between bricks or stones in a chimney stack or wall. Repointing is the process of raking out failed mortar and replacing it. On pre-1930s East London properties, lime mortar must be used — cement mortar causes damage.
The horizontal board that bridges the gap between the back edge of the fascia and the wall of the house, forming the underside of the roof overhang. Soffits provide ventilation to the roof space and prevent birds and insects from entering.
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Nail sickness is when the iron nails holding roof slates corrode and expand, cracking the slate and causing it to slip. Common on Victorian properties. Fix: re-fix with copper nails.
GRP stands for Glass Reinforced Polyester — a fibreglass flat roofing system with a 25-year guarantee.
GRP is rigid and walkable, best for extensions and balconies. EPDM is flexible rubber, best for garages and larger areas. Both vastly outperform felt.
Lime mortar is softer and allows moisture to evaporate through joints. Cement is too rigid for pre-1930s brickwork and traps moisture, causing spalling damage.
A mechanical system that fixes ridge tiles with clips instead of mortar. Cannot crack, erode, or blow off. The modern standard for all ridge work.
A modern underlay beneath tiles/slates that allows moisture vapour to escape while blocking rain and wind. Reduces condensation and keeps insulation effective.
Fascias are the vertical boards at the roof edge that support guttering. Soffits are the horizontal boards forming the underside of the overhang. Both protect the roof structure.
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