Plain-English definitions of every roofing term South London homeowners encounter — from nail sickness and butterfly roofs to building regulations and lead codes. Written by local roofers, not a textbook.
Roof Types & Structure: Pitched Roof · Hipped Roof · Gable Roof · Butterfly Roof · Mansard Roof · Dormer
Roofing Materials: Welsh Slate · Spanish Slate · Plain Clay Tile · Concrete Interlocking Tile · GRP Fibreglass · EPDM Rubber · Torch-On Felt · Code 4 Lead
Roof Components: Ridge Tiles · Hip Tiles · Valley · Flashing · Soaker · Fascia Board · Soffit Board · Bargeboard
Problems & Conditions: Nail Sickness · Ponding · Delamination · Spalling
Standards & Specifications: Breathable Membrane · Dry Fix Ridge · Lime Mortar · BS 5534 · Building Regulations Part L
Processes & Techniques: Make-Safe · Sarking · Re-Battening · Torching
A roof with one or more sloping surfaces, as opposed to a flat roof. The angle of slope is called the pitch, measured in degrees. Most residential properties in South London have pitched roofs — typically 30–45 degrees on Victorian and Edwardian stock, and 20–35 degrees on 1930s onwards.
A roof where all sides slope downward from the ridge to the eaves, with no vertical gable walls. Standard on 1930s semi-detached houses across Bromley, Croydon, Eltham and outer South London. Creates four vulnerable ridge and hip junctions that need periodic mortar maintenance.
A roof with two sloping sides meeting at a central ridge, with vertical triangular walls (gables) at each end. Common on Victorian terraces across inner South London. Simpler than hipped roofs with fewer junction points, but the gable wall itself is exposed to wind and rain.
A Victorian roof design with two inward-sloping pitches meeting at a central valley, creating a concealed box gutter behind a parapet wall. Common across South London terraces in Brixton, Peckham, Lewisham and Camberwell. The hidden box gutter is the most common and most damaging failure point — when it leaks, water enters between party walls where damage is invisible until severe.
A roof with two slopes on each side — a steep lower slope and a shallower upper slope. Originally a French design, common on Georgian and some Victorian properties in South London. The steep lower section often contains rooms or loft conversions. Re-roofing a mansard requires specialist scaffold and careful material specification.
A structural extension projecting from a sloping roof, creating additional headroom and window space in the loft. Rear dormers are the most common loft conversion type across South London. The junction between dormer cheeks and the existing roof is the most frequent leak point — typically caused by incorrect flashing installation.
Natural roofing slate quarried in North Wales — the dominant roofing material on Victorian properties across London. Dense, non-porous, and extremely long-lasting at 100–150+ years. Specified by size (e.g. Countess, Duchess, Ladies) and thickness. Reclaimed Welsh slate is the gold standard for period property repairs and costs £80–120/m².
Imported natural slate, primarily from the Galicia region of Spain. The most common alternative to Welsh slate at 40–60% of the cost. Good quality Spanish slate lasts 60–80 years. Colour varies from blue-grey to dark grey. Suitable for most repairs where Welsh slate matching is not critical.
A flat, rectangular roofing tile made from fired clay, typically 265mm × 165mm. Double-lapped for weathertightness — each tile is covered by two others. Standard on Edwardian and interwar properties across Streatham, Tooting and the 1930s suburbs. Handmade versions are required in conservation areas. Lasts 60–80 years.
A machine-made roofing tile with profiled edges that interlock with adjacent tiles, requiring only single-lap coverage. Introduced in the 1950s and dominant on post-war housing estates across Croydon, Bromley and outer South London. Cheaper than clay but shorter-lived at 30–50 years. Becomes porous with age and vulnerable to frost damage.
Glass Reinforced Polyester — a seamless flat roofing system created by laying glass fibre matting into liquid resin, which cures into a rigid, walkable, waterproof surface. The premium flat roof specification. Requires ambient temperature above 5°C for installation. Lasts 30–40 years with minimal maintenance.
Ethylene Propylene Diene Monomer — a flexible synthetic rubber membrane for flat roofs. Supplied in large single sheets to minimise joints. Bonded or mechanically fixed to the roof deck. Cannot be walked on regularly. Lasts 25–35 years. Best suited to large uninterrupted areas like garages and single-storey extensions.
A three-layer bituminous roofing felt system applied using a gas torch to melt the underside adhesive. The traditional flat roofing method, now largely superseded by GRP and EPDM on new installations. Lifespan 10–15 years. Still used for repairs and budget-constrained replacements.
Lead sheet weighing 1.80mm thick — the standard specification for roof valleys, aprons and larger flashings. Lead is specified by code number (weight). Code 3 (1.32mm) is minimum for small flashings and soakers. Code 4 for valleys and standard flashings. Code 5 (2.24mm) for flat areas, box gutters and high-exposure locations. Using too light a code is the most common specification error in cheap lead repairs.
The tiles or caps that cover the apex where two roof slopes meet. Traditionally bedded in mortar, which cracks and fails after 25–40 years. Modern dry-fix ridge systems use mechanical clips and foam seals — no mortar means no mortar failure. Ridge tile replacement is one of the most common repair calls on 1930s housing across South London.
Tiles covering the external angle where two roof slopes meet at a corner. Found on hipped roofs — standard on 1930s semis. Like ridge tiles, traditionally mortar-bedded and subject to the same mortar failure. Loose hip tiles on a three-storey property are a genuine safety risk to pedestrians below.
The internal angle where two roof slopes meet, forming a channel that concentrates water flow. Lined with lead, zinc or a proprietary trough. Valleys handle more water per square metre than any other part of the roof and are a common failure point. Lead valleys should be Code 4 minimum. Blocked valleys — from moss, leaves or mortar debris — cause water to back up under tiles.
Weatherproof strips — typically lead, zinc or proprietary material — used to seal the junction between the roof covering and an abutting wall, chimney, dormer or other penetration. Step flashings follow the roof slope in steps. Apron flashings run horizontally. Back gutters sit behind chimneys. Flashing failure is the second most common cause of roof leaks after tile displacement.
An L-shaped piece of lead or zinc placed under each tile or slate where the roof meets a side wall. Works in conjunction with step flashings to create a double barrier against water penetration. Often omitted in cheap repairs — the flashing alone looks correct but leaks in driving rain without soakers behind it.
The vertical board fixed to the rafter ends at the eaves, to which guttering is attached. Traditionally timber, now commonly uPVC. Rot in timber fascia is the hidden cause of most guttering failures — the bracket screws pull out of soft wood and the gutter sags. Always inspect fascia condition before replacing guttering.
The horizontal board that closes the gap between the fascia and the wall, forming the underside of the roof overhang. Contains ventilation strips that allow airflow into the roof space. Without soffit ventilation, condensation builds in the loft, rotting timbers and soaking insulation. Painting over soffit vents is a common and damaging mistake.
The angled board on a gable end that follows the roof slope from ridge to eaves. Protects the exposed ends of roof timbers from weather. Takes the same weathering as fascia and often needs replacing at the same time. On decorative Victorian and Edwardian properties, bargeboards may have ornamental detailing that needs matching.
The corrosion and failure of iron nail fixings in slate roofs, typically occurring after 100–140 years. The original wrought iron nails rust, expand, and lose grip on the slates. Individual slates start slipping — first one or two, then accelerating as more nails fail simultaneously. The only lasting solution is a full strip and re-fix with copper or stainless steel nails. The slates themselves are usually still sound.
Standing water that remains on a flat roof for more than 48 hours after rain. Caused by insufficient drainage falls, sagged decking, or blocked outlets. Ponding water accelerates membrane degradation, adds structural weight, and can freeze in winter causing ice damage. Minimum design fall is 1:40 (25mm per metre).
The separation of surface layers on concrete roof tiles, exposing the aggregate underneath. Caused by water absorption and repeated freeze-thaw cycles. Common on 1960s–70s concrete tiles across Croydon, Eltham and New Addington where the original tiles are now 50–60 years old. Delaminated tiles cannot be repaired — only replaced.
The flaking or breaking away of the surface of bricks, stone or tiles caused by moisture penetration and freeze-thaw action. On chimney stacks, spalling bricks indicate serious weathering that requires repointing or rebuilding. Using cement mortar on soft Victorian brick causes spalling by trapping moisture inside the brick.
Roofing underlay that allows water vapour to pass through from inside to outside while blocking liquid water from outside. Essential on older properties where the roof structure was designed to ventilate naturally. Non-breathable felt — standard until the 1990s — traps moisture in the roof space, causing condensation, timber rot and insulation degradation. Building regulations now require breathable membrane on all new and replacement roofing.
A mechanical ridge tile fixing system using stainless steel clips, rubber seals and foam ventilation strips instead of traditional mortar bedding. Eliminates the mortar failure that causes ridge tiles to loosen — the most common repair on 1930s hipped roofs across South London. Conforms to BS 8612. Lasts the lifetime of the roof with no maintenance.
A traditional mortar made with Natural Hydraulic Lime (NHL) instead of modern Portland cement. Essential for any pointing, bedding or rendering work on pre-1920 properties. Lime mortar is softer and more flexible than cement — it moves with the building, allows moisture to escape through the joints, and does not damage soft handmade bricks. Using cement mortar on Victorian or Georgian brickwork traps moisture, prevents breathing, and causes the brick face to spall off within 5–15 years.
The British Standard for slating and tiling. Specifies minimum fixing requirements for roof tiles and slates based on wind exposure zone, roof pitch, tile type and building height. Updated in 2014 to require mechanical fixing of all tiles (previously, some could rely on weight alone). Any re-roofing work must comply with the current edition.
The section of UK Building Regulations covering conservation of fuel and power. Requires thermal insulation to be installed or upgraded whenever a roof is replaced or substantially repaired. On flat roofs this means rigid insulation above the deck. On pitched roofs, insulation between or over the rafters. Compliance is mandatory — not optional — and adds £300–£600 to a typical flat roof replacement.
An emergency temporary repair that stops water ingress and prevents further damage until a permanent fix can be completed. Typically involves tarpaulins, temporary tile fixes, boarding or emergency flashing. A make-safe is charged at standard rates — there should be no emergency premium. The permanent repair is quoted and scheduled separately.
The boarding or membrane layer beneath roof tiles or slates. On older Scottish and some English properties, sarking refers to timber boards nailed across the rafters before battens and slates are fixed. On modern roofs, sarking is the breathable membrane layer. The condition of the sarking is only visible when the roof covering is stripped.
Removing old timber battens and replacing them with new treated softwood battens at the correct gauge (spacing) for the tile or slate being installed. Batten gauge varies by tile type — too wide and tiles do not overlap correctly, too narrow and tiles jam. Re-battening is standard on any full re-roof and essential on any roof where the existing battens show rot, splitting or woodworm.
The traditional practice of applying mortar to the underside of tiles or slates from inside the roof space to hold them in place and block wind-driven rain. Common on older roofs built before roofing felt was standard. Torching deteriorates over time and falls away in chunks — the debris in your loft is often old torching. Modern breathable membrane has replaced torching on all new and replacement roofs.
Skip the jargon. Call us and we will explain what your roof needs in plain English — with a free inspection and no-obligation quote.
0203 489 5352