Plain-English definitions of the 24 key terms used across our guides and surveys — from nail sickness to lead codes, dry ridge to ponding. If a roofer uses a term you don't recognise, check it here: an informed homeowner gets better quotes. Questions about your own roof? Call 01743 537230 for a free survey.
The progressive corrosion failure of the galvanised cut nails that fix roof tiles or slates. Once the zinc coating corrodes through to steel, nails lose holding power and tiles slip in multiple locations across the pitch. The permanent fix is a full de-tile and re-nail using stainless steel ring shank fixings — not individual tile replacement.
Natural slate quarried in North Wales, primarily from Ffestiniog (blue-grey, fine surface) and Penrhyn (blue-purple, slightly coarser). The dominant roofing material on Shrewsbury's Victorian and Edwardian housing. Lifespan of 100–200 years — typically outlasting its fixings by 60–100 years.
The mortar cap at the top of a chimney stack that holds the pots in place and sheds water off the stack top. Cracked or crumbled flaunching allows water into the stack core and is one of the most common sources of chimney-related damp.
Glass reinforced plastic — a seamless liquid-applied flat roof system cured to a rigid finish with no joints or seams. Recommended specification for Shrewsbury's wet climate with a 40+ year service life and 25-year product guarantee.
Ethylene propylene diene monomer — a single-ply synthetic rubber membrane for flat roofs. Highly flexible, handling thermal movement well, which suits larger extension spans. Service life of 30–50 years with a typical 20-year product guarantee.
The thickness classification for rolled lead sheet under BS EN 12588. Code 3 (1.32mm) is minimum for soakers; Code 4 (1.80mm) for step flashings; Code 5 (2.24mm) for bay tops, back gutters and flat applications. Undersized lead is the primary cause of premature bay top failure on Edwardian properties.
Weatherproofing — usually lead — fitted where a roof meets a wall, chimney or other vertical surface. Step flashing follows the tile courses up a chimney side; cover flashing dresses over an upstand. Failed flashing is the most common source of chimney-related internal damp.
A small piece of lead (typically Code 3) interleaved between slates or tiles against a wall or chimney, working with step flashing above to carry water away from the junction. Hidden from view when correctly installed.
Renewing the mortar bed that secures ridge tiles along the apex of a pitched roof. Ridge mortar fails through frost cycling and wind loading — deteriorated ridge mortar is the most common cause of storm-displaced tiles.
A mechanical ridge fixing system that clamps ridge tiles with screws and clips instead of mortar. Maintenance-free, storm-resistant and now required on new builds under BS 5534 — increasingly the correct specification when re-ridging older properties.
The edge of a pitched roof at a gable end. Traditionally finished with mortar bedding, which is exposed to prevailing weather and fails through frost cycling. Dry verge systems provide a maintenance-free mechanical alternative.
The internal junction where two roof pitches meet, carrying concentrated water flow. Formed in lead, fibreglass or mortar-bedded tiles. Valleys accumulate leaf debris and are a frequent failure point on Shrewsbury properties with tree canopy.
Vapour-permeable membrane fitted beneath roof tiles that allows moisture vapour to escape the roof void while blocking wind-driven rain. The correct modern specification, replacing traditional bitumen felt (Type 1F) which traps condensation and accelerates nail corrosion.
The horizontal timber strips fixed across rafters onto which tiles or slates are hung and nailed. Must be graded and treated to BS 5534. Rotted or undersized battens are replaced during any re-roof or nail-and-re-fix.
The sloping structural timbers that form the roof frame and carry the weight of battens, underlay and tiles. Rafter condition is checked from the loft at survey and at strip-out during a re-roof.
The vertical board running along the eaves that carries the guttering and closes the rafter ends. Rotted fascias behind the gutter line are one of the most commonly missed causes of eaves damp in Shropshire properties.
The horizontal board underneath the eaves overhang, closing the gap between fascia and wall. Vented soffits provide airflow into the roof void, reducing condensation and slowing nail corrosion.
The lower edge of a pitched roof where it overhangs the wall, comprising fascia, soffit and guttering. Water management at the eaves determines whether rainfall reaches the drain or the wall.
The external sloping junction where two roof pitches meet on properties without a gable. Hip tiles are mortar-bedded or mechanically fixed; hip mortar fails the same way as ridge mortar and is repointed the same way.
The deliberate slope built into a 'flat' roof so water drains. BS 6229:2018 requires a minimum 1:80 (12.5mm per metre); 1:40 is the recommended design fall. Inadequate fall causes ponding, the primary cause of premature flat roof failure.
Standing water remaining on a flat roof more than 48 hours after rainfall, caused by inadequate fall or deck deflection. Accelerates every failure mode — felt blistering, joint stress and surface erosion — and is corrected with tapered insulation at replacement.
Traditional mortar based on lime rather than modern cement. Softer than period brick, so joints fail sacrificially and can be renewed without damage. The required specification for repointing Victorian and Edwardian chimneys — OPC cement on soft period brick causes irreversible face spalling.
The fracturing and loss of a brick's outer face, most often caused by hard cement mortar restraining natural brick movement, or by frost acting on saturated brickwork. Common on period chimneys previously repointed in the wrong mortar.
A designated zone where planning controls protect architectural character. In Shrewsbury this covers the town centre within the Severn loop, Kingsland, and parts of Belle Vue and Copthorne. Material changes visible from a public highway require prior approval from Shropshire Council; like-for-like repair does not.
See also: all roofing guides · roof repairs · roof replacement · flat roofing