When a roofer describes nail sickness on your sarking deck beneath perished bitumen felt, you should not have to guess what they mean. This glossary covers the most common terms in Inverness roofing surveys and quotes — so you can ask the right questions and understand the answers.
Abutment flashing: The lead or aluminium flashing where a roof slope meets a vertical wall — at the side of a dormer or the wall of an adjacent building. Abutment flashings are a common leak source when the lead has lifted from the wall or separated from the tile course below.
Batten: Horizontal timber strips fixed to the sarking (or directly to rafters) to which tiles or slates are fixed. Specified at a gauge — the spacing — calculated based on tile length, roof pitch and required headlap. Rotted or undersized battens are a common secondary finding when a roof is stripped.
Breathable membrane: Modern roofing underlay that allows moisture vapour to escape from the roof void while remaining waterproof from above. Replaces bitumen felt on new and replacement roofs. 50-year design life versus 20–30 years for quality felt. Required by Building Regulations on new Scottish roofing work.
Dry ridge: A ridge tile fixing system using mechanical clips rather than mortar bedding. Eliminates the freeze-thaw failure mechanism causing mortar ridge cracking in Highland conditions. 20–30 year manufacturer warranties standard.
EPDM: Ethylene propylene diene monomer — synthetic rubber membrane for flat roofing. Cold-climate rated to -45°C. Handles Highland thermal cycling without the brittleness causing felt failures at low temperatures. Design life 40–50 years.
Felt: Traditional bitumen-impregnated roofing underlay. Becomes brittle below -5°C — common in Highland winters. Design life 15–25 years. Replaced by breathable membrane on modern installations.
Headlap: The overlap between successive tile or slate courses. In Highland conditions, we use 10–15mm above BSI minimum to prevent wind-driven rain penetration at exposed sites.
Lead code: The thickness classification for lead sheet — code 3 (lightest) to code 8 (heaviest). Code 3 for small sheltered soakers; code 4 for standard flashings and valleys; code 5 for exposed positions, parapet gutters and Firth-facing coastal sites.
Nail sickness: The condition where galvanised steel nails fixing concrete tiles have corroded, leaving tiles resting on battens rather than fixed. Widespread on Inverness post-war estates where 1960s–70s tile is now 50–60 years old. Tiles are unfixed and mobile in storm conditions.
Ridge: The apex of a pitched roof where two slopes meet. Ridge tiles cap this junction — bedded in mortar (traditional) or secured with mechanical clips (dry ridge). Highland frost causes mortar ridge bedding to fail at 15–25 year intervals.
Sarking: Diagonal or horizontal boarding fixed to rafters, over which underlay and battens are installed. Common on Scottish buildings. Provides additional rigidity and a secondary drainage layer. Rotten sarking is a frequent finding during Highland re-roofing projects.
Upstand: The vertical section of flat roof membrane extending up an adjacent wall or parapet to create a waterproof junction. Upstand failure — membrane separating from the wall — is a common flat roof leak source.
Valley: The internal angle where two roof slopes meet. Water from both slopes concentrates here, making volume higher than any individual slope. Valley linings are typically code 5 lead or GRP. Valley failures are among the most common Highland repair calls.
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Call 01463 262835Flashing: Any sheet metal (typically lead, but sometimes aluminium or zinc) used to weatherproof the junction between the roof covering and an adjacent element — chimney, wall, dormer, Velux window, soil pipe penetration. Flashings are the most maintenance-intensive roof element and the most common emergency call-out cause in Inverness.
GRP: Glass Reinforced Polyester — a laminate flat roofing system applied wet and cured in situ. Forms a seamless rigid shell with no lap joints. Better than EPDM for complex geometry; equivalent to EPDM for standard flat roofs but with a narrower Highland installation temperature window.
Hip: The external angle formed where two roof slopes meet at the side of a building — the opposite of a valley. Hip tiles cap this junction and are typically bedded in mortar or secured with a dry-fix hip system. Hip mortar fails by the same freeze-thaw mechanism as ridge mortar.
Interlocking tile: Modern concrete or clay roof tile with a profiled edge that overlaps the adjacent tile to form a mechanical weather seal. The dominant tile type on Inverness post-war housing. Less wind-resistant than natural slate when nail fixings are corroded (nail sickness).
Patination oil: A protective coating applied to new lead flashings that slows the oxidisation process. Particularly important on coastal Highland positions — Nairn, Fortrose, Invergordon — where salt air accelerates lead surface oxidisation. We apply patination oil as standard on all new lead work at coastal and elevated exposed Highland positions.
Re-slate: The complete removal and replacement of a natural slate roof covering. More complex and time-consuming than concrete tile re-roofing due to the individual fixing of each slate and the requirement for specific batten gauge calculation per pitch and exposure.
Soaker: A small lead sheet inserted between each slate or tile at a roof abutment — the junction with a vertical wall — forming a concealed step flashing beneath the surface covering. Soakers are small but critical; a failed soaker allows water to track down behind the adjacent tile course without any visible surface indication.
Tile slip: A retaining clip fixed to the batten below the nail hole position of a concrete interlocking tile, providing additional mechanical restraint when the primary nail fixing is compromised. Used on individual tiles showing early nail sickness as a targeted repair measure, or on exposed Highland pitches as supplementary fixing on new installations.
"Sarking, breathable membrane, nail sickness — all used in the survey report. This glossary let me follow the conversation properly. Should be required reading before any Highland survey."
"Survey identified lead code issues — code 3 in a code 5 position. This glossary explained why that mattered. Changed the repair specification discussion completely."
The nails fixing concrete tiles have corroded, leaving tiles resting unsecured on battens. Tiles are unfixed and can dislodge in storm conditions.
A modern roofing underlay allowing moisture vapour to escape while remaining waterproof. 50-year design life, required by Building Regulations on new Scottish roofing work.
Code 5 is thicker and heavier. Used on exposed positions — parapet gutters, west-facing flashings, coastal positions. Code 4 for standard sheltered applications.
Horizontal or diagonal boarding fixed to rafters beneath the underlay. Provides rigidity and a secondary drainage layer. Common on Victorian Scottish properties.
The internal angle where two roof slopes meet, concentrating water from both pitches. Higher flow rates make valleys one of the most failure-prone elements. Lined in code 5 lead or GRP.
The mortar joint at the face of a masonry or tile surface. In roofing, chimney pointing is the most common application — the visible joint between the chimney brick or stone courses that weathers and erodes over time. Repointing means raking out and replacing the failed mortar with fresh material to the correct specification.
A hip roof has four sloped faces meeting at a ridge, with no vertical gable wall at either end. A gable roof has two sloped faces and vertical gable walls at each end. Hip roofs have no verge mortar (replaced by hip tiles) but have additional hip rafter details that require their own maintenance. Most Inverness post-war housing uses gable roofs; older and larger properties often have hip or hipped-gable designs.
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