Roofing Glossary

Nail Fatigue

Nail fatigue is the progressive corrosion of the original iron nails fixing slates to roof battens. On the Victorian properties of Great Malvern these nails have been corroding for 120 to 160 years. As the nail loses cross-section it can no longer hold the slate against wind uplift, so slates slide free one at a time and then in clusters during storms.

Flaunching

Flaunching is the sloped mortar collar formed around the base of each chimney pot where it sits in the stack. It sheds rainwater away from the flue opening. It is the most weather-exposed mortar on any roof, and cracked or missing flaunching is one of the most common causes of damp appearing on a chimney breast in Malvern properties.

Lead Code

Lead code is the standard measure of rolled lead sheet thickness and weight under BS EN 12588, running from Code 3 to Code 8. Code 4 suits soakers, Code 5 is the minimum for chimney step flashings and back gutters, and Code 6 is used for valley linings on exposed properties such as those above the A449 in Malvern Wells.

Nail Sickness

Nail sickness is the common trade term for the same condition as nail fatigue: failure of the original ferrous slate fixings through corrosion. The phrase is widely used across Worcestershire roofing. A roof described as nail sick has sound slates held by unsound nails, meaning the covering can often be salvaged and re-fixed rather than replaced entirely.

Hydraulic Lime (NHL)

Naturally hydraulic lime is the correct mortar for masonry built before roughly 1920. Grades NHL 2, NHL 3.5 and NHL 5 indicate increasing strength. Lime is softer than Victorian brick, so it erodes sacrificially instead of forcing the brick face to spall, which is exactly what modern Portland cement does to the period properties of Great Malvern.

Dry Ridge System

A dry ridge system fixes ridge and hip tiles mechanically using an aluminium batten and screwed saddles rather than bedding them in mortar. Because there is no mortar, frost cannot crack the bond and the ridge cannot be dislodged by wind. BS 5534 has required mechanical fixing on new and re-roofed work since 2014.

Back Gutter

A back gutter is the lead channel formed between the upslope face of a chimney stack and the roof surface above it. It collects water running down towards the chimney and directs it around both sides of the stack to the step flashings. When the lead fails, water runs straight down behind the stack into the roof void, often undetected for months.

Breathable Underlay

Breathable underlay is the modern vapour-permeable membrane laid beneath battens and tiles. It sheds any water that passes the covering while allowing moisture vapour from inside the building to escape, preventing condensation in the roof void. It replaces traditional bitumen felt on every re-roof we carry out in Malvern and Worcestershire.

Any roofing quote you receive in Malvern should explain its own terms. If it does not, ask. A contractor who cannot explain why Code 5 lead is specified over Code 4, or why hydraulic lime is used instead of cement on a Victorian chimney, is not the contractor for a period property. Call 01684 439601 and we will talk it through.