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Roofing Specialist terms explained — Kidderminster glossary

CompetentRoofer

The government-authorised self-certification scheme for roofing work. A registered contractor can notify building control themselves and issue the compliance certificate, which is faster and cheaper than a separate local authority application.

The 25% Rule(25% rule)

Renovating more than 25% of a roof surface makes it a material alteration under the Building Regulations, normally requiring the insulation to be brought up to current standards at the same time. It is the reason a "like-for-like" re-roof quote can change once scope is confirmed.

BS 5534

The British Standard for slating and tiling, which since its 2014 revision requires mechanical fixing of ridge and hip tiles rather than mortar bedding alone. Mortar-only ridges on recent work are a defect, not a style choice.

Underlay and Ventilation(Underlay)

The membrane beneath the tiles, and the airflow path above the insulation. Fitting a modern breathable underlay without the ventilation detail is a common cause of condensation appearing in a loft shortly after a re-roof.