Simon Hope at CUPA PIZARRAS outlines the legislation affecting mansard roofs and why natural slate cladding on a metal subframe is a compliant, durable solution.
A mansard roof, with its steep lower slope and often dormer windows, offers an effective way to create extra space in heritage terraces, flats, and new builds. However, since regulatory changes in 2018, it is vital for specifiers to understand the fire-safety requirements that govern their construction.
Legislation and classification
The Building (Amendment) Regulations 2018/1230 states that “any part of a roof pitched at an angle of more than 70° to the horizontal” is classed as an external wall if it adjoins an accessible space. Since mansard roofs almost always provide access, they typically fall within this definition.
Further, BS5534: Code of Practice for Slating and Tiling also defines surfaces pitched at 70° or more as walls, regardless of whether the enclosed space is accessed. Best practice therefore requires treating any mansard roof at this pitch as an external wall, applying all related fire-safety standards.
Fire-safety implications
Under Regulation 7, combustible materials are banned on external walls “at least 18 metres above ground level,” directly impacting mansard roofs. This prohibition includes traditional timber battens often used for slate fixing.
Approved Document B (Fire Safety, Volume 1) reinforces this position, advising that for all buildings, “the choice of materials for external walls” should minimise fire spread. Together, these rules confirm that mansard roofs must be treated as walls, meaning combustible materials such as timber battens are no longer suitable.
Compliant alternatives
A practical solution is to install natural slate using a metal subframe system. These achieve the same aesthetic as traditional slate roofs while meeting non-combustibility requirements. Unlike timber, metal subframes resist warping, degradation, and high winds, ensuring long-term performance.
Natural slate Rainscreen cladding systems such as CUPACLAD® use aluminium or stainless steel subframes, are extremely lightweight, and achieve A1 non-combustible ratings. CUPACLAD® has also been approved by the British Board of Agrément (BBA), providing reassurance for both contractors and fire officers during project sign-off.
CUPACLADâ can support a project’s sustainability credentials. Unlike other façade materials, no heat or chemicals are applied during the production process and natural slate has a lifespan of over 100 years. As a result, the environmental impact of slate is considerably lower than other cladding materials. CUPA PIZARRAS has in fact already achieved Net Zero certification, EcoVadis Gold Medal and BRE Global verification of its EPD.
Learn more about CUPACLAD at: www.cupapizarras.com/uk/rainscreen-cladding
