The mansard roof is a distinctive roof form most readily associated with the grand terraced houses of Georgian and Victorian London and with the architecture of 17th and 18th century France. Its particular configuration maximises usable floor area within the roof structure and has made it a popular choice for upper-floor residential conversions and extensions in cities.

Describing the Mansard Form

A mansard roof is characterised by having two distinct slopes on each side of the building. The lower slope is very steep, often approaching the vertical at around 70 to 80 degrees from the horizontal, and typically contains dormer windows. The upper slope is much shallower, often almost flat at around 10 to 15 degrees. This combination of a near-vertical lower slope and a near-flat upper slope creates a maximum usable floor volume within the roof space compared with a conventional pitched roof of the same building footprint.

In cross-section, a mansard roof looks like a very wide, shallow upper deck with steep skirts hanging down at the sides. The dormer windows in the lower slope light the usable rooms within the roof volume.

Origins and Historical Context

The mansard form takes its name from the French architect François Mansart, who popularised it in France in the 17th century, though the form pre-dates him. Its adoption in France was partly driven by Parisian building height regulations that taxed storeys above a certain height but did not count the roof space as a storey if constructed in the mansard form. This fiscal quirk incentivised the mansard configuration and made it the dominant roof form on Parisian apartment buildings.

Mansard Extensions in London

In London and other UK cities, mansard roof extensions on top of Victorian and Edwardian terraced houses are one of the most popular ways to add a storey to a building that cannot be extended in other directions. A mansard loft conversion adds a full additional floor within the roof volume and typically requires planning permission rather than being permitted development, as the steep lower slopes and flat top significantly change the visible roofline of the property.

Summary

A mansard roof has a steep near-vertical lower slope containing dormer windows and a shallow upper slope, maximising usable floor volume within the roof. It is associated with French architecture and Georgian and Victorian London terraces. Mansard loft extensions in cities add a full storey to townhouses but require planning permission as they significantly alter the roofline. Conservation area guidance must be followed for new mansard extensions in designated areas.