What Is a Mansard Roof?

The shape

Think of a standard hipped roof, then break each slope in two. The lower slope is steep, commonly close to seventy degrees, and the upper slope is shallow. The break between them is a line called the curb or the kerb. Because the lower part is almost a wall, the room inside has near-full height right to the edge, with space for furniture and for real windows.

The name comes from the French architect François Mansart, who used the form in the seventeenth century. In Britain it appears on Georgian and Victorian town houses, where it hid an extra floor behind a parapet, and on modern loft conversions where a mansard gives far more room than a simple dormer.

What covers a mansard

The steep lower slope is usually covered in slate or tile, laid in the same way as on any steep pitch, with extra fixings because of the angle. Slate is traditional, and plain clay tiles are common too. The shallow upper slope is often flat or low-pitched, and it takes a different covering, such as lead, a single-ply membrane or a built-up system, because tiles will not shed water on a shallow slope. Our answer on what roof pitch is explains why the angle matters.

The junction at the curb between steep and shallow slopes is a weak spot. Lead flashing is typical there, and it must be dressed correctly to keep water out. Our guide to lead valleys gives background on lead channels.

Dormers and windows

The steep slope allows dormer windows to be set into it. Each dormer has its own small roof, cheeks and flashings, and each is a further junction to maintain. Large dormers on a mansard conversion can cover most of the slope, and they are often built from timber frame clad in lead or tile.

Because mansard conversions add a storey, they usually need planning permission, and Building Regulations cover the structure, fire safety, insulation and stairs. Check with your council planning team, and with whoever will oversee the structure, before you plan one.

Insulation and ventilation

A mansard conversion puts living space where the loft used to be, so the roof becomes part of the thermal envelope. Insulation is normally fitted between and under the rafters, and the build-up has to deal with moisture. A badly designed warm roof can trap damp air and cause condensation on timbers. The upper shallow section, if flat, needs the same care as any flat roof, with a ventilated void or a warm deck design that suits the insulation.

Building Regulations set minimum thermal performance for new and altered roofs, so ask the council building control team or a private approver what applies to you. Fire safety matters too, since a new floor means a new escape route, and the stair arrangement has to comply.

Mansards on older buildings

On a period building, the mansard may be original, and its lead and slate need like-for-like repair. Where a building has listed status, or sits in a conservation area, changes to materials or appearance may need consent, so speak to the council's conservation officer before any work. Matching old slate and lead detail is skilled work, and it is better planned than rushed.

Common problems

The flat or shallow upper roof is the first to give trouble. If it is a membrane or lead roof, it can pond, crack or split at the edges, and a leak there runs into the living space below. The curb flashing can lift or split, and the lead on dormer cheeks can fail. The steep slate or tile section may suffer from slipped slates because the angle puts more strain on the fixings.

Gutters on a mansard are often concealed behind the parapet or set at the foot of the steep slope, so blockages show up late. Damp in the room beside the sloping ceiling, staining at the curb and daylight at the dormers are the usual signs.

Survey and repair

A mansard roof is a mix of pitched and flat roofing, so the survey must cover both. We inspect the slopes, the curb, the dormers and the gutters, and photograph each junction. The repair might be new lead at the curb, a replaced membrane on the top, or re-fixed slates on the steep face. You can read more about our work on the pitched roofing page.