A mansard loft conversion, with two slopes, has the advantage of maximising the available space within your loft.

The lower slope is close to vertical at 72 degrees and the top section of the roof is almost horizontal. A Mansard loft conversion is commonly built by raising the party/gable walls on either side of your house to make the profile for the mansard and then creating the timber frame to suit.

Although common on older properties, especially in cities like London, mansards are not often seen in the suburbs. Flat-roof dormers tend to be a more popular choice for the ‘average’ 3 bed semi or terrace house due to the reduced cost and simpler construction.

A mansard loft conversion will almost certainly require planning permission.

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Mansard Roof Types

Straight Mansard Roof

The upper slope of a straight mansard roof is not normally visible at ground level. It is designed with dormer windows set in the lower slope, which may be almost vertical. In addition to extra space, the dormers provide lighting and ventilation. A mansard straight roof can also have two stories. One drawback of these roofs is that the weight of snow accumulation can cause cracks and leaks from the roof’s surface.

Convex Mansard Roof

The convex shape of the mansard roof curves outward on the lower slope. The base of the convex roof is formed by a wide cornice supported by heavy brackets. Some convex mansard roofs are shaped like an “S”, while others have a bell shape. Convex Mansard roofs are often seen on courthouses with elaborate clock towers.

Concave Mansard Roof

The concave Mansard roof curves inward or can be flared. Some concave roofs have a steep angle on the lower slope. During the second half of the 19th century, these were a popular design for mansions and large buildings.

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