• Mansard Roofs and Roof Extensions in London

    Mansard roofs are everywhere in London. Some are original to Georgian and Victorian houses, many more have been added as roof extensions to create an extra floor, and a large number of top floor flats sit inside one. They make good use of space, but they have more junctions, more flat roof areas and more ways to let water in than a simple pitched roof.

    What Is a Mansard Roof?

    A mansard has a steep lower slope, usually set behind a parapet and covered in slate, with dormer windows set into it, and a flat or very shallow roof on top. In practice, a mansard is a combination of a steep pitched roof, a flat roof, dormers and gutters, all in one structure.

    Common Defects in Mansard Roofs

    • Flat roof coverings. The top section is often felt, asphalt or a single ply membrane. These have a limited life, and blistering, splits, ponding and poor upstands are common.
    • Dormer cheeks and windows. The sides and tops of dormers are a frequent source of leaks, through failed leadwork, cracked render or poorly sealed windows.
    • Parapet and box gutters. The gutter at the base of the steep slope, behind the front parapet, is hidden and has small falls, so blockages and joint failures go unnoticed.
    • Leadwork. Mansards rely heavily on lead flashings and soakers. Poorly sized or poorly fixed lead splits and lifts over time.
    • Party wall junctions. Where a roof extension meets the neighbouring roofs, the junction is often complicated and badly detailed.
    • Condensation. Rooms inside mansards are often poorly insulated or poorly ventilated, and condensation is easily mistaken for a roof leak.

    Added Mansards and Roof Extensions

    When a mansard has been added to an existing house, the quality varies a great deal. Some are well built and well detailed. Others have been built quickly, with shallow falls, poor drainage and lightweight coverings. When we survey a roof extension, we look at how well it has been put together and how it has aged, and we recommend that you or your conveyancer check the planning permission, building regulations sign off and any party wall paperwork.

    Buying a Flat Inside a Mansard

    If you are buying a top floor flat inside a mansard, the roof is directly above your living space. The flat roof on top, the dormers and the gutters all need checking. If the roof is shared with the building, it is usually the freeholder's responsibility, but you will feel the effects of any leak first, and you may contribute to the cost of repairs through the service charge.

    How We Inspect a Mansard

    The flat roof on top of a mansard usually cannot be seen from the street or from the windows. We inspect it with a drone, and use a 10 metre pole mounted camera for the dormers, parapets and gutters. This lets us see the whole roof without scaffolding, and gives you photographs of the parts you would otherwise never see.

    What We Report On

    • The flat roof covering, its falls, upstands and outlets
    • The steep slopes, slates and fixings
    • Dormer cheeks, tops, windows and leadwork
    • Parapets, copings and gutters
    • Junctions with neighbouring roofs and party walls
    • Signs of leaks or condensation inside, and their likely cause

    For a full survey of a top floor flat, see our Flat Buyers Survey, or our House Buyers Survey for a house with a roof extension.

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    Prowle & Co LLP are residential building and roof surveyors covering every London borough and the Home Counties. Prowle & Co LLP is regulated by RICS. Our advice is fully independent. Call 07731 939956 or email lee@prowleco.co.uk with the property address, or read more about our roof surveys.