A mansard roof has a steep lower slope, often close to vertical, and a shallow or flat top. You’ll see them on Georgian and Victorian buildings in London and on many later loft extensions that add a full extra storey. They give a lot of usable space, but they also bring together several different roof types in one place.
This guide looks at each part of a mansard, the faults that tend to show up and what a survey can check.
The steep slopes
The lower slopes of a mansard are usually slate or clay tile. Because they’re so steep, they shed water quickly, but they’re also exposed to driving rain. Slipped slates and loose tiles are common, especially where the original fixings have corroded. On steep slopes, a slipped slate can fall rather than sit in place, so it’s worth acting on.
The steep slopes also hide a lot from inside. Behind them there’s usually a room, not a loft, so there’s little or no roof space to check from within. Damp on the walls of a top-floor room can be the first sign of a fault in the slope outside.
The flat top
The top of a mansard is often a small flat roof, finished in lead, felt, asphalt or a modern membrane. It’s easy to forget because it can’t be seen from the street or the windows. Typical faults are ponding, splits, blistering and failed joints at the edges. Our guide to flat roof problems covers these in more detail.
Where the flat top meets the steep slope there’s a junction, usually with a lead or metal trim. If that junction fails, water can run down inside the slope and appear well below the actual fault.
Dormers and windows
Most mansards have dormer windows. Each dormer has cheeks (the side walls), a small roof and flashings where it meets the main slope. Common problems include:
- cracked or lifted lead on the dormer cheeks and roof
- rotten timber around window frames and sills
- failed flashings at the sides of the dormer
- blocked or undersized gutters on the dormer roof.
Leaks around dormers often show as staining on the window reveals or on the ceiling just inside the window.
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Gutters, parapets and party walls
Traditional mansards often sit behind a parapet with a gutter at the base of the steep slope. On terraces, the party walls rise between neighbours with their own upstands and flashings. As on butterfly roofs, these hidden gutters can block and overflow inside. Lead in these areas ages in the ways described in our guide to lead flashing problems.
Newer mansard extensions
A mansard added as a loft extension is a newer build, but that doesn’t mean it’s without faults. Snags we look for include poor lead detailing, short flashings, missing cappings, and flat tops laid without enough fall. If the work is recent, a new roof snagging inspection can record issues before the final payment or before the defects period ends.
What a survey can see
Mansards can be hard to see. The steep slope blocks the view of the flat top from the ground. We inspect what can be seen safely: from the ground, from windows, from inside where there’s access and, where applicable, by drone. A drone is often the best way to photograph the top of a mansard without anyone going up. The report records each part and prioritises any defects.
Quick answers
Why does my mansard leak only in heavy rain?
Heavy, wind-driven rain tests the steep slopes and dormer flashings hardest. It can also overwhelm a part-blocked gutter. A survey looks for both.
Is a mansard loft extension covered by your survey?
Yes. We look at the coverings, flat top, dormers, flashings and gutters as part of a roof survey, within what can be seen safely.
Can you see the flat top of a mansard from the ground?
Usually not. It’s often visible from a higher building or by drone where a flight is allowed and the weather is suitable.
Related services
More advice
- Flat Roof Problems: Ponding, Blisters and Splits
- Lead Flashing Problems on Older Roofs
- Roof Checks Before a Loft Conversion
- Victorian Terrace Roofs in London: Common Defects
- All advice
This guide is general information about roofs and roof surveys. It isn’t legal, insurance or structural engineering advice; for those, speak to a suitably qualified professional.