The short answer
Mansard roofs usually leak at their joints rather than across the slate or tile. The usual suspects are the flat roof hidden behind the top of the slopes, the gutter that runs behind the parapet, the lead around dormer windows, and old fixings on the near-vertical lower slope. Finding which one is failing needs a look at all four, because water often travels before it shows.
A steep slope on the outside, a flat roof on the inside
Look at a mansard from the pavement and you see slate or tile on a steep face, then a shallower slope above it. What the eye misses is the top. Behind that second slope, or between the two halves of a double-pitched mansard, there is often a small flat roof, sometimes only a few square metres. Its covering might be lead, asphalt, roofing felt or one sheet of membrane, and whatever the slopes above shed ends up on it.
That is the first place to look when a mansard leaks. Flat coverings fail in ways that slate does not: a blister, a split along a seam, a seam that has opened where the covering turns up at an edge. Water finds that gap, runs along the deck below and can turn up as a damp patch on a ceiling in another room entirely.
A flat roof also depends on a fall, the gentle slope that drains it. A figure the manufacturer IKO quotes from BS 6229 is a minimum of 1 in 80, and designers aim for 1 in 40 so that real-world building tolerances still leave the roof draining. Where the deck has sagged or ponded, standing water speeds up wear. Our guide to mansard roof construction shows how the pieces fit together, useful if you are trying to picture the route a drip takes.
The gutter behind the parapet
Many mansards, particularly terraced ones, finish at the front with a parapet, a short wall carried up past the roof edge. Tucked behind it is the parapet gutter, a lined channel taking the run-off to a downpipe. Its cousin the box gutter sits in the trough where two roofs meet. Lead lines most of them, and few owners ever see them.
That hiding place causes trouble. Leaves and moss collect, outlets block and water backs up over the lead. Lead itself can also split. Sheet lead grows in summer heat and contracts in winter. Lay it in over-long pieces, or without drips (the small steps that give each sheet room to move), and it will crack along a fold in time. Thickness is graded in numbered codes, and one cast-lead maker lists code 5 upwards for these gutters.
A gutter fault can pass for a slope fault, since water enters the wall beneath the parapet and surfaces in a bedroom instead of the loft. Only a look from above separates the two.
Dormers and their junctions
A dormer is a small window structure set into the slope. On a mansard it usually sits in the steep face, and it brings a crowd of joints to one patch of roof: the sill, the two cheeks (the side walls), the roof over the top and the line where the cheeks meet the slate. Each is a place where one material meets another, and each relies on lead or a sealant that has to survive years of sun and frost.
The classic fault is at the cheek. Lead dressed up the side of a dormer is tied to the slating by soakers, small folded pieces of lead woven in course by course. Lose a soaker, or let one slip, and water tracks behind the cheek into the framing. Painted timber cheeks can also split along the grain, letting rain straight through. Sealant is a poor substitute for lead at a flashing, since it hardens and cracks within a few seasons and hides the underlying fault while it lasts.
The dormer roof has its own problems. Many are small flat roofs, with the same hazards as the main one. When we survey, each dormer is looked at as a miniature roof in its own right.
The near-vertical lower slope
Slates on a steep face rely on their fixings far more than slates on a normal pitch. On a pitched roof, gravity and overlap do much of the work. On a face close to vertical, the slate is hanging from its nails, with wind pushing against it and little weight pressing it onto the course below. If the nails rust through, the slate slides or drops.
This is nail sickness, the corrosion of iron or poorly galvanised nails that lets slates slip. Stirling City Heritage Trust describes it, and the Northern Ireland Historic Environment Division notes that a British Standard names copper, aluminium and silicon-bronze as the best choice for a lasting job. Where a mansard face has been patched with whatever nails were to hand, expect more slipped slates after every windy winter.
The upper slope is a different problem. It is shallower, so the covering there needs a deeper lap and good fixings to keep water out. A covering chosen for steeper roofs can leak if it is used on a gentler one, which is why the minimum pitch for the material matters. Our page on minimum roof pitch gives sourced figures.
Tracing the leak and putting it right
Leak tracing on a mansard starts inside. Note where the stain is, when it appears (after any rain, or only after wind-driven rain), and whether it spreads. Then look for a path: a mansard has many places for water to run along a timber before it drips.
Up on the roof we go in a set sequence: flat top, gutters and outlets, dormers, then the face. Most faults are local, and repair works where the material around them is sound. A split gutter lining can be cut out and patched in new lead. A dormer cheek can be re-leaded. Loose slates can be re-fixed with copper nails. Where the flat covering is simply worn out, stripping it and laying new to a proper fall is the lasting cure.
Listed houses and conservation-area streets need repairs that match the original. Our page on planning permission for a mansard explains when consent comes into it, and our listed pitched roofs guide covers the materials. For anything leaking now, we deal with the immediate danger, then plan the lasting repair, and we fix the price after surveying. For the whole picture, see pitched roofing.
