A mansard roof is a four-sided roof style with two distinct slopes on each side, a steep, nearly vertical lower slope and a much flatter upper slope, often used to create a full extra story of usable space beneath it. It is closely associated with French Second Empire and some Victorian architecture. Its steep lower slope is typically covered in shingles, slate, or metal, while the flat upper section behaves more like a low-slope roof and needs its own waterproofing approach.
A mansard roof breaks each side of the roof into two slopes stacked on top of each other rather than one continuous plane. The lower slope is steep, in many cases nearly vertical, and the upper slope above it flattens out considerably, often to the point of being walkable.
That two-slope shape is what gives a mansard roof its signature profile, and it is also what makes the design so effective at adding living space. Because the steep lower slope pushes the walls of the top floor almost straight up rather than sloping them inward, a mansard roof can turn what would otherwise be a cramped attic into a genuinely usable full story, often lit by dormer windows set into that steep lower face.
The two slopes are not roofed the same way, and that is the detail that matters most for anyone maintaining one. The steep lower slope sheds water quickly and is usually finished in shingles, slate, or metal, chosen as much for how they look on such a visible, near-vertical surface as for their weather performance.
The flat upper slope behaves like a completely different roof. With little pitch to move water quickly, it needs a low-slope roofing approach, historically metal, and today often a membrane system, along with careful attention to drainage so water does not pond on top of the house.
Because a mansard roof is really two roof systems joined together, with a transition seam between them that needs its own flashing, it is a more complex and more labor-intensive roof to maintain or replace than a single-slope alternative, and the added dormers common on this style add still more seams to manage.
A mansard roof is really two roof systems joined at one seam, and an inspection that treats the steep face and the flat cap as separate systems, each with its own set of vulnerabilities, catches problems that a glance at the visible lower slope alone would miss.
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