Mansard Loft Calculator — Cladding, Flat Roof & Steelwork

A mansard conversion raises the rear (and sometimes side) roof slope to almost vertical (typically 72 degrees), with a small flat section at the top. This maximises internal headroom and floor space. The steep slope is clad in tiles or slate to match the roof. Mansard conversions usually require planning permission.

The 72-degree angle is a planning convention — steep enough to maximise internal space while remaining clearly a 'roof slope' rather than a wall. Some councils accept steeper angles (up to 80°). The angle also allows tiles or slates to be hung properly with gravity keeping them in place.

Almost always yes. Mansard conversions are not usually Permitted Development because they significantly alter the roof shape and height. Full planning permission is required in most cases. In conservation areas and on listed buildings, additional consents may be needed.

A mansard requires a steel frame to support the new roof shape. This includes horizontal beams at eaves and ridge level, vertical posts at intervals along the wall, and connections at the junction between the steep slope and flat top. Party wall beams and padstones at bearing points are also needed. The steel must be sized by a structural engineer to the Eurocodes (BS EN 1993 for steel, BS EN 1990/1991 for loading, with the UK National Annexes) from the actual spans and loads — this calculator gives only an indicative total run for early budgeting, not a beam schedule.

The flat top is typically a warm roof construction with firring pieces for drainage fall (1:40 minimum), 150mm+ PIR insulation, an EPDM or GRP membrane, and plasterboard below. As a new roof element in an existing dwelling, it must meet the AD L 2021 (Vol 1, incorporating 2023 amendments) Table 4.2 limiting standard of 0.15 W/m²K — this single roof figure applies to both the flat top and the steep slope. Drainage is via an internal gutter or outlet to a downpipe.