What is a mansard loft conversion?

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.

Why 72 degrees for the mansard slope?

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.

Does a mansard need planning permission?

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.

What structural steel is 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.

How is the flat top section constructed?

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. The flat must achieve U-value 0.15 W/m²K. Drainage is via an internal gutter or outlet to a downpipe.

TradeCalculator.co.uk

Mansard Loft Calculator — Cladding, Flat Roof & Steelwork

Calculate materials for a mansard loft conversion: 72-degree slope cladding, flat top insulation, structural steel frame, dormer windows and floor area.

Rear Wall Length

Length of mansard wall

Wall Height

Floor to eaves of flat section

Flat Top Width

Width of flat roof section

Flat Top Length

Length of flat roof (usually = wall length)

Dormer Width

Width of dormer windows

Cost Per m² (optional)

All-in cost per m² floor area

How We Calculate This

This calculator estimates materials for a mansard loft conversion based on the rear wall and flat top dimensions.

The formula

Sloped Wall Area = Length × (Height / sin(72°)) × 1.1 waste

Flat Roof Area = Width × Length × 1.1 waste

Steel Frame = Wall Length + 2 × Flat Top Length

Key elements

Sloped wall: 72° angle, tile/slate clad
Flat top: Warm roof, EPDM/GRP membrane
Steel frame: Posts, beams and connections
Dormer: Windows in the steep slope face

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.