Covering a mansard roof: plan the change in slope first

The steeper lower section creates attic space, while the gentler upper section completes the roof. Their junction is a distinct detail, not an ordinary lap between sheets in one plane.

Measure angles, lengths and support positions for each section separately. In refurbishment, check deviations in the existing structure. A plan view cannot supply every dimension needed for flashings and sheet layout.

Each section must meet the chosen profile’s installation requirements. Plan separate sheets and a junction flashing as appropriate. Do not bend a finished profile across the break unless its manufacturer permits this.

Long sheets can reduce joints but must be handled safely on the steep roof. Consider length, damage risk and dormer cuts. A square-metre price does not reflect all handling difficulties.

Modules can be easier to arrange around short sections and dormers. Plan the courses on both levels so the change in slope does not expose an accidental pattern offset visible from the street.

Compare waste, joint count and installation time. A small format does not remove pitch or fastening limits. The steep lower section also requires a planned working method and secure handling of components.

T18 and the deeper T35 create different surface rhythms, but appearance cannot decide the specification alone. Verify the profile against the planned supports. Both still require a designed change-of-slope detail.

Concrete tiles require suitable structural capacity and permitted system pitches. Their greater weight changes the load assessment. Check available accessories and how tile courses terminate at the mansard junction.

The section should show water passing onto the lower covering without a pocket that retains rain or snow. Support the flashing and specify laps for the actual geometry and material.

Check underlay continuity and the ventilation route too. Sealant cannot replace a correctly shaped flashing. Photograph the junction before concealment and compare the work with the agreed section drawing.

For a heated attic, draw a continuous insulation boundary around rafters, supports and walls. The change in angle can be missed during installation. Include the connection to façade insulation in the section.

PIR may help where space is limited, while mineral wool accommodates some shapes differently. Select the whole assembly, including fire protection and lining. Seal vapour-control junctions around windows and dormers as well.

Separate mansard flashings, scaffolding, protection and steep-slope labour in the estimate. Where snow retention is required, include the designed system and fixings. Future maintenance also needs safe access.

Ask the contractor to explain the junction and layer sequence before choosing. Compare proposals for identical geometry and scope. A workable detail is more informative than an impressive covering sample alone.