How do you prepare an existing building for a new sandwich panel roof?

Measure the spacing and width of the supports and check their alignment in the roof plane. The bearings must meet the design requirements for the specific panel. Correct uneven supports before installation. Pulling a rigid panel down with extra fixings can conceal an alignment problem. Where the existing drawings are unreliable, take measurements on site.

Measure the actual fall from the highest point to the eaves, allowing for deflection under load and for local depressions. The required pitch depends on the panel system, slope length, end laps and roof openings. Even on a roof described as flat, check that the proposed arrangement provides the fall the system requires.

Position gutters, downpipes and overflows according to the drainage design. Existing outlets may be too small or at the wrong level. A change in roof thickness alters the eaves and flashings. Draw a section through these junctions to make sure the water discharges into the gutter and cannot run behind the new facade.

Where a lean-to roof meets a higher wall, the upper end of the roof is particularly exposed. Specify flashing and seals that suit the substrate and accommodate movement. A bead of sealant applied to dirty render is unlikely to give a durable junction. Inspect the masonry above the flashing as well.

The new structure may move differently from the existing building. The designer should detail the connection and any movement joints. Check for snow drifting against the higher wall. Panel selection based on a uniform load across an open roof may not cover this local increase.

Converting an unheated store into a heated workshop changes both temperature and moisture conditions. Select the panel to meet the U-value required for the proposed roof assembly. Then work through the eaves, wall junctions and service penetrations. Even a short interruption in the insulation at a support deserves attention.

Decide whether to retain the existing ceiling and, if so, how the void above it will behave. Allow damp layers to dry or remove them before closing the roof. Retaining old membranes or extra insulation can alter the drying potential. Document and assess the entire build-up as one roof assembly.

In an occupied building, keep the period without a weathertight roof as short as practicable. Agree the work stages, protection for the interior and a contingency for bad weather. Identify any components needed for stability before stripping the covering. Base the sequence on the structural assessment and the lifting equipment available.

Check that each panel can be delivered and lifted at the specified length. Locate any necessary end laps as shown on the design drawings. Plan when to form penetrations and fit flashings. The installer needs the final detail before making a cut so the finished coating and seals can be kept intact.

At handover, compare the panels, supports, fasteners and seals with the design. Inspect the wall flashings, ridge, eaves and drainage. Remove debris as instructed for the roof covering. Metal swarf left on the surface can mark the coating and cause local damage over time.

Keep a schedule of the installed panels and photographs of details taken before they were concealed. Agree how the roof will be accessed and inspected after its first season and after unusual events. Before adding equipment later, check its loads and how its fixings will transfer them to the structure.