How do you connect a new sandwich panel roof to an existing building?

Identify the existing members that will carry the new loads. A similar arrangement on a neighbouring building gives no proof of capacity. Have the designer assess the load path all the way to the foundations. Even an extension with its own frame needs a designed connection to the existing structure.

An extension changes roof geometry and the effects of snow and wind. Snow can drift against a higher existing wall, creating a local load. A span table for a simple exposed roof cannot account for that condition by itself. Select panels, supports and fixings for the actual arrangement.

The new frame may settle or deflect differently from the original structure. Design the junction to stay weathertight through the expected movement. Determine movement allowances and any expansion joint from the design. A rigid flashing or an arbitrary gap left on site cannot establish the required movement capacity.

Measure the levels at the wall, supports and eaves. Check the panel manufacturer’s minimum pitch for the chosen arrangement. End laps, rooflights and other interruptions may affect it. Allow for tolerances and deflection so water will not pond on the finished roof.

Coordinate the old and new drainage systems. Trace runoff from the higher roof and check that its discharge point is protected. Size gutters, outlets and overflows for the catchment and building layout. Agree downpipe positions and any penetrations before installation.

Shape the wall abutment to shed water outwards and prevent it from getting behind the flashing. Check the substrate, fixing method and freedom of movement. A surface bead of sealant works only as part of a detail suited to both the roof covering and the wall.

On the inside, continue the insulation and airtight layer across the junction. Metal parts can create a thermal bridge, particularly in a heated extension. Show the core closure and seals on the drawing. Leave them accessible for inspection before closing the ceiling or wall lining.

Sequence strip-out, support installation and closure of the new roof. If the existing building is occupied, protect the interior from water and debris. Arrange temporary weather protection before work begins so the programme can cope with rain while the roof is open.

Check access for delivery and lifting. On a confined site, panel length can determine whether installation is feasible. Resolve the choice of lengths and joints before production. Include lifting equipment, safe work at height and securing the elements in the installation plan. Never use panels as improvised working platforms.

At handover, inspect flashings, seals, core closures and drainage, particularly where new materials meet existing roof layers. Check surfaces around cuts and remove debris as instructed by the manufacturer. Keep photographs of concealed components taken before covering them.

Arrange another inspection after the first rainfall and temperature changes. A crack at the junction can be a sign of movement that needs investigating. Record the as-built detail and limits on subsequent alterations. Reassess the roof before adding equipment or making a new opening.