Define roof use and geometry before the covering
A new flat roof specification starts with the building and the way the roof will be used. An extension, dormer, garage and plant roof can present different spans, penetrations, edge conditions, fire considerations and maintenance access. Confirm whether the area is intended only for maintenance or has another designed use; a waterproof roof is not automatically a terrace. Set out dimensions, adjoining walls, rooflights, outlets, thresholds and future equipment before selecting a membrane around an incomplete drawing.
Set the build-up in the correct order
The design should coordinate supporting structure, deck, vapour control where required, insulation, waterproofing and any protection layer as one system. Each layer needs suitable support and compatible fixings, adhesives or interfaces. The sequence also affects finished levels at doors, parapets, kerbs and neighbouring roofs. Ask who is responsible for structural, thermal, condensation, fire and drainage design, and make sure the roofing proposal is based on those decisions rather than leaving site installers to reconcile conflicting heights later.
Resolve thermal and condensation design
Warm, cold and inverted roof descriptions refer to different arrangements of insulation and waterproofing; the labels are not interchangeable shopping options. The appropriate build-up depends on the structure, internal environment, materials and project requirements. A designer should assess heat loss, thermal bridges, air and vapour control and condensation risk for the proposed work. Current Scottish building standards and any warrant requirements should be checked for the project rather than relying on a detail copied from another property.
Design falls and drainage before the membrane
A flat roof needs a deliberate route for rainfall. Draw the direction of falls, primary outlets, gutters and any overflow provision that the design requires, then test how the geometry behaves around rooflights, crickets, parapets and changes of level. Allow for construction tolerances rather than assuming a nominal fall will remove every low spot. Outlet bodies, leaf guards and connections must be compatible with the chosen membrane and remain reachable for inspection after the roof is complete.
Draw every junction that can interrupt water flow
The main roof area is usually simpler than its perimeter. Abutments, wall chases, edge trims, parapets, thresholds, rooflight kerbs, pipes, ducts and changes between materials need project-specific details. Confirm upstand heights and interfaces before windows, cladding or doors fix the available space. Where several trades meet, assign responsibility for each substrate and final seal. A neat plan of junctions is more useful than a product name because it shows whether the proposed system can actually be formed around the roof.
Select the system and installation method together
GRP, reinforced felt, EPDM and single ply use different substrates, jointing methods, accessories and weather constraints. System choice should include how it is attached, how seams or laminates are formed, how temporary moisture is controlled and what manufacturer instructions govern the work. Roof size, movement, wind design, complex detailing and other trades may favour different approaches. Ask for one coherent specification, not a mixture of components chosen independently because they share a generic material label.
Ask for useful handover information
Before work begins, agree what completion information is expected. Useful records can include the final specification, product and colour references, photographs of concealed stages, outlet positions, approved changes, care restrictions and inspection guidance. Keep them with building records so later repairs do not begin by guessing the membrane or build-up. For an enquiry, share drawings, intended roof use, project stage, known structural information, access constraints and the design decisions that are still open.
