Know what the replacement decision needs to solve
Replacement should address a defined roof-wide problem, not simply exchange one surface for another. The brief may need to resolve recurring ingress, an unreliable patched covering, wet insulation, an unsound deck, ineffective falls, constrained outlets or details that cannot be repaired coherently. Record which concerns are confirmed and which remain assumptions. This keeps renewal focused on the cause and lets an assessor explain whether part replacement, full strip-up or another route deserves consideration.
Establish the existing build-up and moisture risk
Before selecting a new system, identify the visible covering, deck type where known, insulation arrangement, vapour control provision, attachment method and any later overlays. Internal staining and surface condition cannot show every concealed layer. A survey may recommend targeted opening-up or moisture investigation where the result would change the scope. Agree how disturbed areas will be reinstated, and treat old drawings or invoices as useful evidence rather than proof that the roof was built exactly as documented.
Choose strip-up or overlay from evidence
A strip-up exposes retained structure and allows affected layers to be removed, but it also changes waste, sequencing and temporary-weathering requirements. An overlay may reduce disturbance where the existing build-up is dry, stable, compatible and able to receive the proposed system. It should not conceal unresolved water, weak decking or poor falls. Ask each quotation to state what is retained, how compatibility and loading are checked, which details are raised or renewed and what happens if hidden defects appear.
Use renewal to resolve falls, outlets and insulation
Whole-roof work creates an opportunity to coordinate drainage and thermal design with the new waterproofing. The specification should show how water reaches outlets, how low points and overflows are handled where relevant, and how added insulation affects kerbs, thresholds, parapets and upstand heights. Condensation risk and current Scottish building-standard requirements are project-specific design matters. They should be checked for the building and proposed work rather than reduced to a generic thickness or standard detail.
Compare coverings against geometry and use
System choice follows the roof. GRP may suit stable, relatively simple areas where a rigid laminated finish can be detailed correctly. Reinforced felt offers a layered bituminous build-up. EPDM uses flexible rubber sheets with compatible seams, adhesives and terminations. Single ply brings a designed membrane and attachment system that can suit larger or more technical roofs. Compare how each proposal handles movement, penetrations, plant, maintenance traffic, fire and thermal design, outlets, edges and future repair access.
Make competing replacement scopes comparable
Ask for roof area and assumptions, removal or retained layers, deck repairs, insulation and vapour-control work, formation of falls, membrane specification, outlets, edges, rooflights, penetrations, access, waste and internal making good to be identified. Provisional items should explain what triggers them. A headline total without that breakdown can hide material differences between proposals. The replacement cost guide helps organise these variables without presenting a fixed rate that ignores the actual roof.
Plan access, sequencing and the next decision
Share occupancy constraints, permitted access points, parking or loading issues, sensitive areas below the roof and any times when noise or opening-up would be especially disruptive. Ask how the building will be protected while old layers are exposed and how unexpected deck or moisture findings will be communicated before scope changes. The next step is an assessment that can confirm measurements and constraints, not a commitment to a material based only on photographs.
