Most commercial buildings in Britain wear a flat roof, and most of those are single ply membrane, reinforced bitumen or liquid applied. They fail in ways an owner can predict, and the decision between repairing one, laying a new system over it and stripping it back to the deck is nearly always answered by how long the occupier needs the building rather than by the state of the membrane alone.
The systems, and how each one fails
Single ply membranes are welded rather than bonded at the joints, so they fail where a weld was rushed, at upstands, and around anything drilled through the roof after it was laid. Reinforced bitumen membranes are laid in layers and fail at laps and details, usually slowly and usually repairably. Liquid applied systems are seamless and depend entirely on the preparation underneath, so a coat over a dirty or damp surface delaminates. On all three, the outlets and the upstands do more damage than the field.
Repair, overlay or strip
A repair is right where the rest of the roof has years left and the defect is local. An overlay lays a new system over the existing one, which suits an occupied building because there is no open deck at any point, and it only works where the existing build up is dry. A full strip and replacement resets the service life and the insulation, and it is the option that also lets you fix the falls. The wrong one is the cheap patch on a roof whose insulation is already saturated, because the water is inside the build up and will find another way out.
Insulation, falls and what an owner should ask for
On a commercial re-roof the insulation is usually the largest material cost and the one that changes the building's running cost, so its thickness is worth understanding rather than delegating. Falls are the second question: a roof that ponds will fail again whatever is laid on it, and a tapered insulation scheme is the usual way to correct falls without touching the structure. Ask for a moisture survey before an overlay, because it is the one test that tells you whether the existing build up is dry.
Questions people ask about commercial flat roof repair
How long does a commercial flat roof last?
Long enough that the deck, the insulation and the detailing decide the answer rather than the membrane's published life. A well laid roof with real falls on a dry deck outlives a premium membrane over a ponding one.
Can a commercial flat roof be replaced while we trade?
Usually yes, in bays, with each bay made watertight before the end of the working period. It costs more per square metre than an empty building and much less than closing.
Does a commercial flat roof replacement trigger the Building Regulations?
Replacing a thermal element such as a roof brings energy efficiency requirements into play for that element, which in practice usually means upgrading the insulation at the same time.