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.
Commercial roofing 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.
Commercial roofing repairs, overlay or strip: which is right
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.
Commercial roof installation: insulation, falls and what to 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.
What a commercial flat roof replacement costs per square metre
Commercial roofers quote by the square metre. S Jones Roofing gives £60 to £120 per square metre for replacing a flat roof, Total Cladding and Roofing works on £120 to £170 to replace, and Nationwide Industrial Roofing gives £90 to £180 for a strip and replace. Duratite installs a complete GacoFlex coating system from £40 per square metre. Access, insulation thickness and falls explain most of the spread.
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 replacement go ahead 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.
Sources
- Approved Document L: conservation of fuel and power (GOV.UK)
- Approved Document H: drainage and waste disposal (GOV.UK)
- S Jones Roofing: the cost of replacing a commercial roof
- Total Cladding and Roofing: commercial roof repair vs replacement
- Nationwide Industrial Roofing: cost of industrial roof repairs
- Duratite: industrial roof waterproofing