The short answer
Repair a commercial roof when the defects are isolated, the deck and insulation are dry and the covering still has useful life. Replace it when leaks reappear in fresh locations, the insulation is saturated, corrosion or ageing is widespread, or repair bills are approaching the cost of a new covering. A survey with moisture checks settles the question on your particular building.
The questions that decide it
Surveyors tend to run through the same short list. What share of the roof is involved? Is the damage local, like a parted seam, a raised flashing or a cracked rooflight, or has the whole covering worn out? What is under the covering? Is the deck sound and is the insulation dry? How old is the roof compared with the typical life of its system? And how much has already been spent on patching?
None of these questions gives a verdict alone. A twenty-year-old felt roof with one clear defect might be worth repairing, while a younger roof built on a wet deck might not. The answer comes from putting the points together, not from a rule of thumb on age.
It also helps to separate the cost of this year from the cost of the next ten. A repair is cheaper today, but if it only postpones a replacement by a season or two, you will have paid for both. The reverse is also true: replacing a roof with years of life left in most of it is money spent early. For typical lives by system see our answer on how long a commercial roof lasts.
When repairing is the sensible choice
A repair earns its place when the problem is specific and the surroundings are healthy. A lifted lap on a metal sheet roof, a worn seal around a roof vent, a blocked outlet, a split in a lead flashing and a cracked rooflight are all faults a repair solves for years. So is a blister in felt that has not let water into the insulation.
Repair also suits a roof whose covering still has useful life and whose condition is even. If a survey finds the insulation dry across the area, the deck sound and the seams or laps generally tight, then fixing the few failures extends the roof at modest cost. A written note of what was repaired, and why, becomes part of the roof's record.
Good repair is not the same as a quick patch. It means finding the cause, so that if water got in at a failed upstand the upstand is rebuilt, not just smeared with mastic. A patch over a symptom usually fails again within a season, and the visits add up.
When replacement is the better answer
A set of signs points towards replacement. Leaks come back in new places after each repair. The same area has been patched several times. The surface is crazed, cracked or open at many joints. The insulation beneath is soaked, as a moisture survey confirms, and soaked insulation both reduces thermal performance and keeps feeding water to the deck. On metal sheet roofs, widespread rust at the cut edges and fixings tells you that the steel is losing its protection across the whole roof, and our page on cut-edge corrosion describes how it spreads.
Another sign is a roof that no longer drains. If water lies for good because the deck has settled, or the slope to the outlets was never laid properly, no amount of patching will cure it. A new build-up can include tapered insulation to put the falls right.
Where the covering has reached the far end of its typical range and repair costs keep rising, the replacement is simply a question of when. Planning it on your terms is better than being forced into it by a failure in January.
The middle options: overlay, recover and part replacement
Between a patch and a full strip sits a range of choices. A section of the roof, say the worst bay, can be replaced while the rest is repaired. A new membrane can go on top of the existing one, which is called an overlay or recover. It saves stripping and disposal, but it adds weight, can hide wet insulation, and is only possible when the existing build-up is sound. See our page on commercial roof overlay.
A protective coating may add years to a sound roof, which makes it a different job from re-roofing. On a profiled metal roof, coatings are applied once the sheets have been cleaned and treated for rust, and they work only where the metal is still strong enough to carry the coating.
Which option fits depends on the survey. A larger scheme also raises the Building Regulations question, which our answer on commercial roof building regulations deals with.
Costs, timing and what to ask for
Ask any contractor to separate the options in writing: a repair scope, a part-replacement scope and a full replacement, each with its own price. That lets you set the repair cost against the cost of replacing and judge the remaining life of each. A commercial roof cannot be priced from the road, since system, insulation, access, edge details, rooflights and drainage all change the figure.
Timing matters too. Some systems are better laid in warmer months, and our answer on commercial roofing in winter looks at which systems tolerate cold weather. Disruption matters as well, and you can read how work is arranged around a working building in roof work while the business stays open.
Where repair is enough, we recommend it and price it that way. The survey is free, and the quotation that follows is fixed and in writing. The overview is on our commercial roofing page.
