When a commercial roof starts to leak, the first question is rarely the right one. People ask how quickly it can be patched. The better question is what is wrong with the roof, and whether the fault is in one place or all over. A single split in a membrane is a repair. A whole roof of tired sheets with failing fixings is a different problem. In between sits the overlay, where a new covering goes over the old one. This guide sets out how we think through the three options on business premises around Basingstoke, what evidence points to each, and what the choice does to cost, disruption and the time the building is protected.
Start by finding out why the roof is leaking
Water in a building is a symptom, and the cause is often elsewhere. On a flat roof, water travels along the layers below the surface before it appears as a stain. On a sheeted roof, it runs down the underside of the profile and drips at a purlin or a joint a long way from the opening. So a good decision starts with tracing the leak and judging the extent of the wear, rather than treating the first wet patch.
We look at four things on the roof. The first is the pattern of the faults: are the leaks clustered at details such as outlets, upstands, penetrations and rooflights, or spread across the open surface? The second is the age and type of the covering, and what has already been done to it. The third is the condition of what lies beneath, that is the deck, the insulation and the supporting steel or timber. The fourth is the building's use, because a cold store, a printing works and a storage shed ask different things of a roof.
Clustered faults at details usually mean a repair will hold. Widespread wear across the surface means the covering itself is failing, and a series of patches will cost more than it saves. Failure below the covering, such as wet insulation or a corroded deck, changes the answer again, because anything laid on top is built on damaged ground. If you want to know what lifespan to expect from different coverings, our page on how long a commercial roof lasts gives the general picture.
When repair is the right answer
Repair makes sense when the problem is local and the rest of the roof is in good order. Typical examples are a split along a seam on a single-ply membrane, a lifted lap on built-up felt, a failed collar around a pipe, a run of perished sealant on a sheeted roof, and a cracked or missing coping on a parapet. The fix is to expose the fault, cut out or clean the damaged section, and make good with compatible materials. The repair must match the roof: a patch of the wrong membrane, or sealant that will not bond to old bitumen, fails again within months.
On metal roofs, repair covers resealing laps, replacing fixings, swapping individual damaged sheets and treating early corrosion at cut edges. Our guide to cut edge corrosion explains when treatment is enough and when sheets need replacing. If the sheets are only rusting at the edges and the steel is still sound, treatment and resealing can add years.
The honest limit on repair is the pattern. Once new leaks appear faster than old ones are fixed, or the same area leaks after each repair, the roof is telling you the covering has reached the end of its life. We say so, because paying for repairs that cannot hold is the dearest option of all. Where repair works, we repair and tell you how long we think it should hold, as an honest opinion rather than a promise.
Overlay: a new covering over the old one
An overlay puts a new waterproof layer over an existing roof without stripping it. On flat roofs this means a new single-ply membrane or a liquid-applied coating laid over the old surface, often with a separating layer and extra insulation between. On sheeted roofs the equivalent is overcladding, which is described in our page on overcladding a commercial roof. Our guide to commercial flat roof systems compares the single-ply, felt and liquid options in more detail.
The appeal is clear. There is less waste, less noise and less time with the building open to the weather. On an asbestos cement roof in sound condition, overlaying can avoid disturbing the sheets at all. The overlay becomes a good choice where the old covering is dry, firmly attached and flat enough to take a new layer, and where the roof structure can carry the extra weight.
The risks are just as clear. An overlay seals whatever is underneath, so wet insulation stays wet, a rotten deck stays rotten and trapped moisture has nowhere to go. It also adds a layer, and there is a limit to how many layers a roof can carry. A roof that has already been overlaid once is often a poor candidate for a second. Before we recommend one, we take cores or cut inspection openings to look at the layers beneath, and we use a moisture survey to find wet insulation where the surface gives no clue.
An overlay also leaves details such as outlets, upstands and parapets at the old level, so they must be raised or reworked to suit the thicker roof. Those junctions are where overlays most often fail, and they take much of the care in the job.
Replacement: strip back to the deck or the frame
Full replacement takes off the old covering and, usually, the insulation beneath it, so the deck or the purlins can be inspected, repaired and covered with a new system. It is the most disruptive and most costly option, and it is the only one that lets you put right everything that has gone wrong, including falls, insulation and structure.
Replacement is the sensible answer when the covering has failed across most of its area, when the insulation or deck is wet or damaged, when the structure needs work, when earlier overlays have used up the roof's capacity, or when you want a different performance from the roof, for example better insulation, a new pitch for better drainage or a different rooflight layout. It also makes sense when a repair-and-overlay route would cost most of a replacement and give a shorter life.
On price, a cost guide's informal tradesperson answer puts a full commercial flat roof strip and replace between £90 and £180 per m², occasionally higher, and flags it as low-confidence. It is a national figure for orientation, nothing more. Your fixed quote follows the survey. For a breakdown of the elements that move the number, such as access, insulation thickness, waste and the system chosen, see our page on commercial roof replacement cost.
Replacement brings its own rules. The roof is a thermal element, so Approved Document L applies to the renovation of a thermal element in a non-domestic building. Replacing a flat-roof membrane, as opposed to overlaying it, generally brings an insulation upgrade into the job. Small re-covering jobs can fall under the Planning Portal exemption for limited work, but a whole-roof strip is well beyond it, so approval is needed. If the roof is asbestos cement, the removal method and the controls for the sheets must be settled before work starts. We handle the Building Regulations side wherever the work is notifiable.
How the options compare
The table below sets the three side by side. It is a guide, not a rule, because the roof in front of you can fall between columns.
| Question | Repair | Overlay | Replace |
|---|---|---|---|
| Where is the fault? | In a few details or sections | Across the covering, with a sound base | In the covering and the layers below it |
| What is beneath? | Dry, sound insulation and deck | Dry, sound, flat and well attached | Wet, damaged or unknown |
| Disruption to the building | Low | Moderate | Highest |
| Structure and weight | No change | Extra load to check | Can be altered or strengthened |
| Regulatory scope | Usually below building control thresholds | Check against the thresholds | Usually notifiable, with an insulation upgrade |
| Expected result | Fixes the fault, roof ages as before | New waterproof layer, base unchanged | A new roof with a fresh life |
The decision in practice
Three questions settle most cases. Is the roof's covering failing in one place or everywhere? Is what lies beneath it dry and sound? And how long do you need the building to stay in service? If the covering is failing in one place and the base is dry, repair. If it is failing everywhere but the base is dry and sound, an overlay deserves a hard look. If the base is damaged, or the roof has used up its layers, replace. If you rent the premises, the lease may decide who pays and who chooses, so check it before you commission anything. Our page on landlord or tenant roof repairs sets out how repair duties usually divide.
Time matters too. If a lease ends in two years or the building is for sale, a sound repair may be the sensible spend even when the roof is old. If the building will run for another twenty years, a replacement with better insulation can pay back through lower heating bills and fewer call-outs. A good adviser will give you the cheap answer when that is the right one, and the long-term answer when the roof needs it.
How the survey works with Beacon
We begin with a free survey and photograph your own roof, including the details, the gutters and any areas where we find damage. Where we need to look beneath the covering, we say so and explain how. The photo report names the cause of each leak, sets out which of the three options fits and why, and shows what we would leave alone.
The quote is fixed and in writing. If a repair will hold, that is what we quote. If the roof has years left in it, the report says so. Work is carried out with the access and edge protection the roof needs, we clear the site, and the workmanship is covered by a ten-year guarantee that we put in writing. If you manage several buildings, commercial roofing in Basingstoke shows the rest of what we cover. For this roof, phone 01256 213956 or use WhatsApp to fix a survey date.
