ROOF REFURBISHMENT


and keeps the building weathertight throughout. On a live hospital, that last point is usually worth more than the saving.


None of it applies if the insulation is wet. Overlaying


a wet roof does not solve the problem; it seals it in. The trapped moisture keeps working on the deck, the thermal performance never materialises, and the fault comes back three or four years later with a new system sitting on top of it and no obvious cause. So the cores have to be taken across every area rather


than one representative sample and an assumption, and they have to be read correctly. Multiple layers of insulation, added at different times by different people, are easy to misidentify from a single core, and a misidentification at survey stage is a specification error nobody connects back to its origin for years. That is the difference between the two outcomes at


Addenbrooke’s. What the surface looked like pointed to four strip-outs, four skips, and four wards living with a great deal more disruption. What the cores showed pointed somewhere considerably cheaper and quieter.


n What the roof is doing thermally The second is where refurbishment budgets and carbon targets meet. Approved Document L sets a limiting U-value of 0.18 W/m²K for a renovated flat roof. Against that, the 0.39 to 0.52 W/m²K measured at Addenbrooke’s is a lot of heat leaving a building heated 24 hours a day, every day of the year, and it is worth putting in those terms in a capital case, because a hospital is not an office with a heating season. There is a second number worth having. Below a certain level of thermal performance, condensation risk needs actively managing rather than assuming away. The Addenbrooke’s survey identified 0.35 W/m²K as that point, and every measured area failed it. Interstitial condensation is not a dramatic failure mode. It is a slow one, and by the time it shows it looks like a leak. The practical consequence is about timing rather than


product. The insulation upgrade has to happen when the waterproofing does. Doing it afterwards means buying the access twice, on a building where access is the expensive part.


It is also worth recording the existing figures before the


old roof comes off, area by area. They are the baseline you will be measured against later; they are very difficult to reconstruct once the evidence is in a skip, and they are what turns a roof job into something a Trust can put in a business case. A before-and-after U-value by area, the generation figure, and the waste diverted by overlaying rather than stripping, are three numbers that report directly


A roof that has to be replaced anyway is one of the few moments when a Trust can improve fabric performance, add generation, and clear a backlog liability from the same access and the same programme.


against the whole life carbon requirements a refurbishment now has to answer.


Delivering it above a working ward The sequencing questions matter as much as the specification, and they should be answered in writing before anyone starts.


How many areas are open at once, and what is the


weathertight position at the end of each shift rather than at the end of each phase? What happens if the forecast changes at four in the afternoon? Fire strategy during the works is next. On a healthcare


site, the right answer is that hot work has been designed out around vulnerable details rather than managed carefully around them, and the NFRC’s Safe2Torch guidance is the reference point for that conversation. Then the ordinary logistics, which on a hospital are not


ordinary. Deliveries and craneage against ambulance routes. Noise and vibration against wards, theatres, and imaging. Dust and access against infection control. And finally, who is on site holding it together. A specification is a document. Somebody with technical authority present on the works is what turns it into a roof, and on a live hospital that presence is worth specifying rather than hoping for.


The array: what the roof can actually carry With the roof settled, the solar question can be answered properly, and it is the one that most often comes back with a smaller answer than the first estimate. Structural loading comes first. An array, its mounting


system, and, in a ballasted arrangement, its ballast, is a permanent additional load on a deck designed decades ago for something else. That needs checking rather than assuming, and on an older hospital building it sometimes limits the array before anything else does.


Nine questions before a hospital roof goes to tender


n Have core samples been taken from every roof area, and what is the moisture content of each? n What is the existing U-value, area by area, and what is the target? n What is the weathertight position at the end of each shift, and where has hot work been designed out? n Has the structural capacity been assessed for the array, its fixings, and any ballast? n Has the string layout been designed around plant, rooflights, and shading, rather than drawn over them? n Where do the DC cable routes go, and what do they cross? n How will the array be reached for inspection and cleaning in year 10? n Who monitors the system, and who receives the alert? n Who issues the guarantee, who insures it, and what happens to it if the contractor fails?


October 2026 Health Estate Journal 105


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