GREEN ROOFS
That said, the water retention capacity of a green roof is not fixed – it shifts with the season and the existing moisture content of the substrate, and green roofs should never be specified as a direct replacement for properly engineered drainage. Used as part of a broader SuDS strategy, though, the contribution is real and quantifiable.
Regulating temperature in buildings where it matters most For healthcare buildings, maintaining consistent internal temperatures is much more than a matter of comfort – it is a clinical requirement. Extremes of heat or cold can directly affect the health and recovery of vulnerable patients, and green roofs offer a passive mechanism for moderating those extremes while also reducing energy demand. Research by Nottingham Trent University,
published by the Green Roof Organisation, compared temperatures beneath the roofing membrane of a conventional roof and a green roof across different seasons. In summer, the temperature beneath the standard roof reached 32°C – well above the recorded average outdoor temperature of 18.4°C. Under the green roof membrane, the temperature was just 17.1°C. The picture was similarly marked in winter: beneath the green roof, the sub-membrane temperature held at 4.7°C against an average air temperature of 0°C, while the conventional roof sat at just 0.2°C. Those temperature differentials mean real reductions
in the load placed on mechanical heating and cooling systems, with the savings accumulating across a building’s lifetime. On city-centre hospital sites, the urban heat island effect compounds the problem further. Hard surfaces absorb heat during the day and release it slowly overnight, pushing local temperatures above those of the surrounding area and placing additional strain on patients with respiratory or cardiovascular conditions. Green roofs disrupt that cycle.
Getting the specification right All of this said, a green roof will only perform to its potential if the system beneath it has been properly specified. In a healthcare setting, where a roof leak or waterproofing failure carries consequences that go well beyond inconvenience, the quality of the underlying build- up deserves close attention. The waterproofing membrane sits at the heart of the build-up, and in a green roof context it has to do more than keep water out, it needs to resist root penetration over the full life of the system. Bitumen membranes modified with root-repellent additives, reinforced single-ply systems and hot melt waterproofing, are all established approaches for green roof applications, each with characteristics that suit different substrates and build-up configurations. Whichever system is selected, building regulations require waterproofing detailing to finish a minimum of 150mm above the roof surface – a figure that rises in line with the depth of the green roof build-up and needs to be resolved in the parapet and upstand design well before work starts on site. Falls must be incorporated in accordance with BS
6229 to ensure adequate drainage across the roof plane, including beneath the green roof substrate. Poor drainage at this level leads to water ingress, increased structural loading and potential damage to the membrane over time. For insulation, extruded polystyrene (XPS) is well- suited to green roof applications – it has the compressive
Green roofs slow the passage of water into drainage systems.
strength, moisture resistance, and thermal stability needed to perform reliably in an inverted roof build-up where it sits above rather than below the waterproofing layer. Loading needs to be established at structural design stage. An intensive system can reach around 282 kg/m², against approximately 30 kg/m² for a sedum-based extensive roof – a difference that will determine what is structurally viable on any given building. Saturated substrate adds further weight after heavy rainfall, and this variation needs to be captured in the structural calculations from the outset. This is also where early engagement with a specialist manufacturer pays dividends. On a complex healthcare roof with varying falls, multiple drainage zones, and the added demands of a green roof build-up, that input at the front end of a project is considerably more valuable (and considerably less expensive) than resolving problems once the design is fixed.
Planning for the long term Specifying a green roof well at the outset is half the job. Keeping it performing as intended over a 20, 25, or 30-year lifespan requires ongoing commitment to maintenance. Drainage requires particular attention. As vegetation matures and organic matter accumulates in the substrate, outlets can become progressively blocked – with consequences for water ingress and structural loading that worsen the longer the issue goes unaddressed. A maintenance programme agreed and documented at handover – rather than left to be arranged later – is the most reliable safeguard against this. Irrigation is another factor easily overlooked at the design stage. Extensive sedum roofs are broadly self- sufficient, but intensive roof gardens with deeper substrates and a wider plant palette will need a reliable water supply – and as UK summers continue to trend drier, that requirement is only going to become more pressing. Getting irrigation into the building services from the outset is far more straightforward than retrofitting it. Green roofs are not a new idea in healthcare, but
the case for them has never been stronger or better evidenced. For estates teams willing to invest in the specification properly, they offer something relatively rare in building design: a single element that simultaneously serves the building, the environment, and the people inside it.
Mike Vaczi
Mike Vaczi is technical director and an executive board member at SOPREMA UK Ltd, with over 30 years’ experience across roofing, waterproofing and building envelope systems. He leads the UK technical function, with responsibility for technical strategy, compliance, and certification, including oversight of BBA and KIWA approvals. Mike manages a multidisciplinary technical team providing support across CAD drawings, specifications, U-value calculations, roof surveys, contractor training, and warranty processes. He works closely with internal teams, contractors and industry bodies to support the application of SOPREMA systems in line with current standards and regulatory requirements, including Approved Document B and relevant British Standards.
October 2026 Health Estate Journal 169
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