LifeMedGreenRoof — Instrumented Mediterranean Green Roof Stormwater Retention, Msida, Malta

LifeMedGreenRoof (LIFE12 ENV/MT/000732) built and instrumented demonstration green roofs at the University of Malta in Msida, with a comparison roof at Fondazione Minoprio in northern Italy, in a country with almost no existing green roof stock. The coordinating beneficiary was the University of Malta's Faculty for the Built Environment, with the Malta Competition and Consumer Affairs Authority, Fondazione Minoprio and Minoprio Analisi e Certificazioni as partners. Beyond the roofs themselves, the project developed Mediterranean-adapted growing media and a plant palette of local species, and drafted a national green roof standard and a policy proposal for government.
The stormwater evidence is genuinely measured rather than modelled, and this is what distinguishes the case. The monitored Malta plot has a surface area of 32.61 m2, infilled with 200 mm of 'Malta 1' substrate over screed laid to 2% falls, draining through a single 50 mm pipe to a tipping-bucket counter backed by a precision balance for sub-litre flows. Monitoring ran a full hydrological year from July 2016 to June 2017.
Total precipitation over the year was 348 mm. Because the tipping counter failed between 18 November and 4 December, the period to 9 December was excluded, leaving a valid measurement window with 276.4 mm of precipitation and 37.39 mm of run-off. Per-event water retention ranged from 40% to 100% with a mean of 90%. Of 74 rain days, only 8 produced any run-off at all, with run-off volumes between 0.03 mm and 11.84 mm. For comparison, the FFL reference value for extensive green roofs with 15 to 20 cm of substrate is 60% annual retention at a discharge coefficient of 0.40, for regions receiving 650 to 800 mm of annual precipitation.
The Italian comparison roof at Fondazione Minoprio, with 10 cm of substrate, recorded 963 mm of rainfall and 571 mm of run-off over the same twelve months, retaining 392 mm or 41% of total rain, with monthly retention varying between 15% and 65%. That figure belongs to Italy and should not be attributed to Malta.
The authors are unusually candid about the limits of their own result. They record that the winter of 2016 to 2017 was particularly dry, which allowed ample time between rain events for water trapped in the growing medium to evaporate, and they note that applying the results to an urban watershed to estimate real flood mitigation was outside the remit of the study. They state only that green roofs in Malta 'can contribute significantly to reducing storm water run-off and mitigate localised flooding', and explicitly identify the watershed-scale question as future work.
The honest assessment is that this is a research plot, not a city programme. The monitored area is 32.61 m2 over a single atypically dry year, monitoring was carried out by the implementer rather than an arms-length evaluator, and no peer-reviewed journal article from the project was located. What raises confidence is the transparency of the instrumentation, the publication of the raw monthly series, and the authors' own statement of the caveats. Its transferable output is the substrate formulation, the local plant palette and the draft national standard, which is the mechanism by which a 32 m2 result could become policy in a Mediterranean island state with minimal green roof stock.

Read the full analysis: https://lifemedgreenroof.org/wp-content/docs/Flood_Mitigation_Report_Malta_&_Italy.pdf