Fire resistance of green roofs in the context of current regulations

In the last decade, green roofs have moved from niche ecological solutions to strategic tools for sustainable urban development. They reduce the urban heat-island effect, improve stormwater management, and contribute to carbon dioxide capture. However, their expansion raises a fundamental question: how safe are these systems in the event of a fire, and how do they relate to the fire-protection regulations of the European Union and Romania?

Vegetation as a potential fuel

Flames do not take the designer’s ecological intentions into account. Experimental studies show that the plants and substrates used in green roofs can act as continuous fuel when dry and exposed to ignition sources. Tests carried out in Canada using a cone calorimeter showed that a 2.5 cm thick substrate containing 15% organic matter, when dry, develops a heat release rate of around 95 kW/m² in the first minute of combustion, but the flame extinguishes rapidly, and the heat-release rate drops below 30 kW/m² after two minutes. When wet, the same mixture presents virtually no ignition risk.

On extensive green roofs, where the substrate is crushed volcanic rock, the deeper layers of the green roof do not ignite even when dry.

By comparison, a standard bituminous membrane commonly used in waterproofing releases seven times more thermal energy and burns for more than three times longer. Therefore, a green roof is by definition safer than a conventional one; the already low risk depends on the substrate’s moisture, the composition of the organic material, and the type of vegetation.

Succulent plants such as Sedum species can retain water and limit the spread of fire. In contrast, dry grasses in biodiversity-type vegetation can act like a wick, especially during drought periods. European experiments have shown that dead vegetation can reach peaks of 300–400 kW/m², but the burning is short — usually under two minutes.

Thermal load and system behavior

To assess the real risk, analyzing fire-load density is essential. An extensive green roof with low vegetation and a thin mineral substrate adds only about 15–20 MJ/m² to the overall fire load — equivalent to 20% of that of a bituminous waterproofing layer. An intensive roof with shrubs and soil rich in organic matter, however, can reach up to 85 MJ/m². Even so, these values remain below the thresholds considered critical for roof fire-safety evaluations, especially when the roofs are irrigated (mandatory for intensive systems) and have high moisture content.

Thus, the issue is not the amount of fuel but its distribution and the system’s ability to limit flame spread. In the case of a localized fire, the mineral drainage layer and non-combustible barriers around technical elements (gravel strips) can prevent the fire from spreading.

International lessons and the role of legislation

The development of green roofs in China provides a clear example of how public policy can shape not only the expansion but also the safety of such systems. Cities like Shanghai and Chengdu have introduced technical design standards, including requirements for load capacity, drainage, and fire-resistant materials. However, the lack of a unified national framework and high maintenance costs have slowed large-scale adoption.

In Europe, the regulations are clearer. The EN 13501-5 standard classifies roofs according to their reaction to fire, with the BROOF(t1) class being the safest, ensuring that the system does not sustain combustion when exposed to external flames.

In Romania, the regulatory framework was updated in 2025 by the Fire-Safety Norm for Buildings, code P 118/1-2025, approved by Order No. 267/2025 of the Ministry of Development, Public Works and Administration, published in the Official Gazette of Romania, Part I, Nos. 204 and 204 bis of March 10, 2025.

This document, which entered into force 60 days after publication, introduced important clarifications: green roofs must be designed so that the vegetation layer, substrate, and auxiliary materials meet the permitted fire-reaction classes, and non-combustible buffer zones must be provided around penetrations and at the edges of buildings.

In addition, the norm introduces the concept of a building’s fire-stability level, correlating the fire resistance of roof components with the building’s height and function — an approach that strengthens designers’ responsibility in choosing green-roof systems.

Modern design solutions include mineral substrates with less than 20% organic content, fire-resistant gravel or ceramic-tile barriers, and automatic safety-irrigation systems. Combined with regular maintenance — removing dry vegetation, checking drains, and cleaning the drainage layer — these measures ensure controlled behavior in case of fire.

Moreover, the development of biosolar roofs, which integrate photovoltaic panels and vegetation, brings a new challenge: the interaction between electrical and biological components. This combination requires updated regulations on grounding and fire resistance for mixed systems.

The fire resistance of green roofs is not a structural weakness but a balance between materials, moisture, design, and maintenance. Experimental data show that a well-designed green roof can be safer than a traditional one, and its ecological benefits make it a solution for the future.

For Romania, full alignment with European standards and the integration of BROOF(t1) and P 118/1-2025 requirements into all new projects — including biosolar systems — is not just a legal obligation but an investment in the safety and sustainability of the built environment.