Fire is important, but unpredictable and powerful force - if not controlled it has destructive consequences for our lives, property and nature itself. Use of non-combustible materials is thus one of key prerequisites in building sector, ensuring safety and health of inhabitants.
Roof is a part of building envelope. It provides protection against rain, snow, sunlight, extreme temperatures and wind. It must be functional and durable, while offering security, thermal protection and fire safety. Its presence is vital and yet, roof often remains unnoticed and is taken for granted.
Nowadays, with scarce ground space, roofs are becoming much more. Beside its architectural and aesthetic aspect, roof can be a terrace, garden, green roof, it can host solar panels or it can be turned into attractive habitable multifunctional (public) place.
Green roofs and fire safety challenges
Green roofs need to be, alongside other building elements and systems, fire-proof. At this point, it is important to be reminded about two general types of green roofs – intensive green roofs and extensive green roofs. Key characteristics of intensive and extensive green roofs significantly determine their response to fire risks.
Intensive green roofs have thicker substrate layer with higher organic content and wider plant selection – from grasses, shrubs, large perennials to trees. Think of highly visible presence or publicly accessed roof gardens, walking terraces, raised bed gardens, etc. - these attractive spots need special rooftop substrate with organic content and should be regularly irrigated and looked after.
Here you need always to keep in mind the following: to mitigate fire risks, FLL guidelines define that the substrate has maximum of 20% of organic matter. The organic material used in green roofs should be peat free to minimize fire risk. Project owner should therefore always check exact substrate characteristics on intensive green roofs for the safety reasons. Additionally, if plants on intensive green roof are not regularly maintained and irrigated, you have a significant risk of rooftop plants drying out. With dried grass, shrubs and perennials on the rooftop, fire could spread out even easily and faster. So, beside substrate composition, the dried grasses or shrubs represent another potential fire risk of intensive green roof.
Intensive green roofs are according to FLL, resistant to flying sparks and radiant heat when they meet requirements for substrates (low organic matter) and are made with firebreaks / un-greened horizontal strips of non-combustible surface materials.
On the other hand, extensive green roofs require less irrigation and maintenance. Extensive green roofs are planted with hardy, shallow rooted plants and do not require high organic content in growing medium. Maintenance still needs to be regular, but less frequent compared to intensive roofs. Majority of extensive green roofs are planted with Sedum plants. Urbanscape green roofs consist of several Sedum species, which provide green cover and colourful flowers through the seasons. Sedum plants are plants in the succulent family. Succulents have thick, fleshy leaves that help to make it drought resistant. Sedums store water in its leaves, stems, or roots which makes it easy for the plant to survive long periods between watering. This is important feature, not just during the drought survival, but also because these plants are not flammable as grasses and shrubs.
More about sedum on Urbanscape green roofs can be found here: https://blog.urbanscape-architecture.com/6-facts-why-sedum-plants-are-best-option-for-extensive-green-roofs
Regardless the green roof type and composition, the regular maintenance is key success factor. In case of fire prevention measures, checking and cleaning firebreaks such as gravel strips or upstands, keeping them free from any combustible materials is of vital importance. They need to be kept clean and without any vegetation.
Fire load of a green roof
Fire hazard of a building or any of its part can be expressed by so-called fire load. The quantity of combustible materials presents a certain fire load, usually expressed as the amount of energy per area (fire load density, MJ/m). It can be calculated by knowing the mass of fuel per area (fuel load) (kg/m) and the energy content of this fuel (MJ/kg). Gerzhova et al. (2020) studied and calculated the fire load for green roofs, taking into account the vegetation type, growing medium and green roof category.
For intensive green roofs, the fire load density was as much as quadrupled value compared to extensive one. This again confirms the higher potential fire risk of intensive green roofs. Fire risk also quickly increases with every percentage of organic matter in the substrate, with the presence of peat in the substrate and with the presence of larger vegetation (shrubs, tall grasses). Detailed results are shown in Table 1.
She studied three green roof categories in her work, see Figure 1 bellow.
From this research it is evident, that intensive green roofs bring higher fire risk compared to extensive one.
Ensure green roof fire safety in few steps
- Check the green roof system certification.
Urbanscape green roof characteristics are described in Technical data sheets (TDS) and environmental impact disclosed in Environmental product declaration (EPD), all published on our website https://www.urbanscape-architecture.com/ . Technical features together with Fire resistance class are seen in Table 2.
Table 2: Urbanscape green roof system characteristics.
Additionally, Urbanscape green roofs achieved the highest standards of fire safety with Broof (t1), (t2), and (t3) certifications[1]:
- Broof (t1): Basic fire resistance, protecting against the spread of fire from external sources.
- Broof (t2): Enhanced fire resistance for more severe conditions, ideal for higher risk areas.
- Broof (t3): The highest level of fire resistance, ensuring maximum safety even in extreme fire scenarios.
2. Choose appropriate plants as a key to safer green roof design
Experts recommend at least 60% of a roof’s vegetation should come from the sedum family of groundcover plantings. It is also best to avoid grasses and mosses, which can dry out and create a potential fire hazard (Clagget, 2019).
[1] The standard is based on the test methods contained in ENV 1187:2002. The test methods (t1, t2, t3 and t4) evaluate the fire performance of roofs/roof coverings under the following conditions: test 1 assesses the performance of a roof exposed to burning branches; test 2 assesses the performance of a roof covering exposed to burning branches and wind; test 3 assesses the performance of a roof exposed to burning branches, wind and supplementary radiant heat.
3. Use peat-free substrate, with at least of 80% inorganic content.
Urbanscape green roof thrives on a light weight mineral growing media. Read more:
https://blog.urbanscape-architecture.com/green-roll-as-a-light-weight-growing-media
4. Maintain your green roof
You should inspect your green roof once or twice a year. Border zones must be free of vegetation, keep fire breaks near and around rooftop equipment and structures clean. For irrigation needs, consult your green roof provider.
Following these four simple steps will lead to lushy green rooftop. Learn more about Urbanscape green roofs https://www.youtube.com/watch?v=ZA0E7oWb5Ds
Literature:
- Clagett, L., 2019. Ways to Design a Fire – Resistant Green Roof System. Architectural Record.
- Gerzhova, N., et al., 2020. Flammability characteristics of green roofs. Buildings 2020, 10, 126.
- Urbanscape TDS and Urbanscape EPD.
