But not all green space designs meet the prescriptive building code provisions set out by different jurisdictions, for example when lining an internal wall with vegetation or when introducing plants and trees into an atrium that needs to meet fuel load limits. Where prescriptive compliance can’t be met, the building owner needs a qualified fire safety engineer to carry out a performance-based fire assessment in order to support the regulatory approvals process.
Balancing architectural vision and life safety
While there have been plenty of studies that have sought to understand the fire behaviour and dynamics of vegetation in bushfires, there’s been little research on the fire performance of vegetation in the built environment.
The lack of data and research in the area offers challenges when it comes to fire engineering and has prompted us to research the topic. Our team is looking into vegetation flammability as part of a Masters in Fire Safety thesis. We’re exploring how different species burn and why. The findings from our research aim to help inform our fire performance analysis and assessment as well as contribute to bushfire management.
As fire engineers, we believe the fire hazard properties of plants are something that should be talked about more both within the built environment and on projects. A more open conversation on fire safety helps build awareness of the risks and opportunities as well as, critically, make sure that buildings integrate life safety with exceptional design.
The fire hazard properties of vegetation
We do know that the fire behaviour of a plant is influenced by its physical and chemical composition, geometry and environment.
Physical and chemical composition
Moisture content is one of the most critical factors that influence ignitability. Plants become more flammable when they have dry leaves, dense foliage, dead leaves and twigs underneath or low moisture. Some plants have more moisture than others and plants dry out when they aren’t maintained.
Some plants contain oils, resins or other flammable compounds that can ignite and burn more readily than others.
Geometry
This refers to the shape of a plant. When vegetation has small components – for example small pine needles, twigs and dry leaves – the speed at which a fire will travel can increase.
We also know that how plants are arranged in a space can influence how they burn and the speed at which they burn. For example, if dense vegetation lines a vertical surface or is in close proximity to other organic (combustible) material, they can create an easy pathway for fire to spread.
Environment
The ignitability of plants is also impacted by external factors like fuel load (how much combustible material is nearby), temperature, access to sunlight and humidity.
Flammability of vegetation is described within the International Journal of Wildland Fire as the combination of four components[2]:
- Ignitability: Time until ignition once exposed to a heat source
- Combustibility: Rapidity of the combustion after ignition
- Consumability: Proportion of mass or volume consumed by combustion
- Sustainability: Ability to sustain combustion once ignited, with or without the heat source
Building Codes that influence the design of green spaces
There are a number of code requirements for combustibility and flammability. These differ depending on the project location and jurisdiction, building height and specifications like if the building contains sprinklers and whether plants are proposed inside or outside the building. Each project team should always seek compliance advice from a qualified code consultant, certifier or appropriate authority.
Where vegetation doesn’t meet prescriptive code requirements, it’s often possible to achieve a performance-based solution supported by a detailed technical assessment and analysis. Our fire engineers at NDY carry out assessments and analysis on projects of this nature globally.
Research into vegetation flammability
Published findings on the fire hazards of plants
Historically, most vegetation research has been heavily focused on forest fires, e.g. pine trees. This is motivated by the economic contribution they make through timber production and potential losses in a bushfire.
Scientists, Dahanayake and Chow, recognised that the overall research and knowledge of indoor plants is limited and carried out a study. Their investigation made a dint in the knowledge fire engineers hunt for regarding plant flammability.
Their study looked at the potential fire hazards of plants in the built environment[3]. They took 3 species of plants – all fresh and green – and left them to dry for 75 days under natural conditions (25°C, 60% relative humidity).
Here are some of the key insights from their research:
Moisture content
This is the amount of wet weight in relation to dry weight.
- The inherent moisture content of the 3 species ranged significantly.
- The plants became ignitable when their moisture content dropped below 30%.
- One of the species became ignitable after just 4 days of no water.
- The moisture content of a plant controls the peak heat release rate and the total heat release.
- The findings demonstrate the importance of maintaining a regular watering system and healthy live plants with a moisture content of at least 35%.
- Inadequate irrigation may lead to drying of the plants and increase the potential fire risk.
Plant choice
The study also showed that appropriate plant choice is as important as proper irrigation and maintenance.
The study’s findings sufficiently prove that fresh live plants are difficult to ignite even when exposed to high radiative heat flux levels. Only once the vegetation started to dry out, over at least 4 days, they began to ignite.
The complex phenomenon of plant flammability
There aren’t any standard test methods to assess the flammability of plants or vegetation and it’s a subject that’s incredibly complex. We need to better quantify flammability characteristics of species to predict their intensity more accurately and control them better. This will limit their devastation to buildings, the environment, people and animals.
As part of our research, we’re investigating different test methodologies to assess consistency of flammability characteristics in vegetation. We’re also looking into the influences of plant structure and geometry and the role these play in flammability.
This research is in its early stages but our findings will be transferrable to our mission of creating safe and well-designed fire safety solutions.
Considerations of a vegetation fire safety strategy
As well as the burning properties of vegetation there are other factors that we consider when undertaking fire engineering evaluations. These include ignition sources, the ability for fire to spread beyond the area of ignition and consideration of potential impacts on egress strategies and fire brigade operations.
Fire engineering strategies may also consider mitigating the consequence of ignition through automatic fire systems, irrigation systems and management regimes, as well as careful selection of containers, support structures and the soil, or other medium, in which it’s grown.
Striving for exceptional and sustainable design
Buildings filled with natural features are welcoming, uplifting and they make us feel happy. However, they need careful planning in order to make them practical, sustainable and lower in fire risk.
Building owners and managers, and project teams, often shy away from developing open green spaces because of the fire risk. Our experience has shown us, however, that creative indoor spaces with vegetation aren’t always an unacceptable fire hazard, although more research would help us quantify this to the relevant authorities and certifying bodies.
Our message to the industry – don’t hold back from creativity. Go green. Go bold. Just make sure you employ a robust fire safety strategy and talk openly about risk to keep your building, people and livelihood safe.
[1] https://www.sciencedirect.com/science/article/pii/S2095263521000479
[2] R. H. White and W. C. Zipperer, “Testing and classification of individual plants for fire behaviour: plant selection for the wildlan-urban interface,” Internal Journal of Wildland Fire, vol. 19, pp. 213-227, 2010.
[3] K. C. Dahanayake and C. L. Chow, “Moisture content, ignition, moisutre content, and fire risk of vegetation in vertical greenery systems,” Fire Ecology, vol. 14, no. 1, pp. 125-142, 201
