Maarten de Groot

Presentation on the Workshops 'Brandschutz und Grün', Wien 24.10.2014


European fire classes

Europe has a system of fire classification for building products, which are specified in Eurocodes. In The Netherlands these Eurocodes are implemented into NEN-EN 13501-1.

The Eurocodes are based on the use of building materials including:

·       Wall materials

·       Insulation

Plants or vegetation do not appear in the definition and are therefore not formally considered to be building materials. Therefore, plants fall outside of the scope of the standard.

Legislation and regulations in the Netherlands


Before I address the fire safety aspect of green exterior walls and how to make them fire safe, I first want to give a explanation of the Dutch legislative situation regarding fire safety.

A nationally applicable Building Code is in force in the Netherlands. This Building Code includes a system of functional and performance requirements. The generally formulated functional requirements are met by complying to these formulated performance requirements. The performance requirements are formulated in technical and usage requirements.

The local authority and the fire service are only allowed to perform inspections to check requirements against that laid down in the Buildings Decree. Here they can assume the 'plausibility principle'. The owner and user always remain responsible for meeting the fire safety regulations.

For new-build projects, the authorities having jurisdiction (local authority) is not allowed to impose additional local requirements. This also applies, with some exceptions, to existing projects. For instance, the local authority can impose higher requirements than those that apply as standard for an existing building, if there is a necessity to do so. This necessity can arise due to changed or intensified use. In this case, the local authority must provide reasons for this based on the 'necessity criterion'.

Equivalent safety

In the Netherlands, it is allowed to use equivalent safety to fill in the functional requirements instead of using the performance requirements. This can be done using the principle of ‘equivalent safety’.  According to this principle, the owner can submit a reasoned request to the local authority. In this case, the local authority has the authority to assess the submitted and reasoned request. The applicable test criterion is that the alternative proposal must be at least equivalent to the standard performance requirements. However, the local authority is not allowed to demand a higher requirement than the 'new-build level'.

Fire classes of exterior walls

In accordance with the Building code, the following fire classes apply for exterior walls:

•         Up to two and a half metres                    Fire Class ‘B’.

•         Two and a half   to thirteen metres         Fire Class ‘D’, escape routes Class ‘C’.

•         Greater than thirteen metres                   Fire Class ‘B’.

Five percent of the total surface area is excluded.

Note: Vegetation is not a building material..!  

Definition of fire safety

Up to now, I have addressed fire safety from the viewpoint of legislation and regulations. In summary, I can say that a system of fire classes applies and that plants and vegetation are not considered to be building materials.

Does strictly complying to the fire classification always lead to a fire safe situation? The answer is no! Certainly not for green exterior walls. This is because there can be location-specific circumstances that result in a fire risk.

So when is it correct? To be able to answer this question, you first need to specify the definition of fire safety for the specific situation. The definition that can be used here is:

‘The situation of an acceptable risk related to the outbreak and the consequences of fire, which is also experienced as such by the people involved'.

Fire risks of green exterior walls

So what are the realistic fire risks?

Fire can occur in the direct surroundings of the building, for instance a car fire, a fire in a waste container or a fire due to arson (vandalism). This can cause a fire in the green wall. This can cause a major risk to the rest of the building.

However, a fire in the building itself can also form a risk. For instance, a fire that has reached the flashover point can spread to the building layers above. The flames can cause a fire in the green wall. This can cause a major risk to the rest of the building.

Such risks will probably not be accepted by the involved.


The question now is are you going to strictly comply with the regulations in which you assume that plants are not a building material, or are you going to recognise the actual risks and design a green wall system taking them into account? In the latter case, FSE can be used as a method to do so.

What is FSE? With FSE, you take the actually to be expected fire scenarios into account and not the fire curves that form the basis for the regulations. What Rate of Heat Release is realistic and how will the fire develop? The location-specific conditions determine these factors. For instance, a kilo of solid wood has the same Rate of Heat Release as a kilo of woodchips, though how the fire develops is essentially different.

In FSE, calculation models are often used to map out the development of the fire. These can be sophisticated (semi-)dynamic calculation models like CFD, but in most cases, simpler models can suffice. The calculation models to use are usually validated using realistic fire tests.


·        To use FSE requires both the applicant and the assessor to have a high level of knowledge.

·        Changed circumstances can directly influence the results from the calculation model that is used making them inapplicable.

·        The actual situation can differ substantially from that represented by the fire test.

·        Representative fire tests are more or less impossible for green exterior wall systems. This is because too many parameters must be taken into account, including the type of plants, planting density, moisture content, number of withered leaves and the state of maintenance.


Suggestions seen from the perspective of FSE

Does this mean that a green exterior wall system is impossible? No, it can be used, but you need to use specific criteria and employ the associated measures and provisions. Can these be implemented redundantly?

From the FSE perspective, I can make the following suggestions:

Vegetation criteria:

·        Guaranteed moisture content of the plants greater than thirty-five percent. The required evaporation energy then is so high that a fire cannot          maintain itself independently.

·        The density of the planting must be such that a relatively high moisture content is created with respect to the fuel and air that is present.

·        Effective maintenance is required, for instance, the removal of dead plant material.

·        An irrigation system can ensure that the wall always has a high moisture content. Frost can make this problematic in the winter.

Surroundings criteria:

·        Keep sources of ignition at a safe distance. For instance, ban parking near the exterior wall.

·        Keep flammable material at a safe distance. For instance, waste containers.

Flammable materials in the wall:

·        Limit the use of flammable materials in the wall system.

·        Use non-flammable wall insulation.

·        Use non-flammable plant containers and fasteners.

Flashover from houses:

·        Prevent a compartment fire (flashover) by using an automatic fire sprinkler system. An international (American) standard is available for this. NFPA 13R Residential Fire Sprinklers. The costs are less high than one would expect. They are similar to, for instance, those for floor covering.



In answer to the question of whether green wall systems can be realised in a fire safe manner, the following can be said:

1.    According to the standards, plants are not building material, therefore, from the legal viewpoint there is no restriction.

2.    However, do not base your reasoning on the legal viewpoint but on fire safety considerations and specify the acceptable residual risk.

3.    The use of FSE provides more insight into the scenarios that can be expected, based on which measures can be taken and provisions made.

4.    Note! Fire tests are never completely representative.

Green walls are fire safe as long as they are realised within the criteria mentioned. This implies a challenge for wall builders!