Environmentally friendly roof-covering options have become increasingly topical in the construction industry over the course of the last decade, and the demand for sedum-herb roofs has increased. Despite this increasing interest, relatively few guidelines exist that relate to how combustible roofs are to be designed so as to not compromise on ﬁre safety.
Sedum-herb roofs do not currently fulﬁl the requirements on ﬁre classiﬁcation (Broof(t2)) that are posed by Boverket’s Building Regulations (BBR), meaning that it is diﬃcult to install this kind of roof covering. As this problem is still relatively new and there is currently no established practise, ﬁre protection consultants oﬅen encounter diﬃculties regarding this kind of roof covering.
Whether green roofs are here to stay or not is diﬃcult to say. However, a simple web search shows that the proponents of this blend of new and old construction techniques are many, and that extensive research into the positive eﬀects of green roofs has been conducted – although there is as yet a lack of understanding of their ﬁre resistance properties. Countries such as Germany, Canada, and Switzerland have gone one step further, passing laws that require a certain percentage of all newly built roofs to be green ones (Oﬃce of the Chief Building Oﬃcial, 2013).
Historically, roof coverings of an organic nature have caused the spread of ﬁres, leading to very extensive urban conﬂagrations. It has been said that the great extent of the Great Fire of London in 1666, which burned down ﬁve sixths of the buildings that lay within the city walls, can be attributed to the thatched roofs of the buildings, which were typical of the day (Koo, Pagni, Weise, & Woycheese, 2010).
The advantages of and incentives for using green roofs
Green roofs have many advantages, and anyone interested can devote many hours to reading scientiﬁc articles that explain the positive impact of green roofs on the climate and environment. Below, a small selection of these positive eﬀects, which taken together can be considered to sum up the rationale behind and incentivisation of using green roofs, are presented (Wong, Chen, Ong, & Sia, 2003):
- Reduction in eﬀects of urban heat islands in con-urban environments
- Insulates the interior of the building
- Assists in management of urban runoﬀ
- Filters pollution during uptake of rainwater
- Reduction in air pollution
- Promotion of biodiversity
- Improved soundprooﬁng
- Extends the life of waterproof membranes
These advantages are directly connected to the structure of a green roof, which is explained in brief below. Describing the structure of green roofs also assists in highlighting the problem of the ﬁre protection of sedum-herb roofs, as the various layers that the roof consists of can generally become involved in a ﬁre to diﬀering degrees.
A sedum-herb roof is generally constructed using a number of layers of varying functions and speciﬁc material properties. A great number of products are currently available on the market, but in general the construction of the roof can be summarised, from the joists upwards, as follows:
A waterproof membrane is placed on top of the joists; this is followed by a root barrier that protects the waterproof membrane from mechanical damage. Then follow, in order, a drainage layer, a waterstoring layer, soil substrate, and vegetation consisting of sedum, grass, and herbs of varying species.
Sedum-herb roofs and ﬁre protection – standards and recommendations
Although a great deal of research has been conducted regarding the positive eﬀects on the climate of green roofs, the topic in relation to ﬁre has been sparsely explored. There is at present no standard or best practice regarding the design and construction of green roofs in Sweden. The current version of BBR contains no regulations regarding how green roofs may be used, leaving only the general roof covering requirement of the BRoof(t2) classiﬁcation. Green roofs are more common in countries such as Germany, Japan, and Great Britain, which is why the guidelines used in these countries are generally better adapted to current sustainability development. Examining these existing guidelines is likely to be of interest when it comes to designing Swedish standards for green roofs.
The current ﬁre-testing method for roof coverings in Sweden
At present, the testing method used for the ﬁre classiﬁcation of roof coverings makes no distinction between organic and inorganic materials. The ability of the testing method to result in a fair classiﬁcation of organic roof coverings can – and should – be questioned. This is exempliﬁed by the fact that prior to testing, a specimen is dried in an oven at a temperature of 105˚C until a constant weight is achieved (SP Technical Research Institute of Sweden, 2016), but in reality this would result in the plants on the surface of the roof covering withering and dying; this procedure does not, then, give a wholly accurate picture of the moisture conditions in sedum-herb roofs in the Swedish climate. A further shortcoming is that, according to current testing criteria, ﬁre propagation along the length of the specimen must not exceed 0.55 m (SP Technical Research Institute of Sweden, 2016). In theory, then, vegetation cannot be allowed to exceed this height if the ﬁre propagation criterion is to be met: The reason for this is that if a 0.6 m-high piece of vegetation were to catch ﬁre and fall on the roof, the roof would be deemed to fail the test.
The suitability of the testing method in terms of green roofs should, with the above problem in mind, be examined in order to provide a clearer picture of, for example, the behaviour of tall sedum plants during a ﬁre.
Conclusions regarding ﬁre protection and sedum-herb roofs
It can be concluded that Swedish rules and regulations are currently not able to factor in the dangers associated with green roofs. It has been found that the diﬀerences in assessments of green roof coverings diﬀer from case to case, and no generally accepted guidance or best practice for the design of such structures can be shown to exist.
This is, however, something that we believe is possible to change. There is much to learn from countries such as Germany, where modern construction techniques for green roofs have developed over the course of many years. Further research regarding safe intervals between buildings with sedum-herb roofs (or roofs with even lusher vegetation) should be performed in order to eventually create ﬁre safety solutions using ‘smart’ engineering.
Roofs covered with sedum herbs and similar must fulﬁl the BRoof(t2) ﬁre classiﬁcation to avoid violating the BBR. As has been discussed above, a sedum-herb roof does not pass this test; this is likely due to shortcomings in the testing method for organic and non-smooth roof coverings, rather than any insuﬃciency in terms of the ﬁre-resistance properties of the roof covering itself.
Tests should be conducted in order to ascertain whether this type of roof covering entails a greater likelihood of ﬁre in itself and, if this is the case, possible protective measures should be investigated.
Sustainable building and climate-smart solutions appear to be part of the future of societal development. Should knowledge of current materials, testing methods, and standards not follow this development, then? Is it not time we looked into how the buildings of the future will impact the way we see safety and building today, rather than taking the opposite perspective?
Texten är skriven av Daniel Håkansson och Alexander Elias från Brandskyddslaget inom ramen för ett examensarbete, och publicerades första gången i Brandposten #56 2017.