Dr Tom Young, Design – Blue and Green Infrastructure Associate at TEP, looks at the performance of green roofs in winter and the impact of colder temperatures on green roof systems…
To the untrained eye, green roofs in the winter might appear a lifeless dull looking carpet, waiting for spring to wake up and start pulling its weight. This is not quite the case, and I’ve dug into the academic literature to show why. I have also tried to find studies that show that green roof design must take into account winter conditions for the long-term health of the roof. It must be said that there isn’t actually much literature on green roofs in temperate climates during winter months, but I have tried to find as much material as possible…
The most commonly researched theme I found was the insulating effect a green roof provides its host building during winter months. The layer of substrate and vegetation acts as a large blanket to the building, reducing heat flux out and also into the building. On cold, clear, sunny days, this actually reduces the amount of heat gained by the building. However, this is more than offset by the reduction in heat loss during the night and on non-sunny days, with a total monthly reduction in heat loss of 8-14% attributed to the green roof (Arkar et al. 2015). Interestingly, a number of studies have shown that when it is colder, this effect actually increases. This is because a frozen substrate has a lower ability to transfer heat through it, and so a building will lose heat at a slower rate (Arkar et al. 2015 & Collins et al. 2017). The effect can occur with moisture, with saturated substrates conducting heat at a faster rate than drier substrates as they act as ‘thermal bridges’ (Collins et al. 2017).
Snow has a beneficial effect on thermal insulation, with large build ups of snow acting as a really good blanket. This reduces the effect of the green roof, as a large amount of insulation is already being carried out by the layer of snow (Lundholm et al. 2014, Collins et al. 2017 & Juras 2022).
In order to optimise a green roof system for insulation (if that’s what you want), certain parameters of the roof can be altered. This is the same for any green roof service, with care needed at the design stage of a roof to really decide what the roof is for, and what services should be optimised. In terms of winter (and most likely summer) insulation, deeper substrate, increased complexity of vegetation and reduced winter moisture are the most important variables (Eksi et al. 2017, Wei et al. 2021). To be fair, get these three right and most green roofs will be successful!
One of the few studies that I know of that looked at plant survival in response to freezing conditions was Boivin et al. 2001. In the study a series of test plots were installed on a roof in Quebec, Canada. Six hardy perennials were installed into the plots, which had three different depths, 2, 4 and 6 inches. Temperatures in the substrate were recorded and showed that shallower substrates were much colder during winter months and showed greater temperature variation. By the second year of the experiment, four out of the six species showed greater frost damage at 2 inches compared to the deeper substrate. This effect was less pronounced at 4 inches, leading the authors to recommend that 4 inches be used as a minimum substrate depth for this type of climate for winter plant tolerance (Boivin et al. 2001).
Water runoff reduction
I could write about this topic many times over and have covered it in previous articles. However, it would be remiss not to discuss how one of the primary benefits of green roofs during winter conditions is to absorb rainwater and slow the flow of water off the building into drainage systems. A common problem with green roofs is that once saturated (which can happen quite quickly with shallow Sedum style systems), water flows off the roof in a similar manner to a normal roof, albeit a bit slower (Berndtsson 2010, Mentens et al. 2006). A method to improve this is to store more water in the green roof’s drainage layer and release gradually. Commonly know as blue roofs, or blue-green roofs, this type of system can significantly increase the volume of water held on a roof (Cirkel et al. 2018).
Long term green roof performance
One of the most interesting papers I found was by Gwozdz et al. 2016, who artificially subjected green roof substrate mineral components (crushed brick, LECA, crushed granite and crushed limestone) to a large number of freeze thaw cycles. The infiltration potential of the substrate decreases after a number of freeze thaw cycles. This decreases between 0-67% after 30 cycles (equivalent to 7.5 years) and 33-86 % after 70 cycles (equivalent to 17.5 years). This decrease is explained by the breaking down of substrate components into smaller fractions, which are packed together in a tighter way, reducing the speed of water movement through them.
In conclusion, winter on green roofs in temperate climates has not actually been studied that much. A lot more research has focused on the effect of summer conditions on plant survival and dynamics. However, it is clear that green roofs still contribute a large number of services to urban areas during winter months, and the design of them should be carefully considered in order to maximise these benefits
Full references available on request.