The green roof is viewed through rose-colored glasses

How audible is a difference in sound volume of 3 decibels? There is a website where you can test that. It’s not much. Yes, if a tone from the mid-range suddenly drops 3 dB in strength, you will hear it. But if you are presented with the same tone now at 50 dB and then again at 53 dB, you will have trouble with the difference. That’s about it.

Three decibels of noise reduction is what such a modern ‘green roof’ produces under special circumstances, if everything goes well. In Eindhoven fell measurements in traffic noise in a courtyard behind a flat was about 3 dB lower in 2012 after a green roof was rolled out on the flat. Since that time, green roofs have been regarded as ‘soundproofing’.

Green roofs have a tailwind, flat roofs on the left and right, on which until now only dead birds have gathered, are being converted into environmentally conscious roof gardens. A complete green roof industry has emerged.

Better Biodiversity

The municipality of Amsterdam allocated millions of subsidies to persuade private individuals to install green roofs. Because they were responsible for storing water during downpours, helping to save energy, keeping the city cool and improving biodiversity. Others had discovered that the roofs cleared the air and the livability strengthened. The last IPCC report even reported that they could sequester carbon and contribute to food production. The foundation Nature and Environment calculated that there is still 21 million square meters of flat roof in the Netherlands that can become green roof. “You should make the construction mandatory.” That will probably happen, because with idealists the evil is eating cherries.

De Volkskrant released a on May 13 sobering article about the natural value of green roofs. The rooftop nature turned out to be little more than a handful of plant species and some invertebrates such as insects, spiders and snails and maybe a single bird that rested for a while and they are indeed nice insects that manage to reach the high roofs (the southern squab, the common blue, etcetera) but the problem is that they are attracted to the wrong plants.

Most green roofs are sedum roofs, mono or mixed cultures of plants from the Sedum genus: maintenance-free succulents that can withstand drought well. The Netherlands has attractive sedum varieties such as stonecrop and white stonecrop, but exotic species and varieties are especially used for green roofs. That’s where the shoe pinches. The exotics threaten to irreversibly mix with our native flora, such as the cursed American bird cherry, Persian hogweed and Japanese knotweed. Florists are sweating at how the wrong diversity is being promoted. The species composition of the green roofs must become more autochthonous.

The Amsterdam municipality never put much subsidy into sedum mats because they held too little water and it has now completely stopped supporting small-scale green roof projects. She seems to have entered a phase of reflection. Perhaps she has realized that too many green roofs have been built in places where no flooding threatens. Or has she realized that the useful water storage during downpours is offset by painful consumption of water in dry times. No green roof owner can watch his green roof wither if it doesn’t rain for a while and in drought he keeps his greenery upright with the last scarce water. There is no green roof without a sprinkler system. If by November that entire green roof has been forgotten, the water pipe freezes in January and the water sprays in all directions in March. Remember that gutters and drains are full of wilted plant debris.

Forest of publications

It is therefore not yet clear how good the green roofs are for water management. There is also little certainty about a favorable effect on heat management. Although there is a forest of publications that emphasize that a solid green roof increases the insulation value of a roof, who would have expected otherwise? The question is what the green roof insulation adds to the existing insulation value of the roof. If enough polystyrene or rock wool had already been incorporated, the profit is only marginal† It is noticeable that the energy savings of the spaces directly under a green roof are usually calculated and rarely measured.

But green roofs at least help keep the cities cool, it is said, also by the IPCC, they fight it Urban Heat Island-effect. Above the greenery it doesn’t get nearly as hot during the daytime as above a flat roof of asphalt paper or white gravel! Well, as far as this applies, it is an influence that extends to a maximum of half a meter above the greenery and it is very questionable what the practical significance is of ‘less warming’ at high altitude for the air temperature at street level.

It is painful that it has already been shown a few times that it does get very hot above a sedum plantation, it has been observed in Utrecht and Braunschweig† This is because sedum plants keep their stomata closed during the day and cannot cool themselves, like other plants, by water evaporation. In Utrecht it became warmer over sedum than over white gravel. Even for this reason, the sedum mass must be replaced by a cover of other plants, preferably native ones.

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