FAQ
Below are some quick answers to the most commonly asked questions about green roofs. If you have a question that is not answered here, you can mail us your question at info@greenroof.se, or ask in the Guestbook.
Click the question to go straight to the answer, or read all answers below.
- What is a green roof?
- What are the benefits of green roofs?
- Can any building have a green roof?
- Do I need a building permission for a green roof?
- What slope on the roof is suitable for green roofs?
- What does a green roof cost?
- Are there any disadvantages in having a green roof?
- Do green roofs leak?
- How do I detect a leak if it occurs?
- Does the retained moisture damage the building?
- Do I need to mow the roof?
- Do I need to weed the roof?
- Do I need to water the roof?
- Can green roofs be used in all climates?
- Which plants can grow on roofs?
- Will soil and other debris get into the drains?
- Are green roofs sensitive to damage by wind?
- Will insects bother people in the building?
- How long have people used green roofs?
- Where can I see a green roof?
- What do insurance companies say about green roofs?
- I have moss on my roof, do I have to remove it?
1. What is a green roof?
The quick answer is: a roof with living vegetation, growing in a thin or thick substrate. For a more informative answer, go here. Other terms you may hear are living roofs, vegetated roofs, eco-roofs (= the American term for extensive green roofs), brown roofs (a term used in the UK for a roof designed for biological diversity, but is not necessarily 100% covered with plants), or roof gardens (which normally have thicker soil layers and are accessible).
2. What are the benefits of green roofs?
Green roofs can help mitigate many of the urban environmental and health problems we have today. They are not the sole solution to these problems, but make a contribution to the solution to many problems. For example they can reduce pollution of waterways, reduce costs for municipal drainage, save energy in the building, protect the waterproofing membrane from ageing, reduce air pollution and noise pollution, contribute to better health for people, and give space for wildlife. There is a whole section describing this in more detail here.
3. Can any building have a green roof?
If the slope is not too great (see below) and the roof construction allows the extra weight, most buildings could be retrofitted with a green roof. If the building has some sort of protection as cultural heritage, you should check with building authorities.
4. Do I need a building permission for a green roof?
In Sweden, if the building is within an urban area, you need a building permission for anything that changes the outward appearance of your building, including changing the colour of the walls or the roof coverage. For Swedish rules, look at Boverket. Check with your local authorities.
5. What slope on the roof is suitable for green roofs?
A prefabricated extensive green roof, with a good armouring in the substrate could in principle be attached to a vertical surface, such as a wall. The problem is, as you may imagine, that it gets hardly any water from rain that way. If you are not prepared to irrigate the roof, it is recommended to limit green roofs to slopes of 1-30 degrees. This recommendation may vary according to the climate where you are, and if the roof slopes to the north or south, etc.
6. What does a green roof cost?
This is a very difficult question to answer. It varies very much between different countries, and is generally higher in countries where there are few entrepreneurs that install green roofs. An extensive green roof is normally less expensive than an intensive. Site-built green roofs are often cheaper than prefabricated mats. Often you get a lower price per square metre if you buy a lot. The freight costs and distances also influence the price, as does the accessibility with cranes etc at your building site. Separate drainage layer is often included in the price, and possibly also instalment. To give you just some vague idea of a price, the Swedish prices for an extensive green roof in 2006 were around 500 - 900 SEK/m2, including drainage layer and installation.
7. Are there any disadvantages in having a green roof?
If you are considering water harvesting on you roof, for example for flushing toilets in the building, the green roof would reduce the amount of water you can harvest. Other eco-adaptations of a building, such as solar energy do not have to conflict with a green roof, but can be combined, to mutual benefit.
The price is higher than a naked roof, if you look at the initial costs, though in the long run the saved energy and roofing materials may even pay the investment back.
The weight may require reinforcement of your building.
8. Do green roofs leak?
People often ask us if having all that moisture on the roof won`t harm the building or the roof membrane. No. The membrane is made waterproof, and it will stay waterproof unless it gets physically damaged or aged. What ages a bitumen membrane is mainly uv-light and temperature extremes. The soil and vegetation cover will protect the membrane from these types of negative influences. Unless the roof membrane is damaged while laying the green roof (don`t ever screw or nail through the membrane!), there is no reason why a roof would start leaking because it has a green roof, any more than a naked roof would. Leaks are less likely, in fact.
9. How do I detect a leak if it occurs?
Leaks are always difficult to pin-point for repair, and of course, under a vegetation cover, this is not easy. If this is a big concern for you, there are systems where you lay a network of copper thread under the waterproofing membrane, which short-circuits where water enters, giving an exact position of the leak. However, once you know where it is, the vegetation mat, if it has some armouring in it or felt material under, is easy to lift aside for repair and to replace afterwards.
10. Does the retained moisture damage the building?
Not if the waterproofing is in good condition, and of a type suitable for green roof application.
11. Do I need to mow the roof?
This depends on the soil thickness and type of vegetation you choose. The very common moss-sedum mats for extensive green roofs never need mowing.
12. Do I need to weed the roof?
Again, this depends on the soil thickness and type of vegetation you choose. Thin soil layers with extremely drought tolerant species, such as moss and sedums (stone crop), are not likely to get weeds, simply because it is too dry for most weeds. In certain spots a roof may be shaded, or get additional water from the drain of a higher roof. There you may get some grass. To avoid this, use non-fertilised gravel in such spots. Normally, you do not weed an extensive green roof.
Intensive, or semi-intensive roof gardens do need weeding, as any garden does.
13. Do I need to water the roof?
- Extensive vegetation and roofs designed for biological diversity: Normally no irrigation necessary. Some manufacturers recommend irrigation in the establishment phase. An unusually steep roof might need water during long dry periods.
- Semi-intensive and intensive roof gardens: You can design a roof garden with drought tolerant plants that will not need irrigation, but mostly, some of the plants chosen will have higher demands on water. A good idea is to have a built-in automatic irrigation system for such plantings.
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Remember that one of the advantages with green roofs is their ability to take care of storm water and reduce the loads on municipal drainage and natural waterways. If you install irrigation on a green roof, it is advisable to make it use recycled rainwater, to enhance the environmental benefits, and not to waste fresh water.
14. Can green roofs be used in all climates?
Probably. Green roofs have been tried in cold, snowy, hot, arid and wet climates. If no one has tried them in your area, and you are a pioneer, the thing you need to do, is to find out what natural sites you have, which have a very thin soil layer. Mountains, coastal areas, dry meadows etc are good places to look (or check with your local university, they`ll know). The plants that can live under such circumstances will normally do well on a roof. Try to make a soil substrate that resembles the natural one. If you are in a very arid place, desert plants could be an option, but avoid the ones which have adapted to drought through having very deep roots.
Local species are always more interesting from an ecological viewpoint. Try to recreate habitats that are disappearing, if you can. Remember, that although you may end up with something that doesn`t look like a homogenous lawn, you are doing a valuable contribution to nature, and you can appreciate its variability and changing seasons.
15. Which plants can grow on roofs?
This varies with the soil depth, climate, etc, and also on the intended maintenance levels. Here is a list of the species that are grown on the Augustenborg Botanical Roof Garden in Malmö, southern Sweden.
16. Will soil and other debris get into the drains?
A correctly built green roof system does not erode soil into the drains. However, some plant matter from the roof vegetation might end up in the drains through the action of birds. This is in our experience not a great deal more than on other roofs, where trees shed leaves onto the roofs. Drains will need to be cleaned in either case.
17. Are green roofs sensitive to damage by wind?
You might not have thought so, but a green roof is less susceptible to damage by storms than many other roofing materials! Experiences have shown that after a bad storm in Malmö, Sweden in 1999, newly laid green roofs in Augustenborg were unharmed, whereas tin sheets and clay tiles on other roofs had blown off. The reason is that the wind can not pull the permeable mats up using suction. And if a corner is lifted by the wind, it collapses again, instead of providing a big flat surface for the wind to catch, such as a tin roof does.
In very windy places, however, you can use some erosion protection such as a net inside the soil, and fasten it well to the roof. Drainage, as well as prefabricated vegetation mats can be glued to the roof. (-But never, ever, screw through the waterproofing membrane! Even using screws with rubber packing are not to be trusted in the long run.)
18. Will insects bother people in the building?
No. Insects adapted to living in that type of vegetation are not the same ones who are adapted to find opportunities in your kitchen. Some might fly wrong, and end up inside, but they will not reproduce or cause problems there. Living in a lush, green, park like environment will lead to more insects flying wrong, than living in a city dominated by concrete, but how many people would choose the concrete for that reason? I might also add that rats etc do not find food on green roofs, and so, do not live there.
19. How long have people used green roofs?
Using vegetation on the roofs is not a modern invention. Turf roofs have been a traditional building technique in many places. The differences between a modern green roof and a turf roof are substantial, and this is mainly due to the different purposes, and the materials available nowadays. The main objectives in the early days was to use the turf as insulation, and to stop the waterproofing layer, which was often birch bark, from blowing away.
20. Where can I see a green roof?
Go here for a list of publicly accessible roof gardens and extensive green roofs around the world.
21. What do insurance companies say about green roofs?
It seems that once they understand what a green roof is, they are generally positive. Be sure to follow your national building rules when it comes to fire protection. See here for Swedish rules (page 44 in the pdf is on roofing materials).
22. I have moss on my roof; do I have to remove it?
Well, the opinions differ on that. Basically, if the waterproofing membrane, for example a tar roofing felt, also called modified bitumen, is of the same sort or quality that you would choose to have under a commercially available green roof, your moss is doing you and the environment you live in a favour, and can not damage your house. Removing it mechanically would do more harm than the mosses themselves. Chemical removal would do harm to our environment.
With other roofing materials, there might be cause to look into how the moss can influence the material. It is important to know, that mosses have no roots. They have no ability to penetrate the surfaces, however shallow. One negative influence mosses may have on some materials is that they retain moisture, and so will delay the drying of the roof surface. If your roofing material is organic, this can increase the rate of biodegradation. If the roof is made of clay tiles, retained water may lead to erosion by ice in winter.
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