succulent roofs

July 2018. The sedum roof in all its glory, 4 years after its installation.

Being a fan of botany and natural vegetation, I have always been fascinated by green (or vegetation) roofs, which are quite fashionable these days, at least in countries like Holland, Germany and Scandinavia. Green rooofs have quite some advantages: they protect the waterproof black EPDM sheet, they insulate the house against heat and cold and they work as a water buffer, regulating the water drainage with heavy rains. This may especially hold true in cities where climate change seems to provoke more irregular and heavy rainfall. Urban green roofs also offer welcome new opportunities for insects, birds and plant life. Another personal motivation for green roofs is that they can be very economical, especially if you do everything yourself with the method I propose. When you hire professional help or buy the ready made green carpets from the garden center, you will have to spent 5 to 10 times as much.

June 2018. Drone impression, with the two roofs of the house on the left and the toilet building in the upper right corner.
The greenhouse in the bottom right corner has a semi-transparent roof of fiberglass.

Over the years I installed three green roofs. Two for the house with its two modules (the first consisting of the kitchen and the second the studio-bed-living room) and a simpler one for the toilet building. All three of them are of the Sedum variant and are often called Sedum roofs, after the plants from the Sedum genus. Sedum species are well suited for the task, especially if the soil layer is only around 5 cm. For more insulation and water buffering you can choose a system with more soil (up to 20cm) but then you need to take care of the extra load in the construction of your house. This last variant features more grasses and needs more maintenance. My Sedum roofs are respectively 10, 5 and 10 years old, haven´t had practically any maintenance and are doing fine.

January 2009. The roof of the first straw bale module is ready and the first layer, the EPDM sheet, has been spread out.

For the two roofs of the house I used more or less the same method which will be described in the next paragraphs.

In the early spring of 2009 the roof of the first module of the house (a kind of garden house, serving as a first essay in straw bale building) was ready to receive the different layers that make up the green roof system, as I had studied and adopted from various sources of literature and internet. Schematically it looks something like this:

The knobbed roll (in two colours) is spread and ready to be covered with the geotextile. The black PE pipe, running along the edge of the roof, has many holes and collects the down running rain water. The chimney needs special auto adhesive EPDM to seal the hole off. The plastic bags contain the ´Arlita´ clay pellets.

The waterproof black EPDM sheet is of course the crucial and also most expensive element of the roof. It has a warranty of 50 years but is still quite vulnerable being only about 1mm thick. The knobbed HDPE roll is basically a cheap protection for the EPDM and can also hold some rainwater in its knobs. The geotextile is another protection layer and keeps the soil in place while keeping the HDPE knobs open for some water storage.

March 2009. The EPDM sheet was in one piece and extremely heavy and was lifted, like the soil, with this tractor onto the roof.

The soil consists of the 10cm top layer of the soil from the very same spot of the house, which was excavated in order to lay the foundations. The expanded clay pellets (´Arlita´) were partly used as substratum and partly mixed into the soil. They mainly reduce the weight of the soil layer while still working as a substratum for the rooting of the vegetation.

Once the soil was in place, many ´books´ of straw (=more or less intact pieces of straw bale) were spread out over the surface. Cuttings of a couple of Sedum species were then planted or ´sown´ into the spaces between the books. The straw maintains humidity and helps the seedlings establish. I used mostly the readily available Sedum species from the local region: Sedum acre, S.sediforme, S.album, S.reflexum, S.anglicum and S.telephium. The last one was bought and was the only Sedum species not to survive after 5 years, and it was the same for Sempervivum tectorum. All the other ones quickly settled and thrived.

It was astonishing to see the vigorous growth and flowering the next spring. For aesthetic reasons and fire risks (with the chimney so near) most of the straw books had been taken away.

April 2010. An astonishing explosion of plant growth just one year after planting. Completely spontaneous is the presence of Iris germanica with its long, somewhat succulent leaves, whose roots were hidden in the soil and later on cut and dispersed. The non-succulent tiny annual with yellow flower heads is Alyssum alyssoides which has irregularly reappeared in following years.

June 2010. Sedum acre has just stopped flowering , S.album and S.anglicum are flowering now (white) and will be followed by S.reflexum (just starting to flower in yellow) and later on S.sediforme (white).
At the back the greenhouse.

April 2019. On both roofs the Iris germanica shows off their beautiful and fragrant flowers.

Remarkable is the appearance and persistence of Iris germanica. Its presence was not planned and in the beginning I was somewhat skeptical and reluctant thinking it was too bulky for the vegetation tapestry and that it wouldn’t survive the long dry summers anyway. But it stayed, and moreover: it massively flowers every spring, much more abundantly than its fellow plants on the ground. And then I really started to appreciate them, even during the rest of the year when their leaves turn partly brown and blend nicely with the rest of the withering vegetation. It is amazing that such a big flowering plant can thrive and spread in a layer of just 6 cm soil, which completely dries out in summer when there is sometimes no rain for 2 or more months. Part of its success will have to do with its 3 to 5 cm thick rhizomes. It is very important not to cut its withering leaves after flowering because the plant seems to reuse all the nutrients inside them. I think that the use of this plant on green roofs is very promising and recommendable. It combines the advantages of grass roofs with their denser and higher vegetation with the low maintenance and low weight advantages of Sedum roofs.

July 2018. I admit it is an acquired taste, but now I really appreciate the green-brown mosaic of the dying Iris germanica leaves in the Sedum tapestry.

March 2017. A record breaking freaky and sticky snow storm just at the beginning of spring. Had I not taken away some, the snow would perhaps have formed a layer of 30 or 40 cm.

The last two images above were taken on the second green roof, which was installed on the new wing of the house in 2013, following the same procedure as for the first roof. The first roof has an angle of 5 degrees and this 2nd roof about 1.5 degrees, quite favorable for the establishment of vegetation on green roofs. Higher angles involve risks with heavy rain that might wash away the soil and plants. But on the other hand, the almost horizontal roofs take the risk of dangerous weight with heavy snowfall. Heavy snowfall is very rare in this region, with snowfalls that haven´t gone passed 20 or 30 cm for the last decade(s). But in March 2017 I climbed up the roof with a shovel to take away some snow when there was already a layer of 20 cm and it continued snowing heavily. But I haven´t noticed any cracks or bending whatsoever in the structure of the house.

July 2018. The Sedum roof of the compost toilet building.

In 2009 I installed a green roof on the compost toilet following a simpler design with only EDPM sheet, knobbed HDPE roll and a shallow soil layer of a few centimeters. I didn´t want to charge the un-plastered straw bale walls with too much weight. Especially Sedum album is doing well there, and also, to a lesser degree, S.anglicum and S.acre.

The next images give an impression of the green roofs in different seasons and weather types and show how it makes the house blend nicely in its surroundings.


grey and dark matter

the treatment of grey water and human excrement

An aerial overview of the present situation with the different elements of the filter and compost systems.

March 2010. The greenhouse on the left and the first ´module´of the strawbale house on the right. The down sloping filter canal is made with black EPDM sheet and filled up with gravel and irregular (´conglomerate´) stones, while the filter pond is lined with smoother river boulders and gravel to avoid damaging the sheet.

As I live almost off the grid, half a kilometer from the village (but am connected to the public water supply system) I had to find ways to treat the waste (´grey´) water from the sink, shower and washing machine and the `dark` material from the toilet. After studying the internet, I decided to start with relative simple systems and make them more sophisticated if needed. After almost 10 years I can say that more sophistication is not needed and that the systems work quite satisfactorily. By that I mean that there are almost no smells and for example the vegetation in the filtering ponds is doing fine. Part of the success is probable due to the use of biodegradable detergent, soap and straw. I will give a brief description of the two filter ponds and then discuss the equally successful compost toilet which treats the ´dark´ waste.

March 2010. Excess water flows into the vegetable garden via two roof tiles. The pond was later filled up with gravel and sand and planted with marsh plants.

The first filter pond was installed in February 2010 to clean the grey water from the shower and the washing machine, located in and next to the greenhouse. As the vegetable garden is near I used the opportunity to lead the grey water through a 6 meter long canal down to the filter pond and then let it overflow into the garden. The canal consists of a sloping trench filled with rough rocks and sand of all sizes which was left over from the foundation of the house. These angular (conglomerate) stones offer nice growing possibilities for moss, algae and other ‘dirt eating’ organisms. Also the irregular flow through the canal will help with the oxygenation necessary for decomposition. The canal and the pond are lined with the typical waterproof EPDM sheet. This sheet must stick out at least some 5 centimeters above ground level to avoid the invasion of plants into the canal. The canal was covered with bigger river stones, mainly for esthetic reasons.

The canal is covered with nice river boulders. Gravity leads the water through the filter canal, the pond and finally into the vegetable garden.

The canal comes out into a small pond lined with smooth river stones to protect the underlying EPDM. This pond is about 4 meters long and 40 cm wide and can hold about 500 liters. The water overflows via two roof tiles into the vegetable garden. First I didn´t introduce almost any soil or sand but this would give the opportunity for mosquitoes to thrive, especially if the water is not too clean and there is no stable and mature ecosystem with sufficient predators. So I later decided to fill up the pond with sand, gravel and a bit of soil. I planted Valeriana officinalis, Lysimachia nummularia, Iris pseudacorus, Lythrum salicaria, and Sparganium erectum

July 2018. The filter pond with the canal on the right. Lythrum salicaria is flowering abundantly.

September 2019. Regrowth of Lythrum salicaria after a dry period in my absence.

With the exception of Sparganium, the plants have survived well, even in periods of almost totally dry conditions in my absence during some summer weeks. Upper parts may die then, but the plants revive after receiving water again. Sometimes when there is too much water inflow, especially when the washing machine is operating, there is some undesirable overflow of soapy water into the vegetable garden. The quantities are small and get spread over a considerable surface because of the sloping ground and I don´t consider it a big issue. With more people and more frequent washing, the system should nevertheless be amplified with a considerable bigger and deeper pond.

March 2015. This filter pond is about 6 meters from the house and receives the grey water from the water basin in the bathroom and the kitchen sink. The size of about 3 by 1 meter and the depth of maximum 60 cm should be sufficient to receive and treat this water from one person.

In 2015, after finishing the additional wing to the original garden house, I installed another similar filter pond for the sinks and shower of the house. It covers about 3 square meters and is 60 cm deep at its deepest point. It was also filled up with alternating layers of sand, gravel and small stones. The design is in such a way that the possible water level is always below the substratum in order to avoid a breeding place for mosquitos. I planted some Lysimachia nummularia, Iris pseudacorus, Lythrum salicaria, and Mentha pulegium. After one or two years there was a welcome spontaneous invasion of Juncus effusus. The grey water flows downwards into the pond via two PE pipes from the kitchen sink and from the bathroom shower and sink. As I prefer to use the shower (with heater) in the greenhouse the inflow from the bathroom shower is practically non-existent. The inflow of fat, oil and bigger organic particles is avoided as much as possible and also the (frugal) use of biodegradable soap helps not to overload the filtering capacity.

August 2016. Mentha pulegium and Lythrum salicaria flowering abundantly in mid-summer.

Again, after almost 5 years, there are practically no smells. Only if you put your nose some centimeters above the surface are you able to discern something. The only maintenance practiced is a yearly mowing of the vegetation, where I try to avoid cutting e.g. Iris pseudacorus too excessively to favour its regrowth the following year. The pond rarely overflows and then most often with very heavy rains.

The compost toilet building was built in 2009 as a temporal solution but it showed to be quite stable and was so practical and cosy inside that I decided to maintain it, for at least 5 or 10 years more probably. A vegetation roof was installed and the front features a firewood store.

Apart from the grey waste water you have to deal with the ´black´ (and ´yellow´) waste material of the toilet. Again, I opted for the simplest method and again with satisfying results. The system in various degrees of sophistication is known as the compost toilet or dry toilet. No clean drinkable water is used for flushing the toilet. Instead, in the system I chose, straw is used to cover the excrement, which after some period are taken to the compost heap where it further composts into a fertilizer for the vegetable garden. Many systems use big subterranean tanks for collecting the excrement, but then you need ventilation and problems with smells, flies and others may arise. I simply use small buckets which are emptied and cleaned on a (two-)weekly basis which avoids most of these problems. Again, smells are minimal, thanks to the strong absorbent qualities of the straw. For practical and better results I crush the straw with an improvised ‘weed-wacker` which is actually an electric drill and cement mixer with some strings of weed-wacker string attached.

Inside the straw bale toilet building. On the left is the closet and the bucket on the right contains the crushed straw.

There are usually questions arising around the use of human excrement for fertilizing vegetable gardens, concerning risks of germs and medicine. There are nevertheless almost no reports about illnesses arising from the use of this kind of fertilizer, but letting the compost heap rest for 2 years (instead of the usual one) should probably avoid all the risks. In China there is a millennium old tradition of using human excrement for fertilizing without any mayor incidents.

February 2016. The miraculous compost heap. Installed in 2009, its level has stayed like this for the last 5 or 6 years, even swallowing toilet paper and orange peels. It´s total size is around 1 cubic meter.

Curiously my own compost heap is
doing very, almost excessively, well as it simply doesn´t grow. For almost ten
years it ´eats´ all the organic waste material from kitchen and garden and it
remains half full as if by magic. It certainly indicates that the composting process
is doing fine, even with all the controversial material I throw onto it, like
white toilet paper and orange peelings. It would be interesting to do an
analysis of the stuff by a professional laboratory, after I let it rest for two

Some words about legal matters. As I am trying to get a legal ´habitability´ status for my house, I had to sign a special document to assume the risks concerning the use of a dry compost toilet. But it should be inside the house and I therefore made some kind of pre-installation in the big cupboard of the bathroom, but prefer to use the toilet outside, partly because of the straw dust and particles.

Concluding I can say that the
systems described work well in all their simplicity. The saving of clean
drinkable tap water is tremendous and the avoidance of accumulating organic
waste by recycling is considerable.

March 2017. On the terrain there are two other ponds but not so much for filtering as for attracting wildlife: insects, birds, wild boar, badgers and foxes. Also quite successful, but with their own specific problems and dynamics which will be treated in another article.