FAQs and answers

Weights and roof load

Why do the pockets weigh no more than c. 5 kg at the time of installation?

Light waste flows (“Post Consumer Waste”) have been combined with a unique design. This results in a very light, but still remarkably sturdy drain panel. In addition, the Sam Groofing® pockets are filled with Perlite, a very light substrate for plants that is regularly used in food production horticulture. When they are planted, the sedum plugs are given a carefully calibrated quantity of water. As little as possible to minimize weight but enough to seethem through transport and construction.

What is the combined saturated weight per m2 of a Sam Groofing® vegetation roof?

It is c. 44 kg per m2.

Will the roof not be too heavy when a layer of snow starts melting?

A fully saturated Sam Groofing® vegetation roof weighs no more than c. 44 kg per m2, making it one of the lightests of its kind. In structural calculation for building the added load of snow and/or melt water is taken into account. The structure has to be strong enough to bear the total load. This is no different for a Sam Groofing® roof than for other green roof system.

Do you have a standard calculation to determine the load bearing capacity of a roof?

No, unfortunately not. The load bearing capacity of a roof depends on many factors. Consequently, it must be determined by an expert, such as an architect, builder or structural engineer. In case of doubt we can refer you to specialized engineers. If required, they can decide, in consultation with the client, whether a a roof will bear the load of a vegetation system.

Water storage (retention)

The water storage capacity (retention) is 30 liters per m2 by default. Can this be increased?

Yes. For flat roofs increasing water retention is easy. The Sam Groofing® drain panels can be stacked. Extra drain panels may be placed under the vegetation pockets. For every extra drain panel the water storage capacity increases by 25 liters per m2.

For sloping roofs we advise not to increase the water storage capacity beyond the default 30 liters per m2. The shearing forces and roof load will become too large. That is why the Sam Groofing® concept does not support this possibility. Where it is necessary or desired, we recommend you realise extra storage capacity for water from sloping roofs in or around the building.

Is it possible to calculate precisely how much rainwater is absorbed by a vegetation roof and does not end up in the sewers?

No. Unfortunately there is no way to calculate this with any degree of percision. It depends on the amount of rainfall per occasion, the interval between showers and the evaporation in between showers. In ordinary circumstances the vegetation roof can absorb 30 liters of water per m2. When rain falls shortly after full saturation, absorption capacity will of course be reduced. After a longer interval it will go up again. What is certain is that a vegetation roof retains much more water than a regular (tiled) roof or a regular bitumen, PVC or EPDM flat roof.

Fire safety and wind sensitivity

How is the fire safety of the roof warranted?

Internal company tests show that the fire safety is guaranteed. Firstly, the vegetation roof will be moist for large parts of the year, which makes it resistant to flying sparks from outside. Next, a moist roof covering is fire retardant. Finally, the Perlite in the pockets is also fire retardant. Perlite is used in industrial applications for the purpose of fire retardancy.
In the course of 2022 the fire safety will be tested and certified by external parties.

Is a vegetation roof sensitive to wind?

No. This has been shown in internal company field tests.
On sloping roofs the pockets are firmly secured to the roof construction by means of industrial velcro. On flat roofs the elements are connected by dovetail joints. Also, the growing sedum plants will form a stable interconnected vegetative cover. Finally, years of experience have shown that green roofs are much less sensitive to  “wind suction” than regular roofs.
In the course of 2022 the wind sensitivity will be tested and certified by external parties.

To what height can Sam Groofing® vegetation roofs be applied, with regard to increasing wind load?

In principle all green roofs have only limited sensitivity to wind suction. This also applies to the Sam Groofing® vegetation roof. Therefore it can be used at greater heights. If required, the panels and pockets may be either joined together or attached to the roof surface with industrial velcro. This will further increase wind resistance.


Why does a vegetation roof insulate during the summer?

During the summer a vegetation roof will be moist most of the time. The moisture evaporates in dry, warm periods, keeping the roof temperature down. Also, the vegetation itself has a cooling effect, because the heat that the plants absorb during the day is given off during the night. Next, the air buffers in the substrate afford insulation. Finally, many traditional roofs are covered with dark tiles. Solar radiation raises the temperature of these roofs much higher than vegetation roofs. All these factors combine to keep the building itself cool.

Can you indicate precisely how much CO2 is absorbed by a vegetation roof and how much oxygen (O2) it produces?

No, this is unfortunately impossible. It strongly depends on the weather and the resulting growth parameters of the sedum roof. As we all know, the weather varies greatly. Wageningen University has calculated that 20 m2 of green roof on average produce sufficient oxygen for one person. Because of its wider plant range a Sam Groofing® vegetation roof will capture more CO2 than regular roofs of sedum mats.

What is the material that the drain panels are made of?

We deliberately manufacture both the drain panels and the “inserts” (pocket frames) out of “Post Consumer Waste”. These are waste flows like milk and juice cartons and recycled shampoo bottles. This way we contribute to a meaningful recycling of waste.

The use of sedum plug plants requires a much smaller growing surface than regular sedum mats. Why is this?

We deliberately employ so-called “plug plants”. These are either individual sedum plants or bunched mixtures of a few sedum plants. Our growing areas contain 450 plug plants per m2. They are planted in the pockets with a density of 30 plants per m2. This way 1 m2 of growing surface is turned into 15 m2 of vegetation roof. Every m2 of growing surface is used at least twice a year, so that we can turn 1 m2 into 30 m2 of vegetation roof per year. In regular cultivation of sedum mats 1 m2 of cultivation area becomes 1 m2 of green roof. This makes our demand on expensive and scarce cultivation soil considerably lower and with it our environmental footprint. To build on this even further, our plug plants are grown chemical-free.

Is the irrigation fabric (the top side of the pocket) sensitive to sunlight or birds?

No. The irrigation fabric remains UV stable for at least 8 years, but long before that it will have been covered by the vegetation and not be exposed anymore to solar radiation. The fabric is also resistant to the pecking of birds. A special casing around the roots of the plug plants anchors them firmly in the pockets. This also minimizes the risk of damage inflicted by birds pulling on the plugs.  

Transport and installing

Do the sedum plants not get damaged during transport?

No. The unique shape of the pockets, together with special recesses in the drain panels, prevent the plants from getting damaged during transport. In addition, the roots have a special casing that anchors them firmly in the pockets. Moreover, sedum plants are very sturdy by nature and therefore less vulnerable.

Are Sam Groofing® pockets available in different sizes?

For flat roofs we work with a standard size of c. 100 cm x 40 cm. Connections to roof penetrations (pipes, skylights, etc.) and to the eaves are made with gravel.
For sloping roofs the standard size is also c. 100 cm x 40 cm. To provide these roofs with a full cover, we offer so-called “fitting pockets”. These are 20 cm wide and are available in different lengths (100, 80, 60, 40 and 20 cm). If necessary the fitting pockets will be made exactly to measure “on the job”. This will ensure neat transitions to, for instance, chevrons, valley rafters, windows and dormers.

How are the edges of a vegetation roof finished?

This depends first of all on whether the roof is flat or sloping. With sloping roofs this is done in consultation with the architect/contractor, to fit the design of the building. The standard finishing for a sloping Sam Groofing® vegetation roof is a black aluminium L-profile at the edges. To ensure proper drainage this profile has been perforated on the gutter side.
On flat vegetation roofs the edges are usually finished with gravel. But here as well the edges may be trimmed with aluminium L-profiles.With or without perforation.

Variety and biodiversity

How do your sedum plants compare with regular sedum mats?

A traditional sedum mat is a relatively cheap product. To manufacture it profitably, 6 to 8 fast growing species are selected. Mostly they are exclusively ground cover plants. Higher growing species are rarely employed. They grow more slowly and have trouble developing in a thin sedum mat. Higher growing species are also liable to break when the mats are rolled for transport. All in all sedum mats have a monotonous and low look.

Our vegetation roofs will permanently contain a much larger number of plants and include higher growing species than regular sedum mats. This makes for a richer and much more varied Sam Groofing® vegetation roof.

What are the differences between the various sedum plants?

We divide the range of plants into three groups:
* Perennial and ground covering
* Semi-perennial and ground covering
* Deciduous and solitary

The plants vary as to flowering season, inflorescence, colour and shape. There are fine-leaved, perennial ground cover species, but also solitary, broad-leaved and higher growing species, up to 30 cm and more. Some have yellow flowers, others white or red. Several species shed their leaves in winter. 

Can other plants be sown among the sedum plug plants?

Yes, this is possible in principle. However, we advise you not to do that. The range of our plants has been carefully tailored by our own breeder. This guarantees a beautifully varied cover of vegetation with different flowerings, seasons and colours. Adding other plants may disturb the carefully created balance. In large projects and with large volumes the client’s specific wishes regarding plant variety may be taken into account.

How many sedum species are there to choose from?

By default our vegetation roof consists of a very extensive range of at least 12 to 16 different sedum species. They have been carefully selected and matched. The selection of plug plants helps to maintain a much wider variety of sedum species in the years to come than on regular roofs of sedum mats. In nearly every situation this range of species performs very well.
In large projects and with large volumes the client’s specific wishes regarding plant variety may be taken into account.

Can a vegetation roof be “custom made”?

Yes. But only in large projects and with large volumes. And always in consultation with our own breeder. Because we want to be convinced that the chosen range is well matched and will ensure a beautiful green roof for many years to come.

Does it take a variety of sedum plants to ensure biodiversity in a green roof?

Absolutely! Greater variation in sedum species by definition means greater biodiversity. Furthermore, a wider assortment with different inflorescences and flowering seasons leads to a greater diversity of insects. In this way, for instance, we can support and advance the bee population.

In the future we shall further boost biodiversity by employing e.g. ornamental grasses and herbs in our vegetation roofs.  

Growth and maintenance

How do the sedum plants take root in the Perlite substrate?

At the time of planting the sedum plants are given nutrients that will gradually be released in the course of the first year. This stimulates the development of roots and plant growth. Perlite is characterised by its hospitability to roots. The end result is a fully rooted substrate layer.

Can roots grow through the bottom fabric?

The bottom fabric of the pockets (the needle felt) has been given a root protection layer (DuPont™ Plantex® gold) on the inside. No roots will penetrate this layer. However, we cannot exclude the possibility that roots will grow through the seams of the pockets. Therefore we always recommend using a root-proof roof covering. Particularly on sloping roofs.
If an existing roof does not have a root-proof cover, a root protection fabric should be put in place first. Such a fabric may also be used on flat roofs as extra protection for the roof surface.

Are the spaces between the drain panels under the pockets sensitive to weed growth?

No. The absence of light will prevent any weeds from growing under the vegetation roof, so that the drainage capacity stays as it is.
To maintain a beautiful vegetation roof and continued good drainage, we do recommend that you service you vegetation roof twice a year. You can either do this yourself or engage a specialised gardener or horticulurist. These offer reasonably priced maintance contracts for a guaranteed lastingly beautiful green roof and dependable irrigation and/or drainage.

Can you stand on the pockets?

A vegetation roof can be walked on, but it is advisable to restrict this to the servicing in spring and autumn. During the first year of development we recommend that you walk cautiously. As the vegetative cover continues to develop, it will become less vulnerable.

Is it easy to replace a plant in the pocket?

Yes. No problem at all. This is usually done during the regular servicing of a green roof. If, in spite of all precautions, a bare patch has formed, new plugs are easily planted to fill the gaps.

Can I buy single plug plants?

Yes, you can. For the maintenance of Sam Groofing® vegetation roofs.

What are the maintenance instructions?

For this we like to refer you to the instructions for use at this website.


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