FAQ

We also advise against gravel around or in front of drains: gravel hampers the draining of rain water and, after a few years, it will be covered with sedum. We advise that you put an aluminium profile (approximately 25 x 25 cm, height 6 cm) and keep the drain free. This will allow you to check it at all times. In the case of extensive green roofs and, therefore, higher structures, we advise that you use inspection boxes which can be closed.

We advise against it:

• osmosis, i.e. the plants’ roots’ sucking up of water, is much more efficient when the plants are in a substrate, as opposed to them being in a container. Compare it to a tree or a shrub: you plant them in the soil, not in a bucket…. As a matter of fact, the FLL’s most recent directives (2002, paragraph 6.4.1) stipulate that the retention of water below the substrate layer is detrimental to the growth of plants in extensive green roofs;
• some boards do not resist frost and/or are likely to be destroyed by the roots of plants;
• we are committed to sustainable building; as such, we try and keep the use of plastics to the strictest minimum;
• the price of boards and the cost of placing them are quite high (a 8 cm thick layer of substrate will retain as much water as a 5 cm thick board and cost you +/- 10 €/m² less)

We firmly advise against the of use of root-resistant foil (usually made out of 0,2 to 0,4 mm thick polyethylene). Instead, we suggest using a root-resistant waterproofing membrane:

• with a roof-resistant foil, a water film would be formed between the foil and the waterproofing membrane; this will inevitably draw the roots towards the waterproofing membrane, exactly where they should not be;
• at the vertical elevations, the foil cannot be glued or welded to the waterproofing membrane; dirt will accumulate between the foil and the membrane; this is the ideal nutrient for unwanted vegetation (e.g. birches), the roots of which will grow in between the foil and the waterproofing membrane, a major threat for the latter;
• in fact, the FLL (Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau = Landscaping and Landscape Development Research Society) strongly advises against the use of separate root-resistant foils

Experience shows, when substrate layers are thin, that the alternate swelling and shrinking leads to the co-polymers’ migrating to the surface where the sun’s UV rays will destroy them in a matter of months.

Extensive green roofs are expected to require little maintenance. We supply a standard sedum mixture which has been making its mark since many years. Depending on local circumstances, some sedums will develop better than others, it’s Nature which decides. Experience shows that, after a few years, “custom-made” plant mixes resemble our standard mix a lot. After each florescence, the plants sow themselves ; after a few years, plants will be mixed across the roof. This is, in fact, better because, should you plant plants in separate lots, and should one particular plant not thrive, you risk ending up with bald patches on your roof.

Extensive vegetation can be walked upon several times per year without any problem ; gravel strips are, therefore, not needed unless in those areas where you frequently need to go to. Gravel strips alongside the roof borders are very dangerous ; it is much safer to walk on gravel strips in the center of the roof than near the borders of the roof. Gravel will be covered by sedum after a few years, anyway.