On these kinds of roofs it is almost always possible to install multiform vegetation at comparatively small expense.

Although these roofs are not always actively useable, e.g. as greened seating spaces, greened roofs in contrast with monotonous gravel, bitumen, or sheet metal surfaces can continually improve the climate, filter pollutants, and save heating energy.

A measurable long-distance effect cannot be attributed to greened roof surfaces; however, the effect of many small individual roofs in a city does add up significantly.
 

While gravel roofs and black bitumen pasteboard heat up to between 50°C and 80°C, the maximum temperatures on greened roofs amount to roughly 20°C to 25°C.

On clear winter nights the temperature of non-greened roofs can sink as low as -20°C. The annual fluctuation in temperature thus amounts to about 100 degrees. Greened roofs cool in winter only to slightly below 0°C, so that the annual fluctuation amounts to only 30 degrees.

All open areas of vegetation are capable of storing surface water. According to the type of vegetation, water from precipitation is retained for various durations in the upper layers and then flows out, minus the amount lost in evaporation and transpiration. Table 6/2 below shows the proportion of rainwater that is carried off by drainage (i.e. discharge factors).

According to § 74 (3) 2. of the State Building Ordinance (LBO), municipalities enact a statue for the entire municipal area or a portion thereof requiring that facilities for retaining precipitation water must be constructed in order to relieve drainage systems, avoid danger of flooding, and preserve water resources. Even though aspects of water efficiency are of primary concern here, measures of this type also work against soil capping and its disadvantageous climatic consequences.

Roof greening can be established as legally binding in a site plan. The roof form (flat roof) is based on § 74 LBO and the greening on § 9 (1) 25 BauGB, which empowers the municipality to require planting on parts of built facilities.

As with every other regulation, this can only be implemented after fair consideration of all affected interests (§ 1 (6) BauGB). To be considered, for example, are fire protection, humidity and corrosion protection, and the costs of the planting inclusive of potentially higher construction costs resulting from the additional burden on the roof. These must be incorporated in the reasoning for the site plan.

Following are examples for these types of regulations:

"Greened flat roof; the roof surfaces are to be planted and maintained with a dirt layer of at least 40 cm. Exceptions can be made for light fixtures, glass sections, and terraces, if these serve the building’s purpose of use and are subordinated (§ 9 (1) 25. BauGB)."
or:
"Flat roofs (0 to 15 degrees inclination) are to be planted over a proportion of at least 60% of the roof surface – with the exception of surfaces for technical roof systems – with a substrate layer of at least 8 cm of grasses, soil-covering plants, and wild herbs, and are to be so maintained (§ 9 (1) 25. BauGB). Exceptions for solar energy facilities can be allowed."

Regulations for roof greening can also be issued as localized construction specifications according to § 74 (1) 1. LBO.

The regulatory possibilities for defining the purposes, placement, and design of roof surfaces and facilities according to the design specifications of the LBO also support the goal of erecting solar energy apparatuses (in the form of collectors or photovoltaic devices) when these are intended for long-term effective use (BUNZEL et al., 1997).