RECOMMENDATIONS FOR PLANNING
6.1.4 Roof Greening

In addition to larger and smaller green spaces in the city, roof greening can also reduce urban climatic deficits in relation to humidity and the thermal milieu (DEUTSCHER DACHGÄRTNERVERBAND, 2011). There are also advantages to roof greening from the perspective of building design. Roofs in cities and towns offer reserves of surfaces, largely unused up until the present day, that can be employed for the creation of green spaces (Figure 6/5, Figure 6/5a and Figure 6/5b). While residential, office, and industrial buildings present themselves for greening in built-up areas, garages and auxiliary buildings located in more rural areas typically exhibit flat or low-angle (up to 15 degrees) roofs.

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.

Climatic Effects

The positive thermal effects from roof greening are found predominantly in the reduction of temperature extremes throughout the year (KOLB, 1989). Figure 6/6 shows an example of the temperature characteristics of various construction materials for roof surfaces on a summer day with intense sunlight.

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.

In summer a large part of the sunlight that a green roof receives is converted to evaporate water (cf. Chapter 2.4). The evaporation of 1 liter of water at normal air pressure requires 2,250 kJ without a rise in temperature. The same amount of energy, however, can heat 100 m3 of air by 18 degrees Celsius. Green roofs are altogether an effective measure for the protection of underlying spaces against summer heat. In winter, the vegetation and the roof substrate reduces the amount of escaping heat and thus increases the heat insulation of the building below.

Effects on Water Resources

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).

80% to 100% of the precipitation on standard roofs is carried off by drainage, whereas the amount is only 30% on green roofs. The remainder is released back into the air via evaporation and thus contributes decisively to reducing the lack of humidity in the city that results from soil capping. A further advantage of roof greening is the delayed release of precipitation water, which substantially relieves the city drainage system and reduces the danger of flooding (RÜNGELER, 1998).

The updated version of DIN 1986-100 "Drainage systems on private ground – Specifications in relation to DIN EN 752 and DIN EN 12056" from May 2008 contains all recent requirements and approaches in the field of rainwater drainage. Thus it constitutes a compact set of regulations for the planning and installation of drainage systems in Germany with all major specifications for the practical use.

Type of the surface	Runoff coefficient
1) Waterproof surfaces
Roof surfaces/concrete surfaces/ramps	1,0
Compacted surfaces with joint sealing/bituminous surfaces (asphalt)/paving with cast grout	1,0
Gravel roofs	0,5
Green roof surfaces for intensive/extensive planting with a minimum system thickness of 10 cm	0,3
Green roof surfaces for extensive planting with a maximum system thickness of 10 cm	0,5
2) Partially permeable and weakly discharging surfaces
Concrete paving/paving stones laid in sand or cinder	0,7
Paved surfaces, with a joint portion > 15 %, e.g. 10 cm x 10 cm and smaller	0,6
Water-bound surfaces	0,5
Playgrounds with partial pavement	0,3
Sports grounds with drainage (synthetic surfaces, artificial turf)	0,6
Sports grounds with drainage (cinder fields)	0,4
Sports grounds with drainage (grass fields)	0,3
3) Water-permeable surfaces without or only negligible water drainage
Parks and vegetation areas, gravel and cinder soil, loose gravel, also with partial pavement	0,0
Garden paths with water-bound surface	0,0
Driveways with grass pavers	0,0

Table 6/2: Runoff coefficients of different surfaces, DIN 1986-100 (2008)

Sewer fees can also be fixed on the basis of these values, with graded categories depending on the relief impacts for the urban drainage system. Such measures of indirect funding of existing buildings can be complemented by direct subsidies (e.g. support programme for green roofs).

Thorough analyses and scientific proofs for the efficiency of green roofs in regard to ecology, building physics and urban development suggest that the measure should be more strongly recommended. The German Association of Roof Gardeners ( Deutscher Dachgärtnerverband ) provides a brochure on this topic and further information on the Internet (http://www.dachgaertnerverband.de).

Legal Bases

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).

Conflicts between the use of solar energy and roof greening are not expected as the roof inclination for the solar use is steeper than that for the greening.

The combination of a greened flat/pitched roof and a roof-mounted photovoltaic plant, however, could actually be advantageous: As the efficiency of solar cells depends on the temperature and can be decreased by the strong midday heat in summer, the strategic placing of a photovoltaic plant could cool down the roof and thus increase the power output.

But the scientific proof for this correlation is yet to be delivered. What needs to be ensured is the water supply and sufficient light/insolation for the plants below the modules. The demands for vegetation below photovoltaic modules should be changed from full-sun to all-purpose plants. The total number of species on photovoltaic roofs will rise through the growing number of possible locations. This makes tending inevitable, e.g. the quick removal of plants which grow too high. Figure 6/7 shows a successful example of a green roof installed on a solar plant in Dresden.