Design

A person considering laying a green roof would be advised to take the following three factors into account :

Structural factors
In principle, any deck type is suitable for a green roof.
These include wood, profile metal and reinforced concrete.
The load-bearing capacity of the roof structure must allow the design load of the green roof. You must of course also consider the structure in a fully saturated condition and the weight of the plants when making the calculation. Some peak loads like trees and planting troughs must be considered separately.
The load-bearing capacity of the roof structure must certainly be considered in refurbishment projects. It should also be established whether the green roof can be used for recreational purposes or not.
Budgetary factors
The budget available naturally defines the type of green roof. The roof structure and the diversity of the type of vegetation make an intensive green roof considerably more expensive than an extensive roof. A large roof requires more materials and preferably a larger diversity of plants. The height and accessibility also define the installation costs. The installation costs reduce proportionately, the lager the roof is. The design of an intensive green roof is usualy the responsibility of a landscape architect and those costs must also be included. The price estimate must also take account of the possible subsidies awarded by various municipalities for extensive green roofs.
Technical maintenance aspects
A person choosing a green roof must realise that an intensive green roof requires more maintenance than an extensive green roof.

Once these three factors have been considered carefully, the design and execution phase begins.
The following checklist for the architectural elements, the vegetation technology and the safety aspects may serve as a guide.

Architectural Checklist

Roof structure
The bearing capacity must be suffiecient for a permanent load on the green roof.
The loads to be anticipated are approximately :

30-100 kg/m² - extensive
100-400 kg/m² - simple intensive
> 400 kg/m² - intensive

Slope of the roof
The minimum required slope of the roof is 1°; the maximum slope is 35°. From 10° special measures are required against sliding and erosion.

Elevation heigh
The elevation height of the architectural elements must be at least 150mm above the completed green roof.

Waterproofing
The waterproofing must be protected against root growth. Tests of this must be carried out according to CEN prEN13948.

Insulation
In principle, green roofs can be installed on both non-insulated and insulated roofs. Due to the permanent load of the green roof, account must be taken with insulated roofs of sufficient resistance against the compression of the insulation. No account needs to be taken in the calculation of the respective R-value of that of the green roof.

Wind load
The structure must comply with the NEN norms in force relating to wind load.
The number of rainwater drains must be calculated in relation to the water retention capacity of the green roof and the annual rainfall. They must be free from vegetation and always accessible for inspection.