Design and Development of a Roof Garden
- Design Parameters -
(page 5/10)

Irrigation
The relatively thin, well-drained soil mixtures used in roof garden construction cannot provide the plantings with the subsurface water normally available to ground level plantings. Care must be exercised to prevent the soil mass from drying out and causing damage to the plant materials. Hand watering is too labor intensive and is not cost effective. Therefore, we recommend the installation of a sensor controlled drip irrigation system. Drip irrigation is preferred in roof garden applications because the effects of wind can cause above ground systems to perform inconsistently.

Mulch
Drying and overheating of the soil can be prevented by the application of 2-3 inches of shredded hardwood mulch. Besides providing protection of the plant materials this mulch serves to hide the drip irrigation lines and emitters.

KEY POINTS: Planting Provisions
An important consideration regarding roof and deck garden construction is the substrate supporting the plantings.

• The existing roof drainage system is adequate to support the installation of a roof garden.
• Planting media and the drainage course should provide for fast percolation of water and be free of fine silts that can clog the filter blanket and block drainage.
• Planting media should contain sufficient mineral content to stabilize the plantings and maintain soil mass.
• On a regular basis soils require topdressing to replenishment decayed organic material.
• Drip irrigation is the prefer method of providing moisture to the planting.

Climatic Considerations
Climate and exposure can be prime contributing factors in the success or failure of any outdoor space. This is both a consideration in the selection of plant materials but also a factor in human use and comfort. Wind, sun and shade, and extremes of temperature, as well as long dry or wet periods, snow loads and frost are much greater problems for roof gardens than for other landscapes.

Climate
The Minnesota climate is one of extremes and therefore care must be exercised to make provisions for these extremes. As much as possible, increasing the depth of the soils will mitigate some of these extremes. A minimum of 12” of soils is required for sod, groundcovers and annual planting areas. For perennials and small shrubs a