Design and Development of a Roof Garden
- Design Parameters -
(page 4/10)

Planting Provisions
Like the roof on which the garden is to be built, a roof garden is constructed in layers. Just as failure in the roof components can cause significant damage to the building, so too can failure of the planting components causes significant and costly damage. Therefore, it is imperative to take care in choosing and installing materials of the highest quality and species conductive to a rooftop’s environment.

The Roof Drains
The existing roof drains are appropriate for use within the roof garden. Some minor modifications may be required to accommodate the new waterproofing and topping slab. Nevertheless, the four roof drains and their size are adequate to support the foof garden’s needs. Special care should be taken and accommodations made to allow access to those roof drains should there ever be the need to access them for cleaning.

The Drainage Layer
The drainage layer, directly above the concrete protective slab, should be very porous to permit water to pass easily through it. It must be permanent and continuous over the entire roof surface and strong enough to support the weight of the plant materials and hardscape above it. This layer must be kept free of any materials that could prevent the free flow of water to the drains. Because of its lightweight and integrated filter fabric, McCaren Designs recommends the use of Enkadrain for this drainage layer. Further, its .75 inch thickness allows for more planting media in areas next to paved areas.

Filter Fabric
To prevent the planting media from going into solution and being lost in or clogging the drainage layer and roof drains, a water permeable barrier of filter fabric is needed. As mentioned previously we recommend Enkadrain because the filter fabric is integral with the drainage course.

Planting Media
The critical criteria in the formulation of a suitable planting media for roof gardens include: lightweight; the ability to hold nutrients; adequate moisture holding capacity; and the capability of developing a firm (for plant stability) yet easily drained soil structure. There are several ready mixed medias available that meet these requirements. However, care must be taken in selecting these lightweight soils to be sure there are adequate non-organic components incorporated into the mix. Soil mixes consisting of solely organic material will decompose, losing nearly 30% of its mass every year, thereby requiring frequent topdressing to maintain the soil mass. Therefore one should incorporate sand and expanded shale into these prepared planting medias. When this mixture is properly moist it will weigh approximately 60 pounds per cubic foot.