Design and Development of a Roof Garden
- Design Parameters -
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Protection of the Roof and Structure
The single most important element in rooftop garden construction is protecting the integrity of the roof and the structural components under the garden. For this reason there must be waterproofing of exceptional longevity to prevent damage and to reduce the possibility of long term expensive reconstruction. For this reason it is recommended a completely new waterproofing layer be added to the existing structure to insure the longevity and integrity of the waterproofing system.

Load Bearing Capacity
The structural engineer should verify the maximum load bearing capacity of the existing structure. These figures should be available from the records of the previous construction of the helipad. Typically, a minimum additional dead load limit of 150 psf between columns is needed to accommodate the construction of a roof garden. Loads above columns and at the roof’s edge can be considerably higher, however a structural engineer should be consulted to establish the load bearing capacity of those areas. These higher load bearing areas should be used to accommodate larger specimen plantings and trees.

Waterproofing
As mentioned before, a completely new waterproofing system should be installed to protect the building’s structure. There are several types of waterproofing available, however, elastomeric materials offer the greatest protection. Bituminous waterproofing should be avoided. Over time the organic components in bituminous waterproofing interact with the soils and the plant materials and therefore increase the likelihood of system failure.

A properly installed waterproofing system can last the lifetime of the building, however a single small leak may require the removal of the entire garden to find and repair the damage. Therefore, in order to insure theintegrity of the waterproofing it is recommended a protective topping coat of concrete be applied, as soon as possible, following the installation of the new waterproofing.

KEY POINTS: Protection of the Structure
The single most important consideration regarding roof and deck garden construction is protecting the roof and structure from damage due to excessive loading or leaks.

• A structural engineer should always be consulted prior to roof garden landscape design and construction.
• Rooftop structures must typically be able to support a dead load of 150psf to accommodate the construction of a garden.
• The roof must be completely covered by an elastomeric material and protected by a concrete topping slab.