GreenSpec Green roofs design guidance

GreenSpec Green roofs design guidance

Location

The location of the green roof plays an important role in the design process. The height of the roof above the ground, its exposure to wind, the roofs orientation to the sun and shading by surrounding buildings during parts of the day will have an impact. The general climate of the area and the specific microclimate on the roof must also be considered. Views to and from the roof may also determine where certain elements are located for maximum effect.

Consultants

Structural Engineer to calculate loading capacity of the roof

M&E to calculate the heating and cooling implications and to integrate existing and proposed mechanical equipment and drainage needs.

Structure

Plants

Construction / Installation

Be aware of installation issues. The installers should have experience with green roof systems. In fact, it may be preferable to have one company handle the whole project, from re-roofing to planting, thus avoiding scheduling conflicts and damage claims between the various trades. It will also bring single point responsibility post-construction. Methods for getting the materials up to the roof should be discussed to determine cost and potential equipment rentals. Timing is also important.

Maintenance

Both plant maintenance and maintenance of the waterproofing membrane are required. Depending on whether the green roof is extensive or intensive, required plant maintenance will range from two to three yearly inspections to check for weeds or damage, to weekly visits for irrigation, pruning and replanting. To ensure continuity in the warranty and the upkeep, it is recommended that the fees for three to five years of this service be included in the original tender, and that maintenance contracts be awarded to the company that installed the green roof, or to an affiliate. Intensive systems typically require more maintenance than extensive systems due to the greater diversity of plants.

Maintenance and visual inspections of the waterproofing membrane can be complicated by the fact that the green roof system completely covers the membrane. Although the green roof protects the membrane from puncture damage and solar radiation, doubling its lifespan, leaks can still occur at joints, penetrations and flashings, due more to sloppy installation than to material failure. Regular maintenance inspections should be scheduled as for a standard roof installation, especially just before the warranty period expires. Some companies are recommending the incorporation of an electronic leak detection system between or underneath the waterproofing membrane to pinpoint the exact location of water leaks. Access strategies include keeping the sensitive areas free of plants and growing medium (gravel skirts, etc), and dividing the green roof into distinct compartments for ease of removal. Eventually, after 30-50 years, the membrane will have to be replaced. Depending on the roof size, building height, type of planting, and depth of growing medium, the system will either be removed and reinstalled over the new membrane, or replaced entirely. If the green roof can be removed and stored on the roof while the membrane is being replaced in sections, then the additional cost is labour only, and comparable to the original installation cost; if the green roof has to be moved off the roof, and then brought back up, costs will increase accordingly, and the arguments for starting afresh with new growing medium and plants, become more convincing.


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