Page 51 - Hong Kong Green School Guide

3. Green School Guide - 3.1 Green School Sites
Hong Kong Green Building Council 49
• Consideration of wind load to avoid the roof top greenery being uplifted by
strong wind; and
• Long-term and on-going maintenance commitment.
c. Solutions
• Select roof area that has access to sufficient daylight;
• Assess the structural loading allowance of roof by Authorised Person (AP)
and Registered Structural Engineer (RSE);
• Select plants that do not require much soil to grow or use lightweight soil
which would reduce soil weight. Please consult Landscape Architect and/or
Landscape Contractor and refer to Useful Reference (9) for details;
• Place heavy dead loads of bulky vegetations and landscaping feature over
structural members, such as columns and beams, where practicable;
• Lightweight turf system may be suitable for existing roof and structural
strengthening is not necessary, but the maintenance is labour intensive.
Lightweight turf system shall include water proof membrane and drainage
composite to prevent water seepage problem. Please consult Authorised
Person (AP), Registered Structural Engineer (RSE) and turf systemcontractor
to assess the practicality;
• Install a protection layer (or moisture retention mat) between the water proof
layer of the roof and the plants to avoid leakage. Please consult Authorised
Person (AP) and Registered Structural Engineer (RSE) for details;
• There are two basic types of green roof systems, known as ‘intensive’ and
‘extensive’;
• Intensive green roofs are generally of greater weight, require higher capital
costs, more plantings and higher maintenance requirements of which the soil
depth is at least 200mm. Deeper soil allows more diverse selection of plant,
but due to the heavier weight of soil, structural requirement is more stringent;
• Extensive green roofs are generally of low weight, require lower capital cost
and minimal maintenance of which the soil depth is between 50mm and
150mm. Plants selected are usually low in height and hardy. Some examples
for extensive green roofs include pre-grown blanket system and tray system,
etc;
• Pre-grown blanket system is a pre-grown vegetation covering. The blanket
is a layered system designed to achieve instant vegetation coverage, with
a rich variety of species. Advantages and disadvantages of the system are
detailed in Useful Reference (1);
• Tray system is a modular grid like system that is self-contained and has
drainage connected directly to roof outflow pipes rather than relying on the
existing waterproofing. The system is easier to be maintained and is suitable
for roof retrofitting. Please refer to Useful Reference (1) for details;