Objetivos de Largo Plazo

The Importance of Trees

6.2.1 Any urban greening proposals, including green roofs, should consider the inclusion of trees. Compared to other types of greening, trees are generally the most effective in terms of:

♦ the amount of greenery they provide versus the ground surface area they occupy (though roots may occupy substantial underground space);

♦ their provision of shade;

♦ their rates of evapo-transpiration;

♦ their total leaf surface area and ability to filter air-bourne particulates and gaseous pollutants; ♦ their life-expectancy;

♦ their sensitivity to climatic and air quality fluctuations ♦ their visual mass;

♦ the ecological habitats they create;

♦ their long term maintenance and water requirements; and ♦ their price

6.2.2 Trees greatly enhance the various benefits offered by green roofs. Trees define spaces and provide micro-climates conducive to the creation of amenity spaces on rooftops, which may ultimately enhance the value of the property. A plant selection matrix of trees, palms and palm-like trees applicable for intensive green roofs in Hong Kong is presented in Appendix II.

Trees in Roof Gardens

6.2.3 When including trees in roof gardens the following key aspects should be considered.

♦ Provision of adequate soil volume for healthy growth and anchorage is critical. A simple rule of thumb is that 1.2m soil depth should be provided, although smaller trees may grow in shallower depths, provided that the lateral extent of soil is widened to compensate.

♦ Soil depth may be locally deepened at tree locations (either by mounding the soil surface or locally deepening the planter). Trees may be located over structural columns to take advantage of loading efficiency.

♦ Tree anchoring may be by staking or tying down of the root ball but in all cases must not interfere with the integrity of the waterproofing.

♦ Species selection is critical. Trees need to be selected which have a strong branching system, are flexible in high winds and have leaves that do not appear battered when exposed to strong winds.

Calculating Plant Weights

6.2.4 Calculating plant weights at maturity (particularly for trees) is an important consideration during the initial design stages of a green roof. There is no industry standard for calculating tree weights. The simple method presented below in Figure 6.2 was adapted from the successful techniques used for the importing, transporting, and installing of large trees to Penny’s Bay and Hong Kong Disneyland.

6.2.5 Tree weight calculations consists of three components; stems, branches and roots. Weight calculations for roots can simply use the entire volume of the soil multiplied by an average soil density (1750kg/m³). The proportion of the roots (around 20%) can be ignored because the heavier density of soil builds in an adequate safety margin. The above-soil weight calculations are based on a tree being simplified into a consistent cylinder, where the volume is easy to calculate (¶.r² x H), multiplied by the density of hardwood (650kg/m³). The estimated radius at the branching height (usually ⅓ of the estimated total height) is considered a usable average for the volumetric calculations of the assumed cylinder.

Figure 6.2 Calculating Plant Weights at Maturity

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6.2.6 It should be noted that the above calculations do not factor in wind loading which needs to be added to the weight loading calculations. Additional wind loading results from the horizontal force of the wind being transferred to the roots of the tree, resulting in a downward pressure on one side and an upward pressure on the other side. This is best left for suitably qualified engineers and experts to determine. Also not included are tree supports and anchorage which may become exceedingly heavy with larger trees. 6.2.7 Determining plant weights during construction is also important. These weights need to be calculated to

determine labour and transport costs and to ensure the balancing of cranes that may be used to lift heavy trees. Table 6.2 below presents a very general indication of typical weights for a variety of plant materials.

Table 6.2 Plant Weights

Plant Type Description Root Ball Size and Depth

Estimated Planting

Weight

TREES

Light Standard Tree Stem dia. 35mm, total height 2.5m Root ball dia. 350mm, depth 300mm 54 kg Standard Tree Stem dia. 60mm, total height 3.5m Root ball dia. 350mm, depth 300mm 58 kg Heavy Stand. Tree Stem dia. 150mm, total height 3.5m Root ball dia. 400mm, depth 350mm 119 kg

Semi-mature Tree Stem dia. 200mm, total height 5.5m Root ball dia. 1000mm, depth 600mm 961 kg Specimen Trees Stem dia. 200mm, total height 7.0m Root ball dia. 1000mm, depth 600mm 991 kg Stem dia. 250mm, total height 8.0m Root ball dia. 1200mm, depth 800mm 1884 kg Stem dia. 300mm, total height 10m Root ball dia. 1500mm, depth 1000mm 3640 kg PALMS & BAMBOO (well formed and with good fronds)

Palms Stem dia. 30mm, clear trunk ht. 300mm Root ball 200mm, depth 250mm 14 kg Stem dia. 40mm, clear trunk ht. 700mm Root ball 200mm, depth 250mm 15 kg Stem dia. 80mm, clear trunk ht. 1500mm Root ball 350mm, depth 300mm 57 kg Stem dia. 100mm, clear trunk ht. 3000mm Root ball 500mm, depth 450mm 174 kg Stem dia. 120mm, clear trunk ht. 4500mm Root ball 1000mm, depth 600mm 881 kg Stem dia. 150mm, clear trunk ht. 7500mm Root ball 1300mm, depth 800mm 1997 kg Stem dia. 170mm, clear trunk ht. 9500mm Root ball 1800mm, depth 1000mm 4721 kg Bamboo Palms Stem dia. 175mm, clear trunk ht. 1500mm Root ball 350mm, depth 300mm 75 kg

Stem dia. 210mm, clear trunk ht. 2000mm Root ball 350mm, depth 300mm 97 kg Bamboo Stem dia. 50mm, overall height 600mm container dia 400mm, depth 450mm 103 kg SHRUBS

Small Shrub Stem dia. 10mm, Overall height. 400mm container dia 130mm, depth 150mm 4 kg Medium Shrub Stem dia. 15mm, Overall height. 600mm container dia 150mm, depth 200mm 6 kg GROUND COVERS

Small Gnd covers Average dia. 350mm Pot grown dia.125mm, depth 150mm 3 kg

Medium Gnd covers Average dia. 500mm Pot grown dia.150mm, depth 200mm 6 kg

Large Gnd covers Average dia. 1200mm Pot grown dia.200mm, depth 300mm 17 kg

Note: Calculations use thick-diameter trees, a wood density of 650kg/m³ and soil density of 1750kg/m³. (Source: Adapted from values provided by Asia Landscaping Ltd.)

7

EXTENSIVE GREEN ROOFS (ECO-ROOFS) IN HONG KONG

7.1 CONSTRUCTION DETAILS

Figure 7.1 Details of General Roof Build-up

(Source: Adapted from technical drawings for Eco Park, Hong Kong, from Sarnafil Ltd.)

Figure 7.2 Details of Parapet Edge Treatment

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Figure 7.3 Details of Typical Drainage Inspection Chamber

(Source: Adapted from technical drawings for Eco Park, Hong Kong, from Sarnafil Ltd.)

Figure 7.4 Detail of Typical Penetration of Roof-top Utilities

(Source: Adapted from technical drawings for Eco Park, Hong Kong, from Sarnafil Ltd.)