What is roof greening | green-roof-toppers

Definitions of roof greening

Roof greening systems

There are generally two types of green roof system, namely intensive and extensive system. The table below shows the comparasion between the two systems.

Basic Stucture of a Green Roof

There are different layers constutiting a green roof, each carrying their own functions. With the variety of materials and technology used in each layer, the design of green roof can fit from a typical roof in Hong Kong to that in other cities with contrasting climate.

Vegetation

From lawn to shrubs or trees, the choice of vegetation gives the final appearance to the roof. Ability to resist corrosion and drought, and be able to withstand strong sunlight and wind must be considered when choosing the vagetation to be planted as these are the main condition faced on a roof.
Shorter plants should be planted on shallower soil, thus, only mosses to grass can be grown in the extensive system.

Engineered Soil-based Growth Medium

The selection and combination of materials for the formation of the soil mix depends on the type of plants grown, moisture retention ability, weight that the structure can hold...
The thickness of this layer varies upon the system used, with shallower soil for extensive system and deeper soil for intensive system.
Lava rocks, drainage rocks, pea gravels, sand and compost are combined in different ratio for different plant use.

Filter Fabric

This layer is a "geotextile" used to restrict the flow of fine soil particles and other fine substances to pass through while letting the water to pass freely, so that the drainage will not be clogged by the soil particles.
Tightly woven fabric like polyester or polysthylene is used for this layer.

Reservoir Layer (Optional for extensive system)

This layer retains moisture for plant growth above and the materials used depends on the types of plants grown.
Aggregates are used in the reservoir layer for intensive system as large amount of water is needed.
It typically consists of polyethylene-bases panels formed into a 3D array of water reservoir cups and drainage channels, which can hold a specific amount of water with overflow holes.

Moisture Retention Layer

This layer functions as moisture storage for plant growth. The materials used depends on the water retaining ability of the upper layers.
An absorptive mat with recycled polypropylene fibers is used for this layer.

Aeration Layer (Optional)

This layer is required only when thermal insulation layer is used as it retains the R-value of the insulation layer.
This layer allows air to move across the top surface of the insulation layer and act as the moisture resistance layer.
The composition of the aeration layer is very similar to that of a drainage mat.

Thermal Insulation Layer (Optional)

Buildings with minimal human activities like carpark, will not need to have thermal insulation.
This layer can keep the soil true to the outdoor environment and the indoor environment away from the effect of outdoor temperature. The loft will thus not be heated up in the afternoon and heat loss by conduction through the roof in winter from indoor (heating) to outdoor (low temperature) will be minimized.
A minimum of 2.7atm compressive strength should be used as the layers on top will cause the change in thickness of the materials if the compressive strength is not high enough, which then affect its thermal insulation ability.
Extruded polystyrene boards (XPS) (R-value: 0.2 per mm) is suggested for this layer.

Drainage Layer

This layer provides a location for the moisture to move within the system laterally and enhance the durability of the waterproofing layers below.
The run off from the drainage layer will be removed by the drainage system installed on your roof, which is outside of the green roof system.
Drainage mat with waffle-like plastic core and a filter fabric; or an insulating drainage panels with grooved drainage channels that are covered with a geotextile on one face are used for this layer.

Root Barrier

This layer prevents the plant roots prenetrating the layers below and damage them, causing water leakage.
This layer can sometimes can combine with the protection course or the drainage course.
High density polyethylene boards with heat-welded seams, granulated modified bitumen membranes with root-inhibiting additives and heat-welded seams or o.25mm minimum of polyethylene sheets are used.

Protection Course

This layer protects the waterproofing layer from damage from the installation of green roof as well as other activities on it.
The heavier the topping objects, the thicker the protection course.
Asphaltic sheets, extruded polystyrene boards (XPS) or PVC sheets are used for this layer.

Waterproofing Layer

This layer acts as the last barrier to protect the roof deck, prolonging the life of the roof as it successfully blocks the penetration of water vapour to the concrete roof structure.
1.5-2.5mm thick PVC is used for this layer.

What is roof greening?

structure

systems

casestudy

Green roof has appeared in the Northern Europe as early as prehistoric times in another form called sod roof, which is basically placing a layer of turf or sod on a few layers of birch bark above the wooden roof boards, it is the pioneer of modern green roof. Before the invention of the modern roof with industrial materials like metal, it was the most common roof across the Europe, especially in the Scandinavia district. Historic publications like the Hebrew Bible have also included the concept of roof greening, in the phase, "the grass on the housetops" (2 Kings 19:26), showing the appearance of green roof as early as that time of the bible. The first modern green roof we see nowadays evolved from the 1960s and it has been replacing this sod roof and traditional greening methods like planning an enormous park in the city centre and is catering the need of a modern society better, as it needs no extra space in a densely planned city, where buildings are merely enough to accommodate all citizen’s daily and financial activities.