Construction Engineering

Roof substructure

Roof substructure

Green Roofs can be realised on different types of roofs. Wooden constructions, metal sheeting as well as reinforced concrete decks can be considered as appropriate roof substructures. The base for the Green Roof is a waterproof roof construction with appropriate load bearing capacity.

WaterproofingWaterproofing

Over a period of time roots can damage the waterproofing and roof construction if there have been no corresponding protection measures taken. The root resistance of the waterproofing is determined from the "Procedure for investigating resistance to root penetration at green-roof sites" by the FLL (The Landscaping and Landscape Development Research Society). Over 70 different waterproofing products meet the requirements of this test. If the waterproofing is not root resistant an additional root barrier has to be installed. Aside from the roof surface, the upstands, perimeters, joints and roof edges also have to be protected against root penetration.

Roof parapetsRoof parapets, roof penetrations and adjacent building parts

For Green Roofs, the following upstand and perimeter heights have to be considered:

  • Upstand height for adjacent building parts and penetrations: minimum of
    150 mm (6 in).
  • Upstand height for roof edges: minimum of 100 mm (4 in).

Important: The upstand height is always measured from the upper surface of the Green Roof system build up or gravel strip. Clamping profiles guarantee reliable protection and a tight connection of the upstand areas. In special cases ("Wheelchair accessible apartments", for example) reduced upstand heights are possible, in this case special constructions are necessary. Roof penetrations (e.g. water connections, building parts for the usage of the roof area, etc.), when possible, should be grouped in order to keep roof penetration to a minimum.

Roof slopeRoof slope

Using modern technologies it is possible to install a reliable Green Roof system build up not only on conventional flat roofs, but also on saddle roofs, shed roofs and barrel roofs. Special technical precautions for the mitigation of existing shear forces and erosion are only necessary for a roof slope over 10°. Roofs with a slope of more than 45° are normally not suitable for a Green Roof system build-up. Roofs with a slope of less than 2% are special roof constructions on which puddles often develop. In order to avoid Extensive Green Roofs from being damaged by water retention, specific arrangements for the roof drainage are necessary. In contrast, it can be beneficial for Intensive Green Roofs to design the roof construction without slope to allow for dam up irrigation.

Roof constructionRoof construction

Assumed loadAssumed load

The maximum load bearing capacity of the roof construction must be considered when installing Green Roof system build-ups. Therefore, the water saturated weight of the Green Roof system build-up, including vegetation must be calculated as permanent load. Simple Extensive Green Roofs weigh between 60-150 kg/m2 (13.0-30.0 lb/sq.ft.) depending on the thickness of the Green Roof system build-up. On most gravel roofs, once the gravel has been removed, Extensive Green Roof system build-ups can be installed without increasing the assumed load.

Trees, bushes and construction elements such as pergolas and walkways cause high point loads and, therefore, have to be calculated accordingly. This calculation is also necessary for the dammed water on Intensive Green Roofs. The so called traffic and snow loads are to be considered separately.

Wind upliftWind uplift

A Green Roof must be tight to the roof, especially in cases of strong wind. When designing and installing the Green Roof, safety measures against wind uplift are to be considered. This is especially important when the Green Roof provides the load for a loose laid waterproofing and root barrier. The actual influence from the wind depends on the local wind zone, height of the building, roof type, slope, and area (whether corner, middle or edge) and the substructure.

Roof DrainageRoof Drainage

Green Roof systems store a major part of the annual precipitation and release it to the atmosphere by transpiration. Depending on the thickness of the Green Roof system build-up and rain intensity, surplus water may accumulate at certain times and must be drained off the roof area. The number of roof outlets and the penetrability factor, or more precisely, the water retaining capacity of the Green Roof system build-up, has to be adjusted to the average local precipitation. Roof outlets are to be kept free of substrate and vegetation and have to be controllable at all times. For this purpose "inspection chambers" are installed over the roof outlets. Due to safety precautions, roof areas with inlayed drainage must always have two drainage outlets or one outlet and one safety overflow. For facades and roof areas, gravel strips, gullies and grids provide fast drainage of rainwater into the drainage system.

IrrigationIrrigation

Simple Extensive Green Roofs with drought resistant plant species have to be irrigated only during planting and installation maintenance over the first year. After it's establishment, the annual rainfall is sufficient to sustain the vegetation. In contrast, the requirements are more involved for Intensive Green Roofs with lawn, shrubs, bushes or trees. An adequate number of precisely dimensioned hoses with automatic irrigation units make plant maintenance during drought periods more manageable. The water supply for roof gardens with no slope can be increased through additional dam-up irrigation. In order to lower the consumption of drinking water, roof gardens can also be irrigated with cistern water.

Fire PreventionFire Prevention

As a part of the "Hard Roof" classification, Intensive Green Roofs provide preventative fire protection in the case of sparks and radiating heat. The criteria that Extensive Green Roofs must meet in order to be considered fire resistant, are already met by most Green Roof systems that are offered by suppliers. Openings within the Green Roof (e.g. skylights) need to be installed with a vegetation free zone (approx. 500 mm or 20 in). On larger roof areas a vegetation free zone (e.g. gravel strip or concrete slabs) are to be installed at least every 40 m.

Accident preventionAccident prevention

Working on roofs bears a higher risk for accidents due to the exposed location. This has to be taken into consideration during the planning, design and installation phases. Securing measures are necessary at a height of more than 3 m (9 ft). Fall protection measures include: railings, scaffolding or nets as well as ropes and chains. Various systems provide fall protection by using the weight of the Green Roof system build-up and also avoiding the penetration of the waterproofing.

Access to the roofAccess to the roof

Access to the roof has to be guaranteed throughout the installation and for annual maintenance and service of the Green Roof.

Additional functionsAdditional functions

Green Roof system build-ups can improve the environmental and energy balances of the roof:

Approval

In regards to approvals, Extensive Green Roofs do not require any; whereas, Intensive Green Roofs normally do. The local building authorities are able to provide corresponding requirements.