May 17th

Green roofing for roof pitches over 10 degrees

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Typically green roofing is specified for ‘flat roofs’, that is those with a pitch under 10 degrees, but it’s becoming more and more common that green roofing is required on projects with roof pitches over 10 degrees.  Specified to help satisfy planning requirements and to achieve a project’s sustainable credentials, green roofing continues to grow in popularity.
In order to help you specify green roofing you should take into account

1.     Waterproofing layer

It has been proven that over time roots from green roof vegetation can damage the waterproofing and roof construction if a root resistant membrane is not used or no protection measures are 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.  Therefore it is essential that a root resistant membrane is selected for your green roof application.  If the waterproofing is not root resistant an additional root barrier has to be installed.  Attention also has to be given to the upstands, perimeters, joints and roof edges which also require protection against root penetration.

2.     Drainage layers

The most critical role of the drainage layer is to ensure there is adequate flow of water off the roof during and after rainfall.  Current designs of drainage layers allow a small proportion of the rainwater to be retained on the roof for longer periods of time without oversaturation.  Care needs to be taken to ensure the drainage layer is high enough to prevent contact between the filter layer and standing water. Should contact occur, the filter layer acts as a wick, soaking up the water.  This then causes excess water attenuation on the roof and increased weight on the structure.

3.     Weight of the system

One of the fundamental considerations when specifying a green roof system is the saturated weight of the proposed build up.  The full saturated weight must be obtained and confirmed with the structural engineer before the specification is completed.  Green roofs put a greater loading on buildings than cold and warm roof build ups. Ensure a complete green roof system is selected from a manufacturer who can provide product datasheets on every component of the particular green roof build up.

For pitched or barrel roofs you also need to consider the following

1.     Type of sedum

It is important to take into account the buildings location, climate and roof slope when selecting the type of sedum to utilise on a green roof.  Although sedum are renowned for their drought tolerance and ability to withstand extremes in climate it is important that a species is selected which is most suited to the conditions it will have to withstand on the roofs location.  In pitched/sloped roof applications it is important a quick establishing species is specified.  For added stability of the sedum a non-biodegradable backing must be specified.

2.     Fixing method for roof pitch over 10º

On pitched roof applications the correct installation and sufficient fixing of the complete system is paramount.  The membrane must be mechanically fastened instead of adhered to ensure maximum stability for the subsequent green roof layers.
Following installation of the membrane, it is essential retaining bars are installed at ½ metre intervals (or less depending on the pitch) to hold the drainage and sedum layers in place.
Where necessary the sedum should be stitched onto the roof using a retention system.  This will ensure that whilst it becomes fully established there is no risk of it becoming loose.

3.     Irrigation

With pitched green roof systems an efficient and effective irrigation system is required to compensate for the increased water runoff as subsequently the sedum dries out quicker.  When considering the irrigation system, the correct drainage layer must be specified which is suited to retain a sufficient amount of water on pitched applications.
To find out how Bailey Total Building Envelope helped meet the construction challenges of installing a green roof onto the barrel roof at St Andrews Golf Club read the case study

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