The secret of successful green roof construction lies in understanding what plants need. This article explains more.
Understanding green roof construction
There’s a lot involved in specifying and building a living green roof. First, you need to understand the strengths and weaknesses of your building. Then you need to know how to adapt that building into what is essentially a rather large planter with a thriving ecosystem.
Enviromat Sedum Matting and Meadowmat Wildflower Turf are both products that are frequently used in green roof construction. But they need very different growing conditions. To find out which green roof system is best for your building you need to know
- The weight bearing capacity of the roof
- Slopes and angles of the roof
- Type of waterproofing
- Where the sun falls at any time of day
- Any planning restrictions
- Accessibility for construction AND maintenance
Weights and loadings for Green Roofs
Plants need a growing medium in order to survive and so you must consider how much a green roof weighs before starting to build it. The growing medium is heavy – especially when it’s wet. How many substrates can your roof support? Bearing in mind that you also need to factor in live loading – i.e. snow or someone walking on the roof to maintain it.
To support lightweight sedum green roof construction, your roof needs a loading capacity of at least 120 Kg per square metre. For a wildflower roof with a 150mm depth of the substrate, work on loading capacity of 250Kg per square metre.
Slopes and angles for Green Roofs
The pitch of your roof determines how fast rainwater runs off it. A steep slope will shed water so fast that it cannot be absorbed by the growing medium and therefore will not benefit the plants. A completely flat roof on the other hand, will not drain well which potentially means that plants will end up sitting in water.
The ideal slope for living green roof is between 1 and 5 degrees. Over 10 degrees and you will need to consider some form of irrigation system. If the slope is more than 20 degrees you are likely to have problems with anchorage, wind scorch and dehydration. Probably best to avoid the steep slope scenario altogether.
Type of waterproofing for Green Roofs
Modern waterproofing materials like fibreglass or EPDM are impenetrable to all but the most robust root systems. That makes them perfect for sitting beneath a green roof build-up.
Traditional roofing felt, however, will need protecting. It’s great for waterproofing but can be damaged by plant roots. That’s not a problem. Simply pop a layer of thick polyethene over the roof before green roof construction begins.
Plants harness energy from the sun’s rays to help them grow. Some plants need more sun than others and this is something you need to research before investing in your green roof build-up.
A sedum roof is lightweight but it will not thrive in deep shade. Sedums need sun for at least half of the day. Likewise for most wildflowers although woodland plants such as champions are a little more shade tolerant.
You WILL need to access your roof 2-3 times a year for simple maintenance. Green roof construction needs to consider how you will get a) people and b) equipment on and off the roof safely. Bear in mind too that you may need to bring spent vegetation down from a wildflower roof. How will you do that?
How to go about green roof construction
Make sure you understand the build-up. There are several different green roof construction techniques. NEVER mix and match features from more than one build-up. That would be the equivalent of trying to fix your Porsche with parts from a tractor. The two just won’t work together. So for an Enviromat roof, we would only recommend using Enviromat drainage mat.
Ensure you’re ready before the materials arrive. That means the roof is strengthened (if necessary) and with waterproofing intact. Drainage outlets and edgings need to be in place too. Any delay in unrolling either Enviromat or Meadowmat will mean that the plants will deteriorate.
Distribute weights evenly across the roof. Stacking everything together means a lot of loading in one place. In turn that could lead to the collapse of the roof.
This video shows the construction of a simple, lightweight sedum roof. The sort of thing you would have on your garden shed or garage.
For technical details on either Enviromat or Meadowmat, please contact the Turfonline team who will be very happy to help.
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