The positives of building a DIY green roof for your garden are two-fold. For one, you now have a new patch of plants that will encourage wildlife and colour into your backyard, and if it replaces some downtrodden roof material then the aesthetic will be significantly improved. Plus, the personal satisfaction of doing it all yourself is endlessly gratifying.
However, as fantastic as they look, a green roof does require some consideration and pre-planning. For one, what is the ideal slope for a green roof? And how much load can my roof withstand?
Lastly, the secret of a successful long-term green roof lies in understanding what plants need. This guide will give you a complete run-down of all you need to know to create a long-lasting green roof.
Understanding Green Roof Construction
There is a lot involved in specifying and building a living green roof. First, you need to understand the strengths and weaknesses of your building. This will allow you 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 require 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 to survive, so you must consider how much a green roof weighs before starting to build it. Growing mediums can be heavy, especially when wet. How much substrate can your roof support? Keep in mind that you should factor in live loading, that is people on the roof and moving around; as well as snow and the correct depth of soil / substrate for the product you have chosen.
To support lightweight sedum green roof construction, your roof needs a loading capacity of at least 120kg per square metre. For a wildflower roof with a 150mm depth of the substrate, work on a 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. As with any garden project, it requires a balance; a steep slope will shed water so fast that it cannot be absorbed by the growing medium and won’t benefit the plants; on the other hand, a completely flat roof will not drain well which may leave plants sitting in water.
The ideal slope for a living green roof is between 1 and 5 degrees. If the slope is over 10 degrees, you will need to consider some form of irrigation system. More than 20 degrees and you are likely to have problems with anchorage, wind scorch and dehydration. Probably best to avoid the steep slope scenario altogether.
Types of Waterproofing for Green Roofs
Modern waterproofing materials like fibreglass or EPDM (a highly durable rubber material) are impenetrable to all but the most robust of root networks.This makes them perfect for sitting beneath a green roof build-up.
Traditional roofing felt is great for waterproofing but will need additional protection. To avoid it being damaged by plant roots, simply pop a layer of thick polythene over the roof before green roof construction begins.
Factoring in the Sunshine
Plants harness energy from the sun’s rays to help them grow. Every species is different, though, and some plants need more sun than others. This is something you should research before investing in your green roof build-up.
A sedum roof is lightweight but will not thrive in deep shade. Sedums need sun for at least half of the day, likewise for most wildflowers. Woodland plants such as champions are more shade tolerant, so these can be placed in less direct sunlight than sedums and other wildflowers.
You will need to access your roof 2-3 times a year for simple maintenance so please think about how you will get both people and equipment on and off the roof safely. Also, bear in mind that you may need to bring spent vegetation down from a wildflower roof. Ask yourself beforehand – how will I do that?
How to Start Green Roof Construction
Before beginning green roof construction, make sure you understand the build-up. There are several different green roof construction techniques, and you should 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. For an Enviromat roof, we would only recommend using an Enviromat drainage mat, for instance.
Next, ensure you’re ready before the materials arrive. That means the roof should be strengthened if necessary and have its 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 quickly.
Then, distribute the weight evenly across the roof. Stacking everything together means a lot of loading in one place, which in turn could lead to the collapse of the roof. The following video shows the construction of a simple, lightweight sedum roof, similar to those found 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 you with any query.