Water Retention Mat
Most roofs have some sort of slope to them. Even a “flat” roof has a bit of a fall to allow for drainage. This is vital. If the subject of your planned green roof has puddles laying on it for more than 24 hours after a rainstorm, this needs to be addressed before adding the green roof buildup.
The green roof buildup sits on top of conventional waterproofing. Make sure that’s watertight before installing any of the green roof layers.
For a “flat” roof. That is to say one where the slope is less than three degrees. It’s important to start your green roof buildup with a layer of drainage matting. If the slope is greater than three degrees, this layer isn’t needed.
Irrigation vs water retention
The sedums on your green roof are incredibly drought tolerant and they hate having soggy feet. But, they are living things and they do need to have access to water. Just not too much of it.
There are 2 ways of tackling this.
- Install an irrigation system so that you can water the plants during long periods of drought (you will need to water them regularly for the first few weeks after installation anyway)
- Put a layer of water retentive material up on the roof so that the plants have some water available but the substrate is not sodden.
Option 2 could involve a deep layer of growing medium or it could be a matter of using water retention matting.
Deep substrate is preferable. However, it’s not easy to lift onto the roof and once it’s there it can exert quite a heavy load on the building. If the building has been designed to take the weight – excellent. If you’re not sure – get expert advice.
Water retention matting can normally be bought at the same time as sedum matting, In fact Enviromat All-In-One Sedum Matting has the water retention matting sewn to the back of the sedum mat. That saves you a job!