What is a Green Roof Drainage Layer?
A green roof drainage layer is usually an HDPE (high density poly ethylene) membrane that stores water in cups so that plants can draw on it in times of need. Whilst excess rainwater passes through perforations between the storage cups.
The amount a drainage layer can store is ultimately down to cup size, but typically extensive green roof drainage layers can store between 1.5l – 5.5l / m².
A 12mm layer that stores 1.5l / m² is usually adequate for extensive UK living roofs. But obviously, the more intensive the roof the larger the drainage solution and storage capacity required.
The 6 Best Green Roof Drainage Layers
Oldroyd XV20
Best overall drainage layer
The Oldroyd XV20 is manufactured with patented multi-layer technology that allows a higher proportion of recycled plastic to be incorporated without compromising on product quality.
It’s also significantly lighter than comparable aggregate-based drainage layers, which makes it ideal for DIY green roofing projects.
I have to say if I had made this product – unlikely I admit – I would’ve added another 1.1% recycled plastic so I could legally say ‘primarily made from recycled plastic.’ But I guess that’s the marketing in me.
If you’re searching for a more all-encompassing solution, Oldroyd also have a 20m x 2.08m version – somehow made with MORE recycled plastic.
Evadrain Drainage Reservoir Board
Best value drainage layer
Evadrain
Drainage Reservoir Board-
1.91m x 0.95m
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5 star rating
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Made with recycled polystyrene
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1.38kg / m²
The Evadrain Drainage Reservoir Board is designed for both intensive and extensive green roofs – perfect for sturdy DIY jobs.
Completely made from recycled polystyrene, with a water holding capacity of 10l / m², has a pretty extreme loading capacity. And – at 1.36kg / m² your shed or annex roof is in good hands.
Wallbarn Protecto Drain 8 Drainage Membrane
Best drainage layer for large DIY projects
Wallbarn
Protecto Drain 8 Drainage Membrane-
20m x 2m
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4.7 star rating
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20mm drainage system
Wallbarn’s Protecto Drain 8 Drainage Layer is a single HDPE membrane is designed to direct water away from the surface of the structure towards the appropriate drainage system. Hence its applicability to green roofs.
Super lightweight, abrasion-proof and cupped in structure in order to create a permanent air gap between the roof or wall and the backfill. This helps to reduce pressure points and helps in damp reduction and building ventilation.
Whilst this layer can create a viable waterproofing membrane in itself. It is best suited as a drainage layer that provides additional waterproofing and structural support.
Wallbarn Protecto Drain Drainage Membrane
Best drainage layer for larger projects
Wallbarn’s Protecto Drain Drainage Membrane is a chunky product originally developed for use as a drainage layer and reservoir in commercial green roof construction.
The HDPE membrane is made with 20mm cups, but the area between the cups is perforated to allow for optimal drainage – especially on a horizontal plane.
Water is stored in the cups to support life, whilst the excess water drains away. And the water in the cups can be drawn up through the filter layer to feed your rooftop plant layer.
A hard-wearing, flexible membrane that’s easy to handle with a damp and rot-proof guarantee.
It’s even safe with drinking water. Although we’d question your sanity if you’re drinking your green roof runoff.
Sedum Supply 12mm Drainage System
Best scalable drainage layer
Sedum Supply
12mm Drainage System-
4.7 star rating
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12mm drainage system
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1kg / m²
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Extensive use of recycled polymers
Whilst you may see a £7.59 price tag popup on your screen for Sedum Supply’s 12mm Drainage Layer and think why the hell have Green Roof Guide said it costs £9.72? They have in fact excluded VAT. Which I’ve always found annoying. Anyway.
Their extensive use of recycled polymers is, of course, right up our street. And Roofdrain’s perforated HDPE core allows for optimal water attenuation (reduction to you and me) and drainage. Alongside:
- Protection of the waterproof membrane
- Filtration: To prevent any particles blocking the drainage layer
- Separation of layers
And it includes a pre-bonded filter fleece attached to each side to provide additional elemental protection.
They also supply a 20mm layer, which allows for greater water attenuation, protection and is ideal for more complex, wildflower and sedum extensive / semi-intensive living roofs.
Monkroyds Drainage Layer
Best recycled drainage layer
Monkroyds
Drainage Layer-
4.7 star rating
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12mm drainage system
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5.5l / m²
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95% recycled material
Monkroyd’s Roof Drain consists of a perforated (thankfully recycled) HDPE core that makes it incredibly durable, lightweight and easy to utilise.
The large core holds up to 5.5l / m² (which is a lot and will obviously add to the weight) and the perforated core provides optimal drainage for excess rainwater.
- The upper geotextile acts as a filter, preventing any drainage layer blockages
- The lower geotextile layer offers additional protection to the waterproof layer and retains more moisture
Green Roof Drainage Layer FAQs
Yes green roofs need drainage. It depends on the climate, type of plants and size of the green roof when determining how much.
But ultimately green roofs are typically made up of lightweight, hardy plants that don’t need much water. And a sodden green roof will kill the plant life, ruin it’s insulation and increase the weight of your roof through over-saturation.
Which will ultimately completely destroy the roof. So yes you need drainage.
Living roof drainage layers are typically between 12 – 25mm in depth. The more water the vegetation needs, the greater the depth needed.
And, of course, in climates where the rainfall is more substantial, the thicker your drainage layer should be.
Extensive roofs in the UK will typically be fine with a 12mm drainage layer.
I think there are 4 core things to consider when buying a drainage layer – other than the obvious how much does it cost and how big is it – you need:
- Cups: Multiple cups that store water, typically between 12 – 24mm in depth
- Perforated layer: between the cups that helps drain excess rainwater
- HDPE: You need a lightweight, durable material that is, rather obviously, waterproof and long-life.
- Recycled material: This isn’t a pre-requisite, but we want to utilise as much recycled material as possible. Particularly when plastic is involved.
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