Green Roof Construction

A living green roof brings a whole host of benefits to any landscaping or construction project. Just as long as it is properly built. In this article, we’re looking at green roof construction and how to build a sustainable living roof.

A holistic approach to green roof construction.

A living roof is like a marriage between engineering and ecology. The plants need to suit the building (right plant, right place!) and the building needs to suit the plants. And I don’t just mean visually!



Think of a green roof as a giant planter – which is what it is. It must have decent drainage, be able to keep growing medium and roots where they belong, retain enough water to sustain growth AND have good access for maintenance. At the same time, the building or structure your roof is sitting on, needs to be able to accommodate the weight of the green roof buildup as well as cope with live loading. Ie snow, wind, and maintenance people.

So when you are designing a living green roof there are a whole raft of considerations

• Load bearing capacity.
• Slopes.
• Aspect.
• Drainage.
• Waterproofing.
• Edging.
• Irrigation.
• Substrate type.
• Plant choices.
• Maintenance.

Load bearing capacity:

Around 120Kg per square metre for an shallow sedum roof, rising to 250Kg per square metre for a wildflower roof with 150mm substrate depth.

Slope:

How will you retain materials and water? For a sedum matting roof, up to 10 degree slope is feasible. 15 degrees is do-able but the roof will need extra maintenance to keep it looking good. Flat roofs are great but only if there is a slight fall to accommodate drainage.

Aspect:

How much daylight does the roof recieve? Is it exposed to high winds? Will it need extra maintenance to remove leaves falling on it in autumn? How you position your roof will determine which plant species will thrive on it and also impact on maintenance budgets.

Drainage:

Excess rainwater MUST be able to escape easily. You are building a green roof – not a pond!

Waterproofing:

The green roof buildup is not watertight. Your structure will need waterproofing to be intact and in good condition before you begin. It’s VERY difficult to make repairs once the green roof layers are in place

Edging:

What’s to stop your growing medium from washing off the roof? Every project needs some kind of edging, whether it’s an upstand, a bespoke timber edging or an off-the-shelf aluminium edge. Whichever you choose, it must allow free drainage and prevent wind uplift. For larger or higher green roofs, some kind of fall restraint is essential.

Irrigation:

Depending on the type of plant you are hoping to cultivate and the roof slope, some kind of irrigation should be available. A sedum roof will rarely need watering and if it can be reached with a hosepipe that’s probably enough. A wildflower roof however could potentially perish in a drought. The steeper the slope, the more essential irrigation becomes. Over 10 degrees and a watering system is a must.

Substrate type:

NEVER use topsoil for a living roof. Green roof substrate may be more costly to buy than topsoil but there are several good reasons why it’s necessary. 1. Substrate is engineered to weigh less than topsoil – I refer you back to the point on load bearing capacity. Larger particle size means less risk of silting up drainage outlets and losing volume during rain storms. Relatively little organic matter means less growing matter is lost to oxidisation. In turn, that means you won’t need to top the levels up after a couple of years. Well drained – soggy soil puts a whole lot of extra weight onto a structure and many green roof plants prefer good drainage anyway. Think herbs and alpines.

Plants:

Conditions on a green roof are very different to at ground level. There’s a finite depth of growing medium and several degrees difference in temperature depending on the height of the building. Plants need to be able to cope with windy conditions, poor nutrient levels, minimal maintenance, hot summers, cold winters and sometimes drought. That’s why sedums and wildflowers are so popular as green roofing plants. Think too about aspect. If your roof is shaded, choose shade loving plants. But then make sure that the substrate layer will be deep enough to accommodate them. That will impact upon the loading – so check that the building can take the weight…..told you there’s a lot to think about!

Maintenance:

There is not one green roof in existence that doesn’t need maintenance. At the very least, drainage outlets need to be inspected and cleared once or twice a year and plants fed with a specialist fertiliser every spring (remember that substrate has very little organic matter to generate or retain nutrients). A wildflower roof will need trimming back at the end of summer and all of the excess vegetation needs to come down from the roof. When you are designing your green roof, make sure you pay proper attention to access. Scaffolding or a cherry picker are OK for the build but how will anyone get onto the roof to maintain it? Especially if materials or equipment need to be taken up and brought down again.

Constructing a simple green roof 

1. Choose plants that are suitable for the conditions on your roof.  Sedum plugs for example, need to be in full sun for at least half of the day as do many wildflower species. If your green roof is shaded you might want to think about cutting back some trees to let more light in.
2. Check the load bearing capacity of the building. An extensive wildflower roof exerts no less than 240 Kg per square metre to support both the weight of the green roof build up and anyone walking on it for planting and maintenance.
3. Is the slope between 1 and 5 degrees? Yes? Perfect. A steeper slope needs irrigation. Please do not attempt to create a living roof over 10 degrees without getting expert advice.
4. Is the waterproofing intact? Check by soaking the roof and looking for leaks. Look too for puddles that don’t go away. These indicate a dip in the roof surface which should be addressed before adding the green roof buildup.
5. A bitumous type waterproofing can be damaged by roots – protect it with a strong geotextile membrane. Pond liner is perfect.
6. Edgings. The edging should be in place before you start adding green roof layers. Don’t forget to ensure it drains well. Edgeguard has been designed especially for the job.  It goes without saying that edging shouldn’t compromise the waterproofing in any way.
7. Drainage mat: Only needed on flat roofs (2 degree slope or less). This component ensures that the sedum roots are never sitting in water. Sedums HATE having soggy feet. Simply unroll it into place. Down one side you’ll find an extra flap of geotextile. Open that out and sit the next piece of drainage mat on top of it – it’s a built in anchorage system.TIP measure and cut each piece on the ground if you can. If not, use a cutting board to make sure you don’t slice through the waterproofing by mistake. 
8. Substrate. Ordinary growing medium is not suitable for green roofing. Always invest in specially engineered green roof substrate. It's lightweight, well drained and doesn't have tiny particles that can clog up the roof's drainage system.  Some folks like to have a pebble border between the planting and the edging. It looks beautiful but is not suitable for a sloping roof (for obvious reasons). The full spec and prices for wildflower roofing turf.
9. If you are lifting all of your green roof materials onto the roof in one go – be sure to distribute it over a wide area. A whole pallet full of landscaping supplies sitting in one place plays havoc with the loading on the roof.
10. Keep plants well watered until they are established.  Remember, that conditions on a green roof can be harsh, the baby plants will need plenty of help until they get their roots settled in.

Any questions?

As ever, if you have any questions at all about green roof construction, please don’t hesitate to get in touch. I have two ladies on my team who have a lot of experience with green roofing and will be more than happy to help…..although I can’t quite see @Ladylawn heaving rolls of sedum onto a roof. She’s more likely to be enjoying the view whilst shouting encouragement from the ground.