A sedum roof covering is the lightest form of green living roof. You can grow sedum plants on any flat roof. You simply add a substrate (ie. a layer of material in which the sedum can grow) above the roof’s waterproof membrane and plant the sedum.
We designed and built our own sedum roof on a 5m wide carport. The roof construction is 250mm x 50mm joists spaced 600mm apart with a 18mm plywood deck. There are various layers of sheeting on top to make the roof waterproof. On top of that we added 75mm of mixed topsoil and gravel into which the sedum plants were sown. It’s slightly over-engineered but it is as solid as a rock and still growing beautifully after 5 years.
A green roof needs to be reasonably flat, not more than 20 degrees. Living material will need water, so a steep pitch might require water traps or a deeper water retentive layer. Plants will also need to have direct sunlight for part of the time – constant shadow will kill the plants off resulting in a bare patch. Maintenance will be needed so easy, safe access is an obvious requirement. If you want to integrate a solar light tube or roof dome it is better that this is done before the sedum roof is installed and anything like this should be done professionally so that the integrity of the roof is ensured and insurable.
Sedum is the cheapest, lightest and easiest form of Living Roof, which lends itself to the DIY’er trying to green up on a budget. If done properly it can be planted up and left to its own devices. Often no more than periodic visual checks and minor weeding are required.
We check ours on a quarterly basis, a bit more often between May and August, when things are growing. If anything we do not like takes root, we weed it out by twisting and pulling up (don’t use a fork whatever you do !). If you create a divot, shake the soil off the roots back into the hole and leave it to grow over. If we find a bare patch we plant a new sedum from a small stock we grow at the top of the garden. Trees take a long time to establish, so you will have loads of time to see that it has taken root and to remove it.
Work on the basis of a point loading of 125kg/m2 for a sedum roof. A more intensive roof cover will need a deeper substrate layer and thus the loading will be much higher. If in doubt pay a surveyor to do some calculations for your roof.
You can DIY a green roof although if it is above the house it will need to conform to building regulations. In this case it is best done professionally so it can be properly covered under your home insurance. The first step is to check the structure of the roof and decide what you want to do can take it. Take some measurements and a few photos before seeking professional advice.
Roofs which can support large plants, shrubs and even trees are far more expensive to construct. They require a more substantial roof supporting structure, a thicker substrate and more maintenance. However, if your roof is fit for purpose one nice option is a bio-diverse roof in harmony with the local flora which will provide a natural habitat for wildlife. This system involves little more than covering the roof with locally obtained soil and either allowing it to self-seed from the surrounding area or seeding it with grasses and wildflowers.
A roof garden, where you might want to sit out, play football etc, will require a very substantial supporting structure beneath. It might require an irrigation system and would require a second mortgage for most people.
Done properly, with a roof that is strong enough to take the weight, you should have no problems with a DIY sedum roof. In our case, a thick gravel edge holds back the soil and the roots grow to form a mat that acts like a filter. Rainwater that runs off our carport goes into a large rainwater tank and settles with only the overflow from this tank going into the soak-away. In 5 years we have collected about a cup full of soft muck in the bottom.
You can see the sedum roof pictured above at Tony Almond’s SuperHome at Open Days or by appointment. Tony’s house is one of the homes in the SuperHomes network of older homes that have been renovated by their owners to reduce carbon emissions by at least 60%. Most SuperHomes host public open days in September.