updated 6th february 2013

Do we want to live in water world?

flat green roof in stevenageWith global warming affecting sea levels, storm water runoff being pumped into our rivers and waterways and planners with developers building on flood plains for new housing, it is no coincidence that we are increasingly seeing more flooding around the U.K.. The problem will only get worse as global temperatures rises, but there are solutions to the problems we face.  The easiest by far is to create a green roof, no need for expensive water harvesting systems or a space big enough to put a storage tank in your garden.

A green roof can reduce storm water run-off by up to 80%, this reduces the amount of water being pumped into our waterways and rivers.


I am going to tell you how to create a green roof. This is for flat roofs only.

Whether your roof is big or small there are a few major factors in deciding to have a green roof:

1)      Can your roof take the extra weight of the green roof?

      Your roof needs to be able to withstand the weight of 120kg per square metre.

2)      Is the waterproofing on your roof in a good condition?, Does the roof drain away properly?

      To repair leaks once the sedum matting is installed is very expensive.

3)      Is the surface of your roof flat and smooth? Air pockets can lead to dead patches.

      The matting will settle onto small hills and bumps, but will not cope with sudden changes such as corrugated tin roofs.

laying water retention mat on a flat roofNow that you know your roof is suitable for `greening’, let’s get started with the good bit.

Firstly let’s make sure you can access the roof safely, scaffold towers and hop-ups should be used. Always take extra care when working at height, even relatively low heights.

Final check if the waterproofing is sound, get your hosepipe out and soak the roof, keep checking inside for leaks or damp patches, adding food colouring to a watering can will help to find leaks.

This is your last chance to check before it gets expensive!

The roof will need some form of edging to protect the sedum from `wind uplift’; the edging must still let excess water drain away. Edging can be bought from certain suppliers.

Once you know your roof is water tight we can start to build up the layers we need for the green roof.

flat green roof build upThe first layer you will need is a good quality, strong root barrier, something along the lines of a poly butyl pond liner.

Next up is the drainage layer; this layer can come in a few different designs. Some have an egg cup design with small water reservoirs throughout but these are un-flexible, others come in the form of a filter mat.

The next layer before you get your hands dirty is the water retention mat, this lightweight inexpensive fleece holds 9 litres of saturated water per square metre, this is vital to keeping your roof looking great for years to come.

Now is the time for the sedum blankets, make sure you can lift the sedum onto the roof safely, it may need 2 people to lift it up onto the roof.

Roll the sedum blanket out making sure you have got the blanket as close to the edging as possible, once it is rolled out, remember to unravel the extra piece of matting and lay it flat on the roof. This extra piece helps to secure the sedum to the roof.

Repeat as above, remembering to start the next roll tight up against the previous roll, on top of the extra piece of matting.

Once you have got to the point of trimming your sedum matting, carefully mark the roll where it needs to be trimmed. Make sure to cut the blankets from the non-vegetated side.

Make sure all edges are pushed down along the edging strip and all joints have no gaps.

Once you have your green roof installed make sure to water it, especially if you have installed your green roof during hot weather.

If you prepare fully and follow these simple steps, you too could have a fantastic looking green roof.

green roof officeOnce your green roof is installed, you MUST keep it maintained or all your hard work would have been for nothing.


Watch our video on how to install a sedum roof