How to create a simple low-cost green shed roof

You may have wanted to build a green shed roof for a while but been put off by the cost or it looking too complicated. Well I have got news for you it doesn't have to be expensive or difficult.

But why would you want to build a green roof in the first place?

green shed roof My first attempt at a green roof shed

For me it was just that they looked great. I liked that a space that otherwise would be a desert, could be a space for plants to grow and change with the seasons.

And the plants provide a habitat for insects, which in turn provide food for birds. So building a green roof adds to the biodiversity of my garden.

I was concerned that making a new 'flowerbed' on a shed roof could prove to be a bit of a maintenance liability. And that weeds would colonise this new growing environment. However, I found that the thin soil and harsh environment on a roof means that few plants can survive. I carefully selected plants that were able to thrive in such a harsh environment with direct sunlight, wind and little water.

So how to get started with creating the seed bed for the roof?

green shed roof Layout of small lean-to green roof shed

The first thing to do is to make sure that the roof that you build is strong enough. It has to support the weight of the growing medium that you are going to put up there and more. A shed roof that is not designed for this sort of load is unlikely to have the capacity.

For a first green shed roof I would recommend that you start off relatively small. Partly to minimise the structural requirement and also so you can find out what works for you.

For this shed, I used 3x2 softwood timbers at 600mm (2') ctrs which spanned 1.2m (4') with an 18mm (3/4') plywood deck. This was more than enough to support the roof build up which comprised;

waterproof membrane
drainage medium
filter membrane
peat that I used as a growing medium

The roof also need to be strong enough to support my weight. Whilst I was building the roof and also when I needed to go up there in the future to look after the plants.

For this shed I used nominally 'flat roof'. This was not flat at all, but had a fall of 75mm (3') over its length of 2.4m (8'). This gave a 1:40 slope, which was enough to encourage water to drain.

The slope of a green shed roof needs to be a balance between being too steep or too shallow. A shallow roof pitch wont allow water to drain away. Whereas too steep means that the growing medium slides down the roof. This roof was towards the shallow end of the spectrum and could perhaps have been a bit steeper.

With a sturdy roof deck to support the roof the first step is to keep out the water

Creating the Green Shed Roof growing medium

Layers forming Green Shed Roof

1.0 Waterproof membrane

The membrane I used to protect the roof deck was a 2000 gauge polythene damp proof membrane. The membrane comes in a roll that was xm long and ym wide. This type of membrane is normally specified as a damp-proof course beneath houses. It stops water from the ground penetrating up through the foundations.

I was concerned that it could become brittle through exposure to UV light over time. If the membrane became brittle it could split and leak. During construction I did all I could to ensure that the roof membrane was covered by roof edging, the roof plants and growing medium.

EPDM membranes, which are more commonly used for roofing, don't have this problem but are much more expensive.

After I secured waterproof membrane I placed a couple of timber battens on top of it. This enabled me to walk on the roof without pressing on the stones from the drainage layer and puncturing the PVC.

2.0 Drainage Layer

I then a placed a 25mm layer of 10mm pea shingle on top of the waterproof membrane to form a drainage layer. The drainage layer allowed rainwater to drain off the roof, in to the gutter and away.

3.0 Filter membrane

On top of the drainage layer went a layer of landscaping fabric. The purpose of this is to form filter membrane to stop the washing away of the growing medium that is the next layer up. The landscaping fabric once again is an inexpensive item that came on a roll 4m wide x 10m long. I only needed to use small section of it.

4.0 Growing medium

The final layer for this simple green shed roof was the growing medium. This will depend a bit on the plants that you want to grow. As this was a bit of an experiment I used a mixture of general purpose compost and some sand to aid drainage.