Creating a Shed Green Roof

At our semi-detached house in Halifax, we have built a lean-to shed for our bikes and bins, to free up some space in the garage. So far, so very mundane… but – this shed has been getting lots of attention/compliments – it has a great-looking green roof! It helps to ‘slow the flow’, encourages bees and butterflies, makes us smile, and – IT WASN’T HARD TO DO!

We have previously had our house in Mytholmroyd flooded, and I am part of Slow The Flow: Calderdale’s management team. So, although our current house does not flood, we understand that it is those upstream, rather than those who flood, that need to take action.  This inspired us to include included a green roof and water butts in the shed’s design.

I wanted to note down in a blog post how we’ve done it with the hope that it will inspire others to follow suit – if lots of us do something similar, it could have a big impact on Calderdale’s surface water flooding problems!

My husband, Dan, built a bespoke structure, with a sturdy timber frame to support the additional weight of a green roof. The size of the top of the shed is about 4m x 1m.

To begin with, he lined it with traditional roofing felt – we didn’t know how long it would take us to get around to putting the green roof on. We have left that felt on as an extra layer of waterproofing.

There is a shallow fall on the roof, and it drains to a plastic gutter, which feeds two connected water butts before overflowing to the drain that already existed in the drive. Interestingly, both water butts filled straight away, in our first heavy rain event (perhaps because the planting hadn’t had time to establish yet).  In future, we will leave the tap open ahead of heavy rainfall in future, so that they can act as ‘mini leaky dams’ (i.e. the water comes in a bigger hole than it is allowed out through, thereby slowing the flow).  We have also installed a solar powered irrigation system, that uses water from the water butts (http://irrigatia.com) – a nice optional extra! (We would recommend it, but it does need some tlc – in the dry summer we’ve been having, it has struggled and clogged when the water butt has been empty).

We bought a pond liner (which was actually big enough to use folded over for extra protection, though this is probably unnecessary), laid it, and built a timber upstand, about 10cm deep, around the top of the roof to both hold it in place (with heavy duty staples) and contain the green roof layers. The pond liner laps up on three sides, and down under the timber on one side, to feed the gutter along the bottom edge. We left a drainage gap between timbers on this bottom edge, but water should also naturally seep through at the join.

As a drainage layer, we used some angular gravel (single layer) we had left over from another DIY project and then some lightweight ‘Sportag’ – again a single layer. This drainage layer is about 2-3cm in total. Shed green roofs don’t necessarily need it, if you don’t have the depth/structural strength for the extra weight you could just use soil. It just gives extra water storage capacity, and should prevent the soil getting boggy in wet weather.

The topsoil (multipurpose peat free compost) layer finishes it off – 4 125L bags plus an extra 25L (total 525L) spread thinly after the seed mix, provided a depth of about 5cm.

Again, you could use a thinner layer of soil, but this depth, with the drainage layer underneath, allows us to use the roof to grow small ‘garden’ plants in rather than just sedum – you can see in the pictures that we have used a combination of potted plants for immediate effect, and a seed mix. Plants are chosen that should cope with both dry conditions (much of the time on a green roof!) and occasional water inundation.

We used a mix of seeds – some we had lying around, including a sample of ‘Pictorial Meadows’ pastel mix, and some purpose bought – we’ll look forward to seeing what does well! A ‘Pictorial Meadows’ tip about mixing seeds with sand, was an excellent way to sow the seed. It allows a more even distribution, and an easy visual clue as to where the seed has been scattered!

Plant list:

Pots

  • Saxifraga x arendsii ‘Marto White’ (Saxifrage)
  • Saxifraga x arendsii ‘Alpino Early White’ (Saxifrage)
  • Sedum acre ‘Aurea’
  • Thymus x2 – exact species unknown, one gold variegated, one purple (Thyme)
  • Anemone blanda ‘Blue Shades’
  • Pritzelago alpina ‘Ice cube’ (Chamois cress)
  • Viola x2 – exact species unknown, one blue/purple, one pink/purple

Seeds

  • Pictorial Meadows ‘Pretty as a Picture’ pastel mix
  • Nigella damascena ‘Persian Jewels’ (love-in-a-mist)
  • Linaria maroccana ‘Fairy Bouquet’ (toadflax)
  • Scabiosa atropurpurea ‘Blue Cushion’ (scabius)
  • Mysotis sylvatica ‘indigo’ (Forget-me-not)
  • Helictotrichon sempervirens (blue oat grass)
  • Allium schoenoprasum (chives)
  • Wilko ‘mixed white flowers for wildlife’

Once the shed was built, the green roof installation took two of us a few hours’ work. The shed in total, including green roof/water butt elements cost a total of around £650 (£50 on plants/seeds) Hard to separate the roof construction from the shed as a whole – it was a big shed, and sturdy construction deliberately to support the roof.

Our garden features in the new case study library here

We hope this encourages others to do the same.

If you make changes in order to Slow The Flow at Home, however small, please share your experience to encourage others, by submitting a case study of your own here


By Amanda McDermott CMLI

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