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

Urban SuDS Case Study: Hebden Bridge Town Hall – Courtyard Rain Garden Planters

Calder Rivers Trust & Slow The Flow Calderdale

Rain Garden Planters

Slowing the flow of rainwater into the drainage system, thus reducing the potential for combined sewer flooding

The Town Hall Courtyard planters project is a collaborative community
venture, funded by a grant from the Postcode Local Trust.

It is part of a wider initiative to help us
all understand how urban Sustainable Drainage Systems (SuDS) can play a
part in flood alleviation.

The completed storm-water planters

Five large storm-water planters were installed in the courtyard of Hebden Bridge Town Hall in June 2018. Planters are 80cm high, 75cm deep and range from 90cm to 2.4m wide. They are made from tricoya, a sustainable and extremely durable engineered wood panel.

Ornamental and edible plants were chosen to withstand flood and drought conditions and to improve the riverside bio-diversity corridor.

adding the plants

Raised planters are good if space is tight, or ground conditions don’t allow water to soak away. They divert water from drainpipes, to slow / reduce the flow into sewers. They can be built over existing surfaces as long as there is a plan for any overfow. Excess water can continue into the existing system, as before.

With this being in a public space, it is ideal for promoting SuDS, particularly encouraging people to make their own interventions at home

Assembling the planters

Plants
Resilient plants have been chosen to cope with either flood or drought, and promote riverside  biodiversity – also some that the café can use in their delicious food!
Species list available here.

Monitoring
As part of wider research, the effectiveness of the planters at slowing down rainwater will be monitored by measuring the inflow and outflow of rainwater using the measuring gauges in one of the planters.

River Health
Rain gardens and other SuDS features filter the water and reduce the sediment that enters the river from runoff. By reducing peak flow into sewers they can also reduce the amount of serious pollution from Combined Sewer Overflows (CSOs), improving the quality of water in your local river.

Riverside Biodiversity Corridors
Linking landscaped spaces together along the riverside and including a variety of nectar rich species to attract insects and birds helps to create a wildlife corridor along the riverside.

Rain Garden Planter Construction
These planters are made from “extreme” Tricoya (#MTX) which has all the design flexibility of MDF, but is more durable and sustainable – perfect for external, wet and natural environments.
They were manufactured on a not-for-profit basis by local riverside social enterprise, Green Future Building and installed by volunteers.

The project has been funded by a £20,000 grant from People’s Postcode Lottery with collaborative design and implementation from the following:

To submit your own case study go here.

To find out more about how YOU can help to Slow The Flow, go here.

Urban SuDS Case Study: Swale on Tipside by Todmorden Riverside Improvement Group (TRIG)

The Swale in 2018

What? Constructed a small swale to take surplus water away from the path and make a damp area of grass wetter and more biodiverse.

Where? Tipside Todmorden.

When? December 2016.

Design:The swale has been designed to provide a new wetland plant community in an already wet grassy area, and to provide a small storage area for surplus rainwater. The swale is a shallow hollow no deeper than a foot, with gently sloping sides which covers an area about 60 m2. Drainage is poor in this area leading to frequent flooding of the nearby path, so it is intended that surplus rainwater will fill the swale rather than flood the path. 

Following heavy rain the swale will fill with water, this will then gradually drain away or evaporate, leaving the swale empty for the next downpour. The swale has been planted with a number of different wetland species to make the area more attractive both to people and other wildlife. A range of species will give a long flowering period from early species like Marsh Marigold in April, through to late summer species like meadowsweet and purple loosestrife.

Excavating the swale

Benefits/Constraints: We have been really pleased with the establishment of the plants in the swale, though we have to keep the area well managed to keep unwanted species like Himalayan balsam, nettles, docks and bindweed at bay. We haven’t fully resolved the flooding of the footpath yet and have more work to do with additional drains but we hope once this is completed the path will be much drier and less muddy.
Further opportunities: We are looking at another wet area on another part of the site where a marshland community might be established.

Who? TRIG designed, managed and fundraised the scheme, supervised the groundworks and did the planting

Funding: Plant material – native wild flower plug plants: £413 Compost etc to supplement very poor substrate £85 Ground works to excavate swale £ 910. Funds were provided thanks to the people’s postcode lottery.

Planting the swale December 2016
Swale December 2017

To submit your own case study, go here.

To find out more about how YOU can help to Slow The Flow, go here.

Urban SuDS Case Study: Domestic Garden, Halifax

Green roof

What? Front garden: Hedge and planting. Drive: Shed Green Roof and water butts

Where? Family home (1950’s 3 bed semi-detached house), Halifax.

When? Early 2018 (Hedge February, Green Roof April)

Design: Replaced wall and hard surface at the front with a hornbeam hedge, and planted bulbs below. Built bespoke sturdy (to support extra weight of green roof) shed, over existing tarmac. 

The layers of the green roof are (bottom to top): standard shed felting // waterproof pond liner (doubled because we had enough) // about 2-3cm depth drainage layer (angular gravel + ‘Sportag’) // about 5cm depth peat free potting compost // plants / seeds. Any water that filters out of the soil is collected in water butts, via a downpipe.

Benefits/Constraints: The hedge prevents some of the rainfall ever reaching the ground, the roots soak up some of the water, and the soil absorbs and releases it slowly, instead of the wall and hard surface that would have shed water quickly straight into the drain. Similarly at the back, the green roof intercepts rainfall that would have shed straight into the gully – and we can water our other pots with the water butts. In addition, the view both of and from the house are greatly improved at front and back. We have noticed more wildlife, particularly birds, and as gardeners are glad of extra planting space.

Further Opportunities: We plan to install a permeable drive and large planting bed at the front as well.

Who? Husband and wife team! (He’s a bit of a handyman…) (and the baby cheer-led)

Funding: about £150 for the hedge, including soil. About £650 for the materials for the whole shed, including green roof and two water butts. We also added a solar powered irrigation system that uses the water butt water – about another £100.

Building / Implementation: Needed planning and materials ordering in advance.

Hedge: Required wall demolition, digging out a trench, filling with soil, planting.

Green Roof: needed the shed building first! Green roof elements needed hoisting up via ladders, securing down as appropriate, planting/seeding. Slightly careful detailing to drain to guttering. Water butts installed as instructions.

Before!
With the hedge planted

To submit your own case study, go here.

To find out more about how YOU can help to Slow The Flow, go here.

Rural NFM Case Study: Flood alleviation at Oldroyd, Todmorden, Upper Calder Valley

Case Study 1: Flood alleviation at Oldroyd, Todmorden, Upper Calder Valley 

On Boxing Day 2015 the hamlet of Oldroyd (OS 3949 4241) which comprises a street of former mill or farm workers cottages dating from the 19th century, was partly inundated by flood water running off adjacent sloping fields, a total of five properties were affected, similar problems occurred during the summer floods of 2012.  

The residents were keen to install large diameter pipes across fields to collect the excess water and discharge it downstream away from their properties as quickly as possible.  However, such a scheme would prove to be outside of the budget of £25,000 for the works as the cost of the pipes alone exceeded this figure. 

The project was funded by the Repair and Renew Grant Scheme run by Calderdale MBC following the Boxing Day floods, this was a central government grant made available to homeowners and businesses whose properties were flooded or very narrowly avoided being flooded.    A much more cost effective scheme was put forward by Slow the Flow Calderdale civil and structural engineer Stuart Bradshaw which required the excavation of an attenuation pond to temporarily store water, and then slowly release it once the storm had passed.  

This would solve the flooding issue for the residents of Oldroyd yet at the same time it would slow the flow of water to the River Calder with consequential benefits for the downstream communities of Hebden Bridge, Mytholmroyd, Luddenden Foot, Sowerby Bridge and Elland all badly affected by flooding on Boxing Day 2015.  Neighbouring farmers who owned the land that would be used for the works were assured that their fields would be returned to pasture once the works were completed.   

The scheme comprises an interception channel which is simply a trapezoidal cross sectional channel dug across the bottom of the hillside above the properties, this diverts the water away from the properties to flow into an attenuation pond with an approximate volume of 800 m3. 

The attenuation pond has a 150 mm diameter outlet pipe which allows water to flow out of the pond at a rate of around 25 l/s leaving excess water to rise and be stored temporarily in the pond.  The maximum depth of the water is less than 1.0 metre and due to the fall in the land most of this water is stored below the former existing ground levels by digging into the hillside and using this soil to create a low embankment on the lower side.  The attenuation pond should provide protection for up to a 1 in 50 year storm, storms of a greater intensity will over top the pond but water then flows over a weir on the dam crest which directs the water away from properties and onwards downhill on a flow path that takes it away safely. 

Plate 1: Early stage excavation showing the broad shape of the pond, looking north
Plate 2: Early stage excavation showing the broad shape of the pond, looking south east
Plate 3: Later stage excavation looking north
Plate 4: Completed pond
Plate 5: Completed pond showing interception channel above the properties to the west
Plate 6: Completed pond returned to pasture