Making our own small green roof

detail photo of summer planting on green roof

In 2016, we redesigned the back of our Victorian house in Sheffield which included the renovation of an existing uninsulated balcony roof over an off-shot cloakroom and utility space.  We had decided it could be a good location for a small green roof as it is overlooked by bedroom windows and is relatively easily accessible from a level area in the garden.  The roof faces South West and is 2600mm long x 900mm wide (area 2.35m2). 

Our idea was to create a low maintenance green roof, attracting insects and birds, and to test different types of planting.  We spent several months reading around the subject, using text books, magazines and specialist internet sites, plus our knowledge of designing larger green roofs.

Before we commenced the green roof, our builder removed the railings and tarmac surface and the in-situ concrete slab was fully refurbished to include insulation (depth to comply with Building Regulations) and a fully waterproof roofing layer, in this case a ‘Plant E’ capping sheet from Bauder Roofing.  Our green roof is not fixed to or through the waterproofing layer; this is to maintain the integrity of the roof construction. 

We set out intending to source all our materials locally, but unfortunately, we simply couldn’t find a local supplier who was able to provide the very small amounts we needed.  Therefore, our roof design combines a mixture of proprietary green roof materials from specialist suppliers in the UK and locally sourced materials such as timber edging, peat-free compost and plants.

Day One:

We measured and cut the aluminium edging and drainage layer at ground level to reduce working at height.  The combined reservoir/drainage layer, comprising grey fleece layer on the bottom, plastic ‘egg crate’ drainage layer with white filter layer on top was easily cut to size with a Stanley knife.

Green roof build-up (l-r): waterproofed roof: plastic drainage layer, perforated metal edging and geotextile filter sheet lining assembled on ground then lifted into place: growing medium spread over filter sheet up to edging

After research, we decided to make up our own growing medium using a combination of proprietary medium and peat-free compost with the aim to achieve a relatively nutrient poor, free-draining substrate to support a robust plant community.  The proprietary medium comprises composted organic matter, recycled crushed brick, lightweight aggregate and pumice; it’s mostly small pieces of brick and stone. The compost was organic and peat-free.  We ordered 7 x 25kg bags of growing medium and 1 x 50kg bag of compost (proportions roughly 1: 3.5) which we mixed before lifting up onto the roof to give a general thickness of about 70mm, deeper towards the back.

We took the decision to use an additional thin black-coloured filter layer between the reservoir/drainage layer and the roof edging instead of overlapping the white filter layer; this was purely a decision based on how it would look from the ground through the perforations in the aluminium edgings and from above.  This filter layer was bought on a roll from a local garden centre.  At the end of the Day 1, we loaded about 40% of the growing media onto the roof to a depth of about 70mm overall; compacting by foot as we went.

Day Two:

We cut treated timber edgings to form the edge of the pebble strip against the house; this is supported in place between the pebbles and growing medium.  The grey fleece layer was wrapped over the roof flashing before the pebbles were added and the rest of the growing media was brought onto the roof.

Green roof build-up (l-r): fitting timber edging to form drainage strip against building: drainage strip filled with pebbles, growing medium piled up to level of retaining edges and deeper in centre: timber logs added for habitat and shelter, first plug planting commenced

We stapled some more of the thin black filter layer over the timber edging to prevent the growing medium migrating into the pebble strip which we think looks neater from above.  To ensure the timber edging stayed in the correct location, we brought up the rest of the growing medium and pebbles in alternative lifts.  The growing medium was then manually compacted before the initial planting started.

We placed two fairly rotten sycamore logs from the garden towards the back of the roof to provide initial ‘landscaping’ on the bare roof and create shelter for insects.  We planted the roof to try out different types of plants and planting methods.  To start with, we decided to plant chives (Allium schoenoprasum) and houseleeks (Sempervivum) which we already had in a garden pot to give us some immediate interest.  In addition, we sourced a small amount of UK wild flower and grass seed mixtures for green roofs, keeping 50% back for sowing again in Spring if necessary.  The seed was broadcast across the roof at the end of the day.

2016 – 2018:

Green Roof: left, meadow grasses in first spring 2017: right, Birdsfoot Trefoil taking over in May 2018

We continued to add more of the original seed plus smaller alpine perennials over the next year.  Despite broadcasting a large proportion of grass seed over the roof, it did not take over, whereas Birdsfoot Trefoil (Lotus corniculatus) from the seed mix proved a bit of a bully but provided great colour in late summer.  The Allium and Muscari bulbs took very well.  Our green roof plants grew rapidly in the warm early season in 2018, then suffered in the summer heat and the lack of rainfall.  In August, just as the weather turned, we decided to intervene and removed and cut back the larger plant material to expose the plants underneath; the growing media was extremely dry and friable. 

2018 – 2021:

Green Roof: left, planting with sedums and alpines August 2018: right, summer 2019

In order to respond to the ‘boom and bust’ planting, in autumn 2018 we decided to switch to planting to predominately Sedum species and low-growing drought tolerant alpines, all sourced from local garden centres.  Within a couple of weeks, the Sedums had begun to anchor onto the growing media and by spring/summer 2019, the new planting and original bulbs were growing well. 

Since 2020, we have plugged a few gaps with a couple of new sedums and sedum cuttings from elsewhere on the roof.  The sycamore logs fell apart within two years and were removed. With limited maintenance the roof is now pretty much self-sustaining on what seems to be dry growing media for most of the year.  Over the last two years, the bulb and sedum flowering has been amazing, attracting insects from March with Iris recticulata through to late flowering Sedum in October. One of the most interesting outcomes is that areas of the roof we thought may be less successful because of shading, are actually the areas where the Sedums have thrived. Other exposed and sunny areas have been more tricky.

Green Roof: left, July 2020: right, July 2021

Building and growing your green roof is not an overnight project; like gardening generally, you need some patience and be willing to learn from your mistakes.  However, it’s entirely been worth the effort and research and now we encourage everyone else who might have a new or existing roof or balcony to get involved!

We’ll cover the benefits of creating your own green roof and the dos and don’ts in a follow up post.

Planting List:

Sedums:
  • Sedum album
  • Sedum Blue Carpet
  • Sedum forsterianum Elegans
  • Sedum kamtschaticum
  • Sedum oreganum
  • Sedum reflexum Red Form
  • Sedum reflexum Rupestre Gold
  • Sedum sexangulare
  • Sedum spathulifolium Purpureum
Herbs and alpine plants:
  • Armeria maritima
  • Sempervivum spp.
  • Thymus citriodorus Green Lemon
  • Thymus praecox albiflorus
  • Thymus serphyllum Goldstream
Bulbs:
  • Allium schoenoprasum
  • Iris recticulata
  • Muscari armeniacum
  • Muscari azureum

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