If you’re thinking of a green roof, here’s how much they cost in Ireland today and what you need to consider if you plan to install one, from planning permission and structural requirements to drainage and planting choices.
In this article we cover:
- What is a green roof and how much does it cost?
- Detailed infographic showing all the layers of a green roof
- Do you need planning permission?
- Structural requirements
- Drainage and windbreaks
- Finding a theme and planting choices
Here’s what you need to know to add a green roof to your new build plans, or to retrofit one on your existing home.
Ballpark Costs: How much does a roof garden cost?
Small balcony garden with paving, decking and electrics already in place: from €2,500 (£2,100) to €3,500 (£2,900)
Small roof garden with paving, decking and electrics already in place: anywhere between €25,000 (£20,800) and €50,000 (£41,600)
Roof terrace build from scratch: from €120,000 (£100,000)
Planning permission
You will need planning permission to add a roof garden. For a new build project, this will be taken into account at the design stage and the design will form part of the overall planning application for the house.
Whether building new or renovating, both a structural engineer and preferably a specialised company should be involved at the design stage.
Note that a design that will allow you to see into the neighbour’s garden (or upper storey windows) will lead to issues securing planning permission in both NI and ROI.
As with any project that requires planning permission, make sure to let your neighbours or local resident’s association know of your plans before lodging your application.
Structural requirements
In addition to securing planning permission, you will need to comply with building regulations and health and safety regulations in both NI and ROI.
This will involve an application to building control in NI; they will carry out inspections of the work. In ROI you must also comply with building regulations and may need to file a commencement notice, depending on the scope of work.
An architect or a structural engineer will ensure the design complies with building regulations. Crucially, their report will calculate the roof’s structural tolerance and determine whether structural changes must be made to support the roof. The report will also specify the waterproofing layers.
Garden design firms can help map out the building work to be done too. “We primarily build roof gardens on pedestal and frame or tray systems that [lift] the main body of weight off the waterproof layer or allow for additional insulation beneath,” garden designer Tony Woods says, explaining that “this also allows space for running pipes [and] cables”, used for rooftop lighting and irrigation, underneath the garden.
“It’s all about the research and figuring out what is possible first,” says garden designer Damien Keane. He says that hiring landscaping professionals is advisable as they can oversee every aspect of the design and installation.
Manoeuvring machinery and materials onto the rooftop is no mean feat. Tony says: “Access to the roof is one of the most challenging aspects. In an urban environment, if a crane cannot be used, then the design will need to consider what size of materials can be transported through the building to the roof.” If passage to the roof is narrow, “materials such as decking boards will have to be cut off site”, he says.
Damien says the landscapers involved may require harnessing, and that safety and logistics are the two big things you really have to worry about.
Drainage
The drainage layer is essential to prevent water from pooling, according to Ashton Lewis of Raaft, as it will allow excess water to flow freely from the soil, avoiding waterlogging, root rot, structural damage, and erosion.
There are three ways to drain the roof: specialised mats or boards that allow water to pass through and be directed to the outlets while providing air pockets to keep the roots oxygenated; gravel or crushed stone in combination with a perforated drainage pipe system; or geotextile fabrics that prevent soil from clogging the drainage system, ensuring effective water flow.
Windbreakers
All that takes root on your roof must be hardy enough to withstand the sweeping gales and scorching sunshine that a rooftop faces. “Everything is magnified massively up there,” Damien says.
Strategically placed slatted screens and hedges can “break the force of the wind”, Damien explains. Solid windbreaks, however, are counterproductive because the wind will flow up and over them, hitting the garden with full force.
For cover, he recommends planting Elaeagnus, an evergreen coastal hedge. “That does very, very well in windy spots,’” he says.
Find a theme
Consider sowing a theme through your planting. Minimalist zen gardens, also known as Japanese rock gardens, are popular, as are Mediterranean gardens, replete with terracotta accents, tiles and perfumed herbs.
This will inform the garden furniture you choose; whatever it is, make sure it is securely fixed so it doesn’t become a risk in strong winds. Note that fibreglass pots are an alternative to the heavier stone or terracotta variety, but they need to be heavy enough to avoid wind lift. Also remember to include a lighting plan for your rooftop.
A final step is figuring out the aftercare your garden will require. “We would probably go back to a roof garden three or four times a year, just to tweak it and make sure everything is okay,” says Damien.
Planting choices
Plant selection is important for many reasons: temperature regulation, water retention and providing shade. Also some plants, especially trees and large shrubs, have aggressive root systems that could compromise the integrity of the waterproofing. Damien recommends sourcing dwarf mountain pine such as Pinus mugo or a large bonsai pruned pine.
Low-maintenance grasses and Alpine plants will flourish on Irish rooftops. Damien says: “As a general rule, anything with a glassy or hairy leaf will usually do well up on an exposed site.” Stipa arundinacea, Miscanthus grasses, Carex, Libertia, Cordyline palms, Trachycarpus, Phormium, Agapanthus, Vinca, Arbutus and ferns are among the other plants he recommends.
As for flowers, choose red hot pokers, named for their pronged shape and flame-like colouring, for summer colour and fuschia or tangerine crocosmias for vibrant autumn colour. Nepeta and lavender, meanwhile, fashion a mist of purple hues.
Those hoping to pluck fresh cooking ingredients from their garden can plant rosemary plants and a bounty of herbs, while vegetables can be grown in troughs.
What doesn’t work? “I would steer clear of any soft plant,” Damien says. Large herbaceous perennials can be blighted by windburn. No-nos include Japanese maple trees, tree ferns and any plant that requires protection in the wind, he says.
Green roof layers
1. Roof structure (base layer)
The roof must be structurally sound and able to support the entire garden system without risk of damage or collapse. A structural engineer will be involved to assess the roof’s load-bearing capacity at the design stage, well before any work begins.
2. Waterproofing membrane
Typically made from rubberised asphalt, EPDM (ethylene propylene diene monomer) rubber, or PVC, the waterproofing membrane should be installed seamlessly to cover the entire surface of the roof.
3. Root barrier
Made of puncture resistant materials such as polyethylene or other specialised root-resistant fabrics, this layer stops plant roots from breaking into the waterproofing layer and thus damaging the roof.
4. Thermal insulation
Often made from rigid foam boards or mineral wool, it helps to minimise temperature fluctuations that could stress plants or the building structure.
5. Drainage
Made of variety of materials, it is essential to prevent water from pooling.
6. Filter fabric (geotextile membrane)
Made from non-woven synthetic fabrics, this layer is porous enough to allow water flow but fine enough to prevent soil from clogging the drainage system and allow water to flow freely through the soil while preventing erosion.
7. Growing medium (soil)
Lightweight, specialised soil mixes, often containing expanded clay aggregates, perlite, or volcanic rock, are used to reduce weight and improve drainage. The growing medium is typically less dense than traditional garden soil.
8. Vegetation
May include sedums, grasses, small shrubs, or even small trees, depending on the depth of the growing medium and the climate conditions.
Finishing (protective layer)
Some roof gardens will require a finishing layer consisting of an organic mulch, gravel, tiles or decorative stories to help with moisture retention, erosion and to enhance the aesthetics.
