How Much Does a Green Flat Roof Cost?

Updated Prices For 2021

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Welcome to Quotation Check

A Look at Green Roofs

A green roof is a great way to improve the appearance of almost any flat roof. Instead of ugly fibreglass, rubber or bitumen, you can, for a reasonable price, look out of your window at a beautiful green roof covering.

You’ll also see an abundance of bees, butterflies and other insects thriving on the roof which also helps to absorb CO2.

There are two types of green roofs:

  • Intensive green roofs – these usually contain shrubs, large plants and even trees.
  • Extensive green roofs – often growing no taller than a few inches, these roofs typically contain sedum and other low-level plants such as wildflowers.

Intensive roofs are usually only constructed on large non-domestic structures while extensive green roofs can be built on garage roofs, extensions and even sheds.

Step by Step Installation Process

Here a step-by-step guide to installing a green flat roof above a domestic property.

  • Take measurements of the existing flat roof, noting the width and length, the joist sizes and spacing and also the type and thickness any decking and insulation.
  • Contact a structural engineer, provide them with the roof measurements and details of the product. You’ll be told if the existing roof can support the extra weight or if extra supports are needed.
  • Contact a flat roofing installer; any competent roofer should be able to install an extensive flat roof. Provided the roof is strong enough to hold the load, the installation is fairly quick and straight-forward:
  • Assuming the roof is now strong enough to support the weight of the green roof, the first step is to lay a protective membrane directly onto the flat roof surface.
  • The second step is to install a drainage layer; this is usually made from plastic and contains small cups that hold enough water for irrigation. Excess water is allowed to drain off the plastic.
  • The next layer to be installed is a filter membrane; this prevents roots, seeds, moss and the growing medium from blocking the drainage channel.
  • Next, the growing medium and vegetation layer is installed. This is typically a carpet of sedum.
  • Finally, for flat roof edges that aren’t protected by wall or upstand, an edging is required. This protects the roof from the wind; without it, the materials might lift or become dislodged in the wind.

Update: Instead of installing the drainage layer and filter membrane, all-in-one pre-grown modular trays are now available. These trays contain a filter and drainage channel under the growing medium and vegetation. The images and videos below explain how this is installed.

Photos and Images

Below you’ll find photos and images of the product and installation.

This isn’t the only way to construct a green flat roof but is the easiest:

Sedum flat roof tray

A pre-grown sedum tray complete with filter and drainage channel at the base. Each tray is 500mm x 500mm so 4 trays are needed to cover 1 square metre. They easily clip together so it only takes minutes to lay a few square metres. Image from Roofing Superstore.

Sedum trays stacked

Stacks of sedum trays ready to be installed. Image from Roofing Superstore.

Green roof edging

Green roof edging system. Image from Roofing Superstore.

Geotextile fleece

Geotextile membrane.

Green flat roof drainage

If you don’t want to use a modular system, you can lay a drainage system such as this. The growing medium and vegetation are placed on top of this. The small cups hold just enough water for irrigation, excess water drains away. The filter layer protects the system from blockages. Image from Permagard.

Green Flat Roof Cost

Recently installed green flat roof. Image from Urbanscape.

Green Flat Roof Installation Video

The video below shows a modular system being installed.

Once any structural work, edgings and landscaping (i.e. slabs or stones) are complete, the project is simple and quick:

How Much Does a Green Roof Cost?

The following prices were sourced in early 2021:

Geotextile membrane – £1.75 per square metre.

Pre-grown modular trays – £55 – £70 per square metre depending on quantity ordered.

Edging bar – £15 per linear metre.

Stones or slabs for edgings – £12 – £25 per square metre.

Structural engineer calculations – £250 – £500

Labour for the installation – No more than one day’s work for a typical domestic project, i.e. an extension or double garage.

Plus any costs for altering the roof structure to accommodate the extra weight. This cost isn’t always applicable but a new roof extra supports and a new covering can be costly.

Example Green Flat Roof Price

For a domestic project, the cost to supply and install the roof will be somewhere in the region of £95 – £130 per square metre including VAT.

This guide price assumes that no structural alterations are required, and the original roof covering can be left in place. 

Smaller roofs will be priced at the upper end of the price range, large projects cost less per square metre due to bulk discounts.

Find an Installer

We hope you found our guide to green flat roof costs insightful.

As green roofs are becoming more and more popular, we expect the cost to come down over time.

To get a custom price for your flat roof project, tap the button below and fill in the contact form:

  • Get a custom green flat roof price based on the specifics of your project.
  • Check vetted reviews and ratings.
  • Compare prices from local tradespeople and specialist firms.

Green Roof:

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Green Roof FAQs

Do I Really Need to Use a Structural Engineer?

The weight of the sedum and the moisture it holds, plus the growing medium, the plastic trays, decking, insulation and ceiling plasterboard can easily weigh over 100kg per square metre. Add the weight of any snow or ice and you could be close to the structural limit of your roof.

Instead of paying for a structural engineer, you could instead contact one of the green roof manufacturers to ask for advice regarding the weight. They’ll have plenty of experience and if your roof design and construction is typical and straight-forward, you might not need to pay for the services of a structural engineer.

Will the Green Roof Damage the Flat Roof Underneath?

With two membranes, neither roots nor growing medium should come into contact with the flat roof covering. We’ve never heard of a green roof damaging a roof covering.

When Were These Green Roof Prices Published?

We sourced these prices in early 2021.

Can I Walk on the Roof?

While it’s possible to walk on sedum green roofs for maintenance, it’s not something you should do regularly. The sedum is a plant and may become damaged by foot traffic.

Do I Need to Maintain the Roof?

Sedum roofs are very low maintenance and apart from an occasional tidy up; you shouldn’t need to spend too much time maintaining the green roof.

Wildflower roofs require yearly deadheading and general maintenance, but the extra work will reward you with more colour and a prettier roof.

How Long Does it Take to Build a Green Roof?

Assuming the structure below is sound and the roof can be reached easily, a domestic flat roof should take no longer than a day.

Can a Green Roof be Built on a Sloping Roof?

Yes, but there’s extra work involved with securing the materials to the roof tiles and sloping roofs are more difficult to reach, often requiring specialist access equipment. The cost will therefore be higher than a flat roof which is a fairly straight-forward project.

Do I Need to Pay For a New Flat Roof?

Green roofs should be installed over flat roofs that are in a good condition, but there’s no need for them to be brand new. The green roof is independent of the flat roof below and can be installed and removed without damaging the roof covering.

How Environmentally Friendly is a Green Roof?
Good quality green roof products are made from recycled plastic and the sedum encourages insects to the garden. The materials also help to insulate the flat roof. The vegetation also absorbs CO2, so yes green roofs are environmentally friendly.
What Type of Flat Roof Can a Green Roof be Installed Over?

Green roofs can be laid over mastic asphalt, bitumen felt, rubber EPDM and fibreglass flat roof coverings that are in good condition.

Where Can I Find More Information?

Discover More Price Guides:

By Quotation Check

Bitumen Flat Roofs

See how much it costs to replace a flat roof with long-lasting and time-tested bitumen felt systems. This flat roof covering is perfect as a base for a green roof.

Rubber Roofs

Rubber EPDM is a popular and modern product that will hold up well in all conditions. It’s suitable for use with a green environmentally-friendly flat roof.

Fibreglass Flat Roofs

From ship hulls and kayaks to pond liners and car parts, fibreglass can be found on thousands of waterproofed products. It also perfect for constructing under a new green roof.

Their prices for roofline installations were correct; we were given quotes from several firms, all were similar to [Quotaton Check’s]

Quotation Check’s guides to flat roof prices are very accurate, they include all the extras many other price guides conveniently exclude.

Why only rely on three or four quotes for comparison? This website called Quotation Check gathered prices from hundreds of firms for various home improvement tasks and published their findings online.

Roofing firms often charge more than they should because they think consumers have no idea about how much the parts and labour should cost. Quotation Check’s guides are easy to read and they explain in detail about the materials needed and how long the work will take.

Go see the prices on the Quotation Checker website; they’re suggesting a price for a new roof that’s much lower than that.