FAQ’s

1. Do Green Roofs help with planning permission?

Yes. There has been a definite move from planning authorities to encourage green roofs and sustainable construction methods in recent years., so incorporating a green roof into the plans for any new build can help with the approval process.

In general, a green roof does not need planning permission. However, it is recommended that owners always check with the local authority beforehand in case the building is in conservation area or if it is a listed building.

Incorporating a green roof into the plans for any new build can nowadays ease the approval process as it is seen as desirable aspect for many councils.
Before you plan your green roof, though, you need to consider there are no specific building regulations relating to green roofs, there may be issues concerning load capacity, fire prevention or the disposal of water from the surface of the installation.

Green roofs can also increase the sustainability scores for new buildings and can gain credits with BREEAM (Building Research Establishment Environmental Assessment Method), particularly within the land use and ecology section.

2. Do I need to place a protection fleece over the waterproof layer?

Yes, its good practice and is part of the GRO Code 2021 and other best practice.

Wallbarn recommends using Re-cycled polyester geotextile 300gsm GE-PE-PECT-0300-Z for this purpose.

3. Does the roof have to be flat to lay a green roof?

Most green roofs are installed onto flat roofs, although the term flat roof is often misunderstood by the public. The roof does not need to be dead flat, in fact the roof should fall to a certain degree to allow free drainage of rainwater beneath the green roof system. Roof decks which fall 1 in 80 to 1 in 40 fall are the most common types.

Wallbarn recommends green roofs are installed up to a 15° pitch without modification being required.

For roof decks with a fall greater than 15° pitch installers would need to ensure the whole system is sufficiently anchored to the sub deck. This can be done through battens (for M-Tray®), cables and ties. Specialist design and specification advice should be sought in this case.

Blue Roofs will have a dead flat roof space / subdeck and the stored water withing the Blue Roof needs to build up with the adjustable drainage outlet providing the exit.

4. What is the minimum depth of substrate?

For extensive green roof systems (including sedum roofs) Wallbarn recommends a minimum of 100mm (4 inches) of substrate. Some green roof suppliers offer much lower build ups of substrate (down to 40-50mm in some cases) but in our opinion this will not be sufficient to sustain the plants in the long term.

Beware low build up systems which offer solutions to planning or cost savings. Always ask for data on long-term performance of a green roof system – what does it look like after 2 years, 5 years?

The 2020 Amendments in Approved Doc B of The Building Regulations 2010 include a fire safety assumption of using a minimum of 80mm substrate, as does the GRO Code 2021; so green roofs should not be constructed with a substrate level thinner than 80mm.

Wallbarn has conducted numerous physical growth tests on its substrate mixes and it can demonstrate how and why a minimum substrate depth of 100mm should only ever be considered.

5. What sort of substrate should be used?

An engineered growing medium designed to encourage strong, healthy growth of sedum and other plants on a roof space. It is made up from sustainable, recycled, mainly UK sourced materials to create a lightweight and free draining substrate free from weeds and contaminants.

Important aspects required are:

a. an optimum level of nutrient to suit the type of green roof being constructed; low organic content;
b. varied aggregate particle size; a balance between draining and moisture retention;
c. resistance to compaction and erosion;
d. avoidance of peat in the mix;
e. avoidance of impurities and contaminants within the mix;
f. adherence to green roof codes, standards and regulations (see section 6);
g. weight / depth;
h. long-term sustainability & healthy growth of plants

6. What depth of drainage should I use?

Fore extensive green roofs (including sedum) our 20mm deep drainage membrane Protecto-drain 20P is ideal and will suit nearly all applications. It is an egg box shaped membrane which will collect a certain amount of water with the excess draining through holes, running away in the air gap beneath the membrane into the roof drain.

For extensive green roofs where extra strength and water reservoir capacity is required we have 40mm or 60mm deep drain perforated membranes. The depth and water capacity of the drainage membrane specified will depend on the depth of the substrate and the types of plants / traffic on the actual roof garden / green roof.

For Blue Roofs special drainage cells are required

Ask us about M-Tray® Blue.

7. What are sedums?

Sedums are succulent plants often known as stonecrops. They grow typically in mountainous regions in rock faces and crevices and are incredibly hardy. They can thrive in a shallow, free draining growing medium and can cope with cold, high wind and drought conditions, making them ideal for use in green roofs.

There are up to 500 different species of sedum, including annuals, herbs, small shrubs and perennials. They have a huge variety of different shapes, colours, sizes and growth cycles.

There are many flowering species, and it is often forgotten that they produce stunning colour throughout the summer months, flowering predominately between May and July and attracting bees, butterflies and a huge variety of wildlife.

8. Can I choose my own sedum mix?

Wallbarn uses a selection of 13 different sedum species in its standard offering, including in the M-Tray® sedum product. The species list is here .

These species were chosen to be hardy and strong growing in the long-term; to give an all year round green covering; to bring variety of seasonal growth, different leaf colour, structure and size; to offering a large variety of flowering colours and to attract bees, butterflies and other wildlife to the green roof space.

Because the sedums take 8-12 months to be mature and established enough to sell Wallbarn sticks to a standard, well proven species mix in its M-Tray® and roll out green roof systems. If clients wish to create their own sedum species mix they need to understand the timescale required to create the finished product and a minimum volume of product including pre-payment of the production / growing process will be needed.

The best option for clients who wish to add their own sedum species is to purchase plug plants or semi mature (P9) sedums and add them to the green roof once it has been installed. Wallbarn can offer this service to clients.

9. Can I buy sedum plug plants?

Yes, Wallbarn offers a range of different plug plants, semi mature sedum plants, cuttings and sedum blankets should clients need to increase the amount of vegetation on the roof. Ask us for more details.

10. What are wildflowers for green roofs?

Is a wildflower a weed? Any plant that is in the wrong place can be called a weed. Any weed that is in the right place, such as on a biodiverse roof, can be called a wildflower.
Wildflower green roofs are also called biodiverse roofs, are chosen because they increase the amount of different plant species available to the local environment and offer more than just sedums. They can replicate to a certain extent the natural flowers and plants that would be growing in that part of the country in similar soil conditions.

The wildflower species used on Wallbarn roofs all British native species and are chosen for their hardiness and suitability. We aim to introduce ambient colour and scents into the location to recreate the native wildflowers that could be found at ground level, thereby attracting bees and butterflies and a whole range of other native and local wildlife.
The wildflower species selected are typically low growing, perennial and suitable for low nutrient, low moisture substrate (soil) conditions, tolerant to dry conditions and suitable for substrate depths of 100mm. The species list is given here .

Wallbarn uses a base of sedum within its wildflower offerings to provide a green cover over the roof space during the periods when the wildflowers are not showing. We also choose species that typically do not produce stalks and flowers higher than 60cm.

It is important that very tall plants and those with a large amount of stalk and other growth that will die off at the end of the season are not chosen. Masses of dead and difficult to rot vegetation presents a fire hazard on the green roof space. Clients should be aware that even a biodiverse / wildflower roof system requires regular maintenance.
Wildflowers can take up to 2‐3 years to fully germinate and flower from seed, and some species are viable only as annual and biennial flowers, so new seeds may need to be broadcast at intervals.

11. Can I walk on a green roof?

Although sedum and wildflowers will tolerate a minimal amount of traffic it is generally not recommended to walk across the green roof repeatedly. Two to three year maintenance visits and other ad hoc instances of walking across the sedum and wildflowers will not present a problem to the integrity of the green roof but if regular traffic is envisaged then hard landscaping access areas should be constructed.

Do not consider using a green roof as a recreational or sporting space.

12. Can I install a green roof in a shaded area?

Generally speaking, no. Sedum and wildflower roofs require consistent access to light and rainfall to grow properly. Semi shaded areas can be considered but the roof space would require access to light for at least half of the time that space gets normal sunlight. Areas covered or overlooked by low hanging trees should or areas in constant shadow caused by tall buildings should not be considered.

13. Is irrigation necessary for an extensive green roof?

For rollout
Yes. Once the sedum / biodiverse blanket is installed onto the substrate it is vital that the whole green roof is irrigated for the first few months, even in the winter. The roots of the sedum need to start to enter the substrate mix and establish themselves, a process that can take many months so the more help they can get via irrigation the better.
substrate blend and because the vegetation is not disturbed during transit from the farm to site.

For M-Tray®
M-Tray® modular green roof system has all the required elements contained within the plastic box and the sedum / wildflowers have been pre-grown for up to 12 months before being delivered to site. Therefore, they have a much higher degree of drought tolerance and are much stronger green roof plants, given they are fully established and rooted into the 100mm deep substrate.

However, for best results it is recommended that they are irrigated after installation and for a period of 2-3 months to give them the best chance to provide the strongest growth.
During the summer UK can experience long periods very hot and very dry weather. Although the sedum is a hardy plant with drought tolerance; it is not completely drought resistant. It will turn red and shrink back during these very dry periods, so irrigation is recommended.

14. Do green roofs carry a guarantee / warranty?

Green roofs are alive, and with most agricultural and horticultural products, do not carry a guarantee or warranty due to the myriad of factors that can cause food and plants to rot, die or become unviable.

The “non-live” elements within Wallbarn green roof systems (empty M-Tray® module, geotextile etc.) carry to the standard 15 year Limited Warranty offered on Wallbarn products. Wallbarn also warrants that the products will arrive on site in a good condition and fit for installation.

If clients wish to protect the green roof in the long-term they can purchase Maintenance Contracts with Wallbarn partners who will ensure the long-term sustainability of the green roof system.

15. What is aluminium Edging Bar?

The green roof should be surrounded on all sides by a wall or parapet / upstand for best results. When constructing green roofs up to walls of occupied buildings a fire break of stones is required. See the GRO Code for more details.

The upstand or wall will enclose the green space and prevent wind uplift and movement of the plants or substrate. If there are open sides to the green roof then an L shaped Aluminium Edging Trim can be supplied by Wallbarn which is anchored down by the weight of the green roof / pebbles to give a secure and tidy edge detail.

16. Accessories

As well as the Aluminium Edging Trim mentioned above, rounded pebbles are normally used around the perimeter of the green space and around objects (such as rooflights, mansafes, drains) to provide a tidy edge detail and to prevent the plants growing over such objects or hazards. It is important that these pebbles are rounded to prevent puncturing waterproofing membranes, and are river washed stones usually 20-40mm in size. Decorative specialist stones are sometimes used.

Access chambers and covers will be required to separate and protect objects such as roof drains (including Blue Roof Outlets), electrical points etc.

Hard landscaping is often required for foot traffic on access areas and paving slabs or decking is often constructed using support pedestals.

17. Do I need to fertilise the sedum?

Fertiliser is essential to maintain the condition of the plants, as little nutrient exists in the shallow growing medium. Wallbarn supplies a slow release granular fertiliser which is safe to handle and easy to apply. Simply sprinkle onto the green roof and water in thoroughly. Fertiliser is best applied twice per year, during spring and autumn.
Nutrients in the fertiliser help the plants to become conditioned and resistant to temperature extremes, also helping the plants to flower. Without fertiliser, the strongest and most resistant species will become dominant and you will end up with a monoculture, losing the variety and aesthetic balance.

18. What are the advantages of M-Tray®?

As explained in some of the questions the M-Tray® modular green roof system has significant advantages than loose lay systems. All the necessary elements required for an extensive green roof have been added to M-Tray® and pre-grown in a British farm, meaning fully established and mature sedum or wildflower plants are fully integrated into the substrate mix, ensuring a strong, healthy and long lasting green roof.

The modules simply click together to create a seamless, instant green roof making constructing a green roof incredibly easy, with far less disruption.

The advantages include:

I. Convenience – mess and disruption are reduced as all the required elements are contained within a plastic tray unit
II. Damage reduction – risks of loose aggregate, spades tec. Damaging the waterproofing membrane are eliminated
III. Quality – M-Tray® is quality controlled throughout its production and growing phases. Only trays in perfect condition are packed. They do not suffer from being damage in transit so the product stays in primary condition
IV. Lightweight – each tray can be carried by one person
V. Irrigation – they require less irrigation due the unique special substrate mix
VI. Easy to install – just click the trays together
VII. Accessible – by carrying by hand roof spaces considered inaccessible for green roof construction can now be greened up. They can even be carried through a window if necessary
VIII. Speed of installation – at least twice as fast to install than loose lay systems
IX. Instant Green Roof – sedum / wildflower are grown in M-Tray® for at least 8 months before delivery to site to ensure healthy vibrant vegetation
X. Futureproof – If the roof deck beneath needs to be inspected the trays can be lifted easily with little disruption to the whole roof
XI. Retro-fit – for the reasons above and others they are the perfect retro-fit green roof product
XII. They can be installed by non-skilled labour because of their simplicity
XIII. Variety – M-Tray® comes either as a Sedum or Sedum and Wildflower mix. Bespoke planting options are also available

19. Can I purchase empty trays?

Usually no. Wallbarn intends to control the quality and consistency of M-Tray® by organizing the production and growing itself. Therefore, for small roof schemes self-filling is not available to clients. However, for established professional crop growers the option is available.

International licensing and distribution rights are also available for the patented, registered and protected design.

20. Maintenance for green roofs

Green roofs are living structures and as such need attention throughout their lifespan.

Some vegetation types such a sedum are planted as they require less maintenance than grass roofs for example. However, regular attention to substrate moisture and feed levels as well as the removal of weeds is essential to maintain optimum performance.

There is a thorough maintenance programme available from Wallbarn for all green roof types. Details can be found here.

Extensive M-Tray® modular roofs – Sedum or Sedum / wildflowers mix:

Spring schedule

• Remove weeds by hand pulling only
• Apply Vitax Q4 slow release granular fertiliser (available from Wallbarn) at a rate of 50 grammes per m2
• Clear debris from drainage outlets
• Generally, inspect vegetation area and monitor poor growth area
• Insert cuttings from good growth areas into bad growth area
• Check irrigation system (if applicable) and replace timer batteries

Autumn schedule

• Remove weeds by hand pulling only
• Clear debris from drainage outlets
• Generally inspect vegetation area and monitor poor growth areas
• Cut back dead stalks or dying vegetation
• Insert cuttings from good growth areas into bad growth areas
• Check irrigation system (if applicable) and drain down for winter to avoid frost damage

Wallbarn offers long-term Maintenance Contracts to ensure the green roof remains in the best possible condition and installation services are also available. Ask us for more details.