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In the final of the series of blog posts regarding retrofitting homes to be more sustainable, PJH are looking at green roof retrofitting and external landscaping. Beginning with an introduction and facts regarding green roofs, pros and cons and associated costs.

Green Roof Retrofit Introduction

As part of the passive measures for retrofitting a dwelling to become more sustainable, a green roof works hand in hand with water saving and the below measures for reducing the heat absorbed and stored within solid materials.

The partial or full covering of a building roof with well-chosen vegetation fit for the local environment and roof structure will have several benefits offered to the dwelling occupier. There are systems that can be installed over the waterproof membrane of a roof or incorporated as part of a larger project. A root barrier and drainage system will protect the roof and the vegetation cover will actually help extend the life of the waterproof membrane by protecting it from the solar radiation. The systems can be incorporated with automated irrigation and rainwater harvesting to provide a completely integrated flow with minimal input.

The urban heat island, discussed below, is also reduced through the reduction in the number of hard surfaces that the sun can store heat within. They also act as an initial store for rainwater that delays the water reaching any harvesting methods and eventually drainage that, when done on a larger scale, can have a significant positive impact on local flooding. The roof also has the added bonus of increasing the amount of evapotranspiration that can occur, this form of evaporation is aided by the increased amount of water that can be stored prior to joining any surface runoff.

Incorporating a green roof system within the design of a roofing system enables the designer to achieve a greater U-value, which in turn aids the reduction of running costs and heat loss.

Green roof retrofit systems can also be installed on most pitch roofs and even have lightweight options that require reduced load capacities to keep structural costs down, although flat roof options are more popular.

green roof retrofit

Sedum green roof systems can be installed in a variety of different roof types however these systems are easiest to retrofit to existing flat roof systems with relative ease, due to not requiring extensive works to be undertaken to prepare the roof. When retrofitting, it is important to understand how the different green roof systems can impact the loading of the roof structure.

Systems come in mats or tray systems, with the trays usually arriving fully grown and not requiring additional layers due to them usually being incorporated as part of the tray systems.

green roof retrofit

Pros and Cons of Green Roof Retrofit

green roof retrofit

Green roof retrofit: Costs

The costs associated with installing a green roof system depend on the type of system being installed and the size of the roof in question. For a M-tray system, it can be expected that costs will be roughly £120 per m². For standard sedum roofs, it would be roughly £65 per m² as per Sedum Green Roof (2023 .

The type of system required may be determined by the roof it is intended to be installed on.

External Landscaping

The external landscaping to a dwelling can have varying results to the comfort levels internally. An increased number of hard surfaces like concrete, tiles, bricks means that the amount of solar radiation absorbed and stored is higher, this then is released back in and around the dwelling once the sun goes down. This effect in more urbanised areas is called the urban heat island. The more infrastructure that can absorb and release the suns heat means that these areas experience higher temperatures than outlying areas that are surrounded by more natural landscapes.

Including additional measures for vegetation cover around a dwelling enables the solar radiation to be reflected and not absorbed to be released later. Taller vegetation also brings the added benefit of shading that decreases the amount of solar radiation that is gained from any windows that get a lot of sunlight.

Adding vegetation into the design also has many health benefits with improved air quality, lower greenhouse gas emissions and enhanced benefits to the wellbeing. By incorporating vegetation into focal points of a design increases the occupier’s connection with nature and the natural colour palette that can have psychological benefits to mood, productivity and general wellbeing.

Landscaping is a very easy and cost-effective way to improve a dwelling’s energy efficiency and privacy. Strategically positioning vegetation will add protection of cold winter winds and shade from the summer sun with the indoors benefitting from lower temperatures of around 6-12 degrees Celsius during summer.


To conclude, green roof retrofit systems offer the occupier reduced running costs through increasing the roof u-value and reducing the urban heat island effect from the vegetative cover. Depending on the roof systems, varying options are available for different budgets that enable home owners to benefit from a green roof. When done in conjunction with external landscaping, these measures aid in the wellbeing and comfort of the occupier too whilst also increasing biodiversity and reducing the home’s carbon footprint.

Ask us for advice

If you have a project in its infancy and want to have an informal chat regarding your options, feel free to contact us, we are always happy to provide friendly and knowledgeable advice. Get in touch on 0161 460 0382 or email the team at enquiry@pjhas.co.uk 

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