Look up from the pavement in almost any large British city and you will start to notice something new on the rooftops. Where there used to be grey felt and gravel, there are now low carpets of green. Some are thin mats of hardy plants. Others are proper gardens with paths, benches and small trees. Birmingham and the wider West Midlands are no different, with green roofs turning up on offices, flats, schools and even the odd bus shelter or bike store.
So what are these roofs actually for? Are they just there to look nice in planning drawings, or do they do something useful for the people who live and work below them? And how do you stop a garden on top of a building from leaking into the rooms underneath? This guide walks through the basics in plain terms.
What a green roof actually is
A green roof is simply a roof that is partly or fully covered with plants growing in a layer of soil or a special growing medium. It sits on top of a waterproof layer that protects the building.
There are two main types. An extensive green roof is light and shallow. It usually uses tough, low growing plants such as sedums, mosses and wild flowers that can cope with wind, sun and dry spells. These roofs need very little care and people do not normally walk on them.
An intensive green roof is deeper and heavier. It can hold lawns, shrubs, vegetable beds and sometimes small trees. It works more like a garden or park in the sky, so it needs regular watering, weeding and upkeep, and the building has to be strong enough to carry the extra weight.
There is also a middle option, often called semi intensive, which mixes the two. You might see a roof terrace with planters and grasses in one area and a simple sedum mat across the rest.
What green roofs do for a city
Cities are full of hard surfaces. Roads, car parks and roofs all shed rainwater quickly, which means heavy downpours can overwhelm drains and cause local flooding. A green roof works a bit like a sponge. The plants and growing layer hold on to some of the rain and release it slowly, either through the drains over a longer period or back into the air as the plants breathe it out. That takes pressure off the drainage system at the busiest moments.
Green roofs also help with heat. Dark roofs soak up the sun and give that warmth back to the street, which is one reason city centres can feel warmer than the countryside on a summer evening. Plants stay cooler and shade the roof surface, so the building and the air around it can stay more comfortable.
The building itself benefits too. The extra layers add some insulation, which can help keep top floor rooms cooler in summer and a little warmer in winter. The planting also shields the waterproofing from sunlight and big swings in temperature, which can help it last longer.
Then there is wildlife. Bees, butterflies and birds need food and shelter, and city centres do not offer much of either. A roof planted with native flowers can become a small stepping stone that helps insects move between parks, gardens and canal sides. Birmingham is famous for its canals and green spaces, and every extra patch of planting helps join them up.
Finally, there is the simple human side. People like looking at green things. A view of plants from an office window or a flat balcony is more pleasant than a view of tar and air conditioning units, and a usable roof garden gives residents somewhere to sit outside in a busy part of town.
How a green roof is built
A green roof is a stack of layers, and each one has a job. Green roof systems are usually designed as a complete package so that the layers work together rather than being picked separately. From the bottom up, a typical build looks something like this.
First comes the roof deck, the structural surface of the building. An engineer checks that it can carry the weight of the finished roof when it is soaked with rain, which is much heavier than when it is dry.
On top of the deck sits the waterproofing. This is the most important part of the whole roof, because once plants and soil are in place it is hard to get at. It has to be reliable for many years and able to cope with water sitting against it.
Next is a root barrier. Plant roots are good at finding small gaps, so this layer stops them from pushing into the waterproofing and damaging it. Some waterproofing products are made to resist roots on their own.
Above that there is usually a drainage layer. It lets extra water run away to the outlets so the roof never becomes waterlogged, while some designs also store a little water for the plants to use in dry weather.
A filter fabric then stops fine soil from washing down and clogging the drainage. On top of this goes the growing medium, which is a lightweight mix designed to hold water and nutrients without being too heavy. Finally come the plants themselves, either as ready grown mats, plug plants or seeds.
Around the edges and outlets there are gravel strips, which help with drainage, make inspections easier and act as a fire break.
Looking after a green roof
Even the low maintenance types need a little attention. A typical extensive roof needs a visit or two a year to clear the outlets, pull out any weeds or tree seedlings, and add a little feed if the plants look tired. Intensive roofs need much more, just like any garden at ground level.
Safe access matters. Anyone working on the roof needs proper edge protection or safety lines, and the roof should be designed with this in mind from the start.
It is also worth keeping good records. Knowing what was installed, when it was checked and what was done makes life easier for building managers and helps spot small problems before they turn into leaks.
Is a green roof right for every building?
Not always. Older buildings may not be strong enough without extra work, and steep roofs need special systems to stop the soil from sliding. Cost is another factor, as a green roof costs more to install than a basic flat roof. But for many new builds and refurbishments, especially in dense parts of the city, the benefits for rainwater, heat, wildlife and wellbeing make a strong case.
If you are a homeowner, smaller versions are possible too. Sheds, garden rooms, bin stores and garages can all carry a simple sedum roof, which is a good way to try the idea on a small scale. Just check the structure can take the weight and get the waterproofing right first.
Conclusion
Green roofs are more than a planning box to tick. They slow down rainwater, cool the air, protect the roof beneath and give wildlife a foothold in the middle of town. They work because of careful layering, with good waterproofing at the bottom and the right plants at the top. Next time you are walking through Birmingham city centre and spot a patch of green above the shop fronts, you will know there is a lot more going on up there than meets the eye.