green roofs: pros and cons

Green roofs (roof-top gardens) have some important advantages over traditional roofing. But they involve higher costs, higher maintenance and some potential problems that should be taken into account.

Green roof in Atlanta, USA Left: Green Roof in Atlanta, on the GA City Hall. Image Source: EPA

Pros of green roofs

Green roofs are an environmental solution. They reduce urban «heat islands» (higher urban temperatures caused by traditional dark colored roofs and pavements, and the materials used in them) and can be an important option to reduce city pollution.

From the homeowner’s perspective, green roofs are a way for reducing the use of air-conditioning (they provide excellent insulation and cooler indoor temperatures).

Other (environmental) advantages:

- green roofs are excellent sound insulators.
- the vegetation element of green roofs may be a good habitat for birds and other small animal life;
- they assimilate large amounts of rain, reducing the needs of sewage systems and mitigating storm water runoff (roof-top gardens have an important role in storm water management, since they act as sponges during rainstorms).
- green roofs vegetation absorbs negative air toxins, purifying urban air.
- extended life of the roof base.

Costs of green roofs

Green roofs involve higher costs than traditional roofing. $8 per square foot is a minimum (includes materials, work costs and installation). Values around $12 to $24 per square foot are common. These values may amount to 5 to 10 times the cost of most traditional built-up roofs (and also much more than roofing based on new cooling materials: see Roof coatings and membranes and Roof clay and cement tiles).

These much higher up-front costs may me at least in part offset through air conditioning energy savings, but the return of the investment may be too long or you may never achieve it.

Possible disadvantages of green roofs

Besides their higher costs, green roofs pose other potential drawbacks:

- higher maintenance costs;
- restrictions involving climate and weather conditions (un example: rooftop gardens are inappropriate in very windy places; plants are fragile and can be blown away).
- complex drainage systems;
- eventually, stronger roof beams in order to support the several green roof layers, namely the soil layer;
- more costly repairs and fixings (finding and repairing eventual leaks is more expensive and difficult).