If you really want to “go green,” a green roof is a great way to create a natural and relaxing environment for your home while being eco-friendly. But before deciding if this roof is right for you, consider the pros and cons of a green roof.
There are many benefits to a green roof, but like with any roofing choice, green roofs have their drawbacks, as well. Green roofs are costly and weigh a lot, which puts stress on the structure of your home – and your wallet.
What is a Green Roof?
What is a green roof? Appropriately called rooftop gardens, a green roof is a layer of vegetation growing on a waterproof membrane.
Green roofs have several layers:
- Structural roof decking
- Vapor barrier
- Thermal insulation
- Root barrier
- Drainage layer
- Filter membrane
- Growing medium
- Vegetation layer
Green roofs started in Germany during the 1960s. The idea has taken hold since then, spreading to other countries and increasing in popularity, especially in major U.S. cities.
Green roofs can be installed on residential or commercial buildings, as long as the roofs are flat or have a low-sloping angle. The angle of a green roof should be no steeper than 45 degrees.
3 Types of Green Roofs
There are three types of green roofs – extensive, intensive, and semi-intensive.
Extensive Green Roof
Extensive roofs have a shallow rooting system, generally only 6 inches deep. At 25 pounds per square foot, extensive green roofs weigh less than alternative green roof types.
Extensive green roofs support only a limited number of plants, generally grasses, mosses, and succulents such as sedums.
Intensive Green Roof
On average, an intensive green roof supports a broader variety of plants, offering homeowners a diverse selection of vegetation. Flowers, mosses, grasses, herbs, shrubs, and trees are all plants that can thrive on a green roof. Intensive green roofs are more expensive and require more maintenance.
Intensive roofs are capable of holding up to 80 to 150 pounds per square foot. Due to the strain the weight of an intensive roof will put on a house, not every roof can support an intensive roofing system.
Semi-intensive Green Roof
Semi-intensive roofs are essentially a blend of extensive and intensive green roofs.
Semi-intensive green roofs have deeper soil and accommodate more diverse plant life than an extensive roof, but they aren’t able to support quite as much weight as an intensive roof. Semi-intensive green roofs can host smaller plants such as grasses, mosses, and flowers, and even shrubs.
Pros of a Green Roof
Lasts a Long Time
A green roof extends the lifespan of your roof by protecting it from rough weather. The vegetative layer covers your roof from exposure to UV rays, wind, ice, snow, rain, and hail. Ultraviolet light from the sun wears down a roof over time, so this protection means your green roof will last longer.
A green roof can last 40 to 50 years, which is two to three times longer than a conventional roof.
Reduces Urban Heat Islands
The natural plant life and soil comprising your green roof reduce heat in your area.
Cities are constructed of concrete, stone, and asphalt, which all absorb heat and create a sweltering heat bubble. This effect is known as the urban heat island effect. With so many heat-absorbing materials crowded in close proximity, it creates an “island” of heat in a city.
Through evapotranspiration, plants remove heat from the air, creating a cooler environment. As a result, the plant life on green roofs reduces heat islands by cooling the temperature and saving energy.
Reduces Waste
Green roofing extends the lifespan of the roof’s membrane. The roof does not have to be replaced as frequently, which cuts back on tear-off waste. This decreases the amount of discarded shingles that end up in landfills. About 11 million tons of asphalt shingle waste end up in U.S. landfills each year.
Green roofs are frequently made from recycled materials, another eco-friendly bonus about green roofs.
Rain Management
Green roofs are helpful for stormwater management in urban areas. Green roofs prevent urban flooding and water erosion caused by rainwater runoff.
Rooftop runoff makes its way to streams, collecting toxic chemicals and pollutants along the way. Runoff water carries these pollutants to local lakes, ponds, and rivers, contaminating local water sources and negatively impacting aquatic life.
In a non-artificial environment, only about 15% of rainwater runoff makes its way into streams, rivers, and other local bodies of water.
The soil and vegetation of green roofs absorb as much as 70% to 90% of rainwater during the summer months, and 25% to 40% of precipitation during the winter season.
However, in areas that depend on stormwater runoff from roofs, this will be more of a liability than an asset. Those areas need rainwater to recharge the groundwater underflow.
Noise Reduction
Green roofs provide excellent insulation against noise. The natural insulative barrier of vegetation and soil minimizes outside noise pollution entering your home. This gives homeowners a quieter, more peaceful living environment.
Rooftop Garden
An intensive green roof can function as a rooftop garden. You can enjoy the benefits of having a natural garden on top of your house. Some intensive green roofs are even used as recreational areas or as parks for people to visit and enjoy.
Temperature
Green roofs work as an energy-efficient insulator. They keep hot and cold air trapped inside your home. The root system and the air trapped between the underdrain layer insulate a building. This helps stabilize the interior temperatures of a home. It saves homeowners money on energy bills by lowering heating and air conditioning costs.
Clean Air
The greenery of a living roof improves air quality. Plants create oxygen, providing the local area with cleaner, better air.
Plants combat harmful emissions such as sulfur dioxide, nitrous acid, and carbon dioxide. People with green roofs benefit from purer air and reduced air pollution.
Wildlife Friendly
The ecological benefit of a green roof is that it increases biodiversity by providing a safe resting area for local wildlife such as butterflies, bees, and birds.
Bird baths, beehives, or flowers that attract butterflies can be added to your roof to make it more hospitable to local wildlife.
Fire Resistant
A green roof is not fireproof, but it does decrease the spread of fire. Plants contain a lot of moisture, making them fire-resistant.
Although if the vegetation on top of your roof is dried out, your roof may be more flammable.
Curb Appeal
Green roofs are an eye-catching roofing choice. If you want a roof that stands out from other roofing materials, a green roof makes your home a memorable landmark.
The natural beauty of growing plants is aesthetically pleasing and a calming sight compared to other artificial roofing materials.
Reduces Stress
Research has proven that being surrounded by plants and the natural world has several cognitive benefits:
- Increases happiness
- Helps you relax
- Decreases stress
- Prevents depression
- Stimulates mental wellbeing
- Boosts your immune system
- Lowers blood pressure
- Improves sleep
- Raises your energy levels
- Healing effect
Enjoying the beauty of nature has many beneficial effects on the human mind and spirit. Adding a beautiful rooftop garden brings the beauty of nature right to your house. Homeowners with this type of roof can enjoy the benefits exposure to nature brings us.
Cons of a Green Roof
Expensive
Green roofs are pricey. Upfront installation costs will be expensive, depending on how much your roof needs to be altered and strengthened to accommodate the weight load of a green roof.
Each roof is different, and size, shape, slope, and location are all factors that will be considered in the pricing of a green roof. Averaging $15 to $50 per square foot, the total cost of a green roof ranges from $13,000 to $50,000.
Weight
The weight of a green roof may be problematic if it causes sagging. Flat roofs are often sturdy enough to hold up under such weight, but some roofs may need extra support added to cope with such heavy weight, particularly for intensive green roofs.
The amount of weight your roof can withstand can be increased by renovations such as adding more trusses and braces to the roofing system or strengthening the existing trusses.
If a green roof weighs too much, it could be a problem for the safety of your house. Before you decide to install a green roof on your home, have it inspected and assessed to determine that your roof can support the weight of a green roof.
Additionally, during rainstorms green roofs absorb a lot of water, so your roof will need to hold up under that extra weight,
Root Damage to Roof
While the flora is lovely to look at, growing roots can cause serious damage to your house. Roots growing into cracks or shingles can damage your home and cause leaks. If plants damage your home, the repair expenses may be high.
Maintenance
Extensive green roofs are low maintenance. Generally, extensive green roofs are left to grow naturally and need maintenance only once a year.
Intensive green roofs require more dedicated maintenance. Intensive green roof care is essentially the same as maintaining a small yard or garden. Like any yard, intensive green roofs require weeding, watering, fertilizing, feeding, trimming, and whatever else is necessary to keep your living roof in good shape.
Limited Plant Options
Due to limited soil depth, you won’t be able to plant just anything on top of your roof. Weight considerations must be taken into account, as your roof can sustain only so much plant life.
An extensive green roof limits the types of plants your roof can sustain, since extensive roofs accommodate only small plants with small root systems.
Leaks
Locating a leak in a green roof is like looking for a needle in a haystack. It is very difficult to locate a leak in a green roof. Luckily, it is possible to use electronic leak detection to locate your roof’s leaks.
If the green roof’s waterproof membrane is penetrated, it will cause leaks, which is going to be difficult and expensive to repair.
Best Green Roof Plants
Photo Credit: Maja Dumat / Flickr / CC BY 2.0
Hardy plants with shallow root systems are generally the best choice for green roofs, especially if it’s an extensive green roof. From bushes to wildflowers, homeowners can choose a variety of plants for their homes.
Sedums are one of the most common choices, as the succulents are tough plants that generally require low maintenance.
Herbs are a great choice too, since they are often aromatic or edible plants with medicinal benefits.
Here is a list of some plants that are a good choice for green roofs:
- Asters
- Birdsfoot trefoil
- Gold sedum
- Houseleeks
- Meadow saxifrage
- Oregano
- Sea thrift
- Two row stonecrop
- White stonecrop
- Widow’s cross
- Yarrow
Things to Consider Before a Green Roof
Before you get a green roof, you need to consider if green roofing is suitable for your home.
Important things to consider are:
- Slope. Green roofs are made for flat or low-sloping roofs. Steep roofs above a 45 degree angle aren’t a good fit for green roofs.
- Structural support. Since green roofs are so heavy, your house must have the structural support to withstand the immense weight of the soil and plants on a green roof.
- Green roof type. You will have to decide which green roofing type is the best fit for your home: intensive, extensive, or semi-intensive.
- Budget. Calculate if such an investment is within your budget. Green roofs are pricey investments.
- Geographic location. Green roofs perform better in rainy climates than in dry, arid areas. Local weather conditions determine what types of plants you can have on your roof.
- Shade. If your roof receives a lot of shade each day, calculate how that will affect vegetation growing on your roof.
FAQ About the Pros and Cons of Green Roofs
A green roof must be flat or have a gentle sloping substrate that isn’t too steep. Green roofs shouldn’t have a slope steeper than 45 degrees.
It is possible to have a green roof in most parts of the world. However, what plants are used for a green roof will be heavily impacted by the local climate.
Without care, plants may wither and die in hot, arid climates or areas that don’t receive much rain. The limited depth of soil makes it hard for plants to thrive in hot climates, so green roofs may not be the best choice for every area of the world.
A house with a green roof will still need a gutter drainage system. While a green roof usually absorbs the majority of rainwater, it will not absorb all of it, so it is important to have a gutter system in place to deal with excess runoff. This will protect your home’s foundation from damage.
Ready to Consult a Roofing Pro?
Green roofs are an eco-friendly choice that cools both exterior and interior environments. They offer relaxing, stress-reducing greenery and they provide recreational garden areas for you to hang out in.
If you have decided that green roofs are the perfect fit for your home, contact a professional roofer near you.
Main Image Credit: Arlington County / Flickr / CC BY-SA 2.0
