Green Roof Pros:
- The plants on a green roof capture the heat and sunlight which cools the roof significantly. A green roof can be cooler than surrounding air temperatures in the summer, whereas a conventional roof is generally hotter. When a roof doesn’t radiate heat in the air, rooftop HVAC units pull in cooler air (closer to the air temperature around the building) and less energy is needed to cool it. There are also savings due to added insulation value for the interior (less cool air is escaping from inside). How much it will save in energy cost is highly dependent on square footage.
- The waterproofing membrane is protected from UV rays by several layers of material, which may result in a longer life for the roof membrane.
- Storm water runoff is more limited (60% run off is “captured”, used by the plants, or has delayed drainage as it filters through the growth media), which means potentially less flooding around your building and parking lot and lower sewer impact fees.
- The extra thickness of the growth media and plants also acts as a sound barrier in urban environments.
- If open to the public, it creates a unique feature which may draw more people to your building.
- The national average payback period for a green roof is only 6.4 years through storm water retention, energy savings, etc. This becomes more advantageous as more regulatory bodies add fees to storm water runoff.
- Any leaks are more likely to be found during construction because there is extensive flood and leak testing on the material before the growth media and other components are added on top of the water barrier.
- A green roof usually has a longer warranty period (20-30 years depending on manufacturer for a green roof vs 10 years SBS).
Green Roof Cons:
- Usually outside of the scope for renovations – structural capabilities to hold the added material on the roof would have to already be in place
- The initial cost is more than a traditional roof – about $13.40/sf more on average nationally in 2014 for extensive (which is a thinner roof with smaller plants like sedums and succulents – see photo) and $19.70 over the cost of a traditional roof for intensive (which is a thicker roof that can support larger plants like shrubs). However, installation costs per square foot decrease as the roof size increases.
- Ongoing maintenance of plants is needed, though low maintenance plants are used (typically $0.75-$1.50/sf per year). Much of the maintenance is in the first few years as the plants establish themselves and create a ground cover on the growth media.
- Weeds - especially in the establishment period. If weeding isn't completed on a regular basis during the establishment period, they can easily take over the roof and choke out the plants you want. While the necessity for weeding will never completely go away, it will become less needed as sedums and other plants take root and have good coverage (about 2 years after planting).
- Pests! If you're building a biome on your roof, you might get pests. One of our projects got ants a year later even though the roof was 45 feet above ground level, but some other side effects can include aggressive birds nesting, beetles, or other pests which would then need to be controlled much like you'd control pests on your lawn.
- A large amount of the roof has to be ballasted (with pavers or gravel around the perimeter) in wind prone areas, which means a small roof cannot house a large planting area.
For Part 2 of the Green Roof series, Green Roof 101: Components, click here.