The term “green roof” covers a wide spectrum of building-topping concepts, so it’s impossible to make a blanket statement about cost-effectiveness. Residential applications will typically be smaller in scale compared to commercial settings. Their goals will almost certainly differ, and the types of benefits that will be built into the calculations will be different. Let’s talk about some of those elements that should be considered when evaluating your own project.
Even though the capital costs of green roofs are almost certainly higher than conventional roofs in most situations, it’s crucial to consider lifecycle costs when considering overall cost-effectiveness. The style of the green roof, the size, the goals for the roof structure should be thought through as well.
The ultimate goals for a green roof project should be considered, since the ability of different styles of roof to meet those goals should absolutely factor into the overall analysis. It’s also important to recognize that choices regarding the style of roof, the relative size, the quality of materials and contractor experience, as well as other variables will make any generic cost-benefit estimates extremely unreliable.
It’s true that green roofs are not the perfect solution for everyone, so we’ll talk about some of the practical considerations that will (or should) affect your own calculations of whether a green roof is “worth it”.
Capital Costs
Calculating capital costs is as far as some green roof projects get. Green roofs have a reputation of being extremely expensive, and perhaps being good for show rather than practicality. But that’s a one-dimensional and very short-sighted view.
In truth, the capital costs for green roofs vary widely. The size of the roof itself will affect the overall cost, given the economics of scale. Other factors like the location of the roof, ease of access, and type of roof installation will contribute to the costs. Don’t forget to consider whether any local or state incentives to install green roofs exist. This might include tax abatements, direct financial incentives, and reduction in stormwater fees.
One of the first choices to make when you’re pricing out a green roof is the type you need. A simple extensive roof, that will have no public access could be as little as 1/10 of the cost to construct a complex intensive green roof that will be used by the public.
An extensive roof will be relatively light since it has a shallower growth zone and is typically designed to be mostly self-sustaining compared to an intensive roof. In contrast, an intensive roof designed for public access will be substantially heavier but will be able to support a wider variety of plants, including shrubs and even trees. Keep in mind that even if an intensive roof is much more expensive, the resulting usable space may quickly turn the balance sheet around.
Other relatively straightforward choices will also affect the overall cost of your green roof, including important components like the choice of growing medium, the type and quality of roofing membrane, the drainage system, potential use of fencing or railings, and the type and quantity of plants.
Cost Factors to Consider
- Availability of labor force experienced in green roof installation
- Ease of roof access for installation and maintenance
- Choice of materials, including the type of waterproof membrane employed
- Need for additional structural support
- Types of available warranties
Cost Abatements and Incentives
In some instances, local policy initiatives may offer incentives that quickly turn a high capital investment into a positive ROI. Cities that offer these kinds of incentives are often doing so in order to reduce the cost of their own infrastructure repairs and upgrades. When enough green roofs are installed in an urban area, for example, city engineers can save on infrastructure related to stormwater management.
- Look for local or statewide policies that offer direct financial incentives to build or replace current roofs with green alternatives, such as tax credits, tax abatements, grants, rebates, subsidies, and reduced stormwater fees.
- Some communities, like Portland OR, offer a variety of financial, technical and educational incentives through its green roof initiative.
- Some communities offer favorable zoning or expedited permitting to encourage new construction with green roofs.
- In some cases, green roofs can be used to earn points for sustainable development plans.
