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How much do greenroofs weigh?
The average extensive greenroof with 3-4 inches of soil weighs about 20 pounds per square foot when saturated. This is slightly more weight than gravel ballast. Intensive or mixed-use greenroofs are custom installations and weights vary greatly.

What is the process for getting a greenroof?
Step 1 - Determine if your rooftop can handle the load. You will need an architect to certify that your roof can hold 20 pounds per square foot in addition to snow loading in order to get your building permit. Please see Are You Roof Ready? to help determine if your roof can handle the weight of a greenroof. If you want to greenroof a new building, you probably already have an architect on hand to do this assessment. If you want to retrofit an existing building, you will need to hire an architect to certify the structural loading. This can be expensive to do. D.C. Greenworks can refer you if you don't know an architect.

Step 1A - Design your greenroof and determine specifications. If you are working with an architect, he or she may draw plans or write specifications for your greenroof system. If not, D.C. Greenworks will provide this service for you as part of contract. We will work with you to design and build the greenroof that fits your needs and your site. If your building can support the weight, we can also build intensive rooftop landscapes, patios, pergolas and the like into your greenroof design.

Step 2 - Waterproof the building with the right membrane for the greenroof and the site. D.C. Greenworks works with local waterproofing companies to make sure your building is watertight and ready for your greenroof. If you don't have a waterproofer, D.C. Greenworks can refer you, but you should always shop for the best price. You can also install a leak detection system at this point. For waterproofing specifications, please download here. Remember that waterproofing your building is a separate cost from building your greenroof. This makes it most cost-effective to greenroof when you are already replacing your waterproofing.

Step 3 - Install the greenroof. Once the waterproofing is sound, D.C. Greenworks will install all of the greenroof components, and plant the roof according to your preferences and the horticultural needs of your site. We can also install patios, pergolas and other features into your greenroof at this time.

How much maintenance is involved?
In the first year, your greenroof plants will be small, and there will be a lot of bare soil that might grow weeds. If your greenroof is beneath trees, this is especially true. During the first year, your greenroof will need weeding at least twice, and may lose up to 10% of its plants. D.C. Greenworks provides two visits in the first year as part of your contract where we weed and replace plants, but if you wish to weed more often (or want us to!), it won't hurt a bit.

Once the plants have grown in, and there is no bare soil visible, the greenroof will be virtually self-maintaining. Yearly fertilization with pelletized compost or slow-release fertilizer carefully spread around the plants should be all that your greenroof needs.

How much do greenroofs cost?
Greenroofs have an economy of scale. The bigger the greenroof, the cheaper the square foot cost. This is because the materials and soil used in greenroofing are specially made just for greenroofing, and are cheaper to produce and deliver in bulk quantities. Another major cost factor is labor. If your rooftop is inaccessible for heavy equipment and vehicles, we may have to build your greenroof by hand, which drives up cost greatly.

In the residential range, extensive greenroofs can vary from $12-35 per square foot, but if you have a tiny greenroof to build, the total cost of the project can be relatively low. In the commercial range, extensive greenroofs can vary from $5-15, depending on the size and the difficulty of construction. Mixed-use and intensive greenroofs are custom installations, and cost varies greatly. Please see Are You Roof Ready? to determine if greenroofing is right for you.

Are there any tax incentives, grants or rebates for greenroofing?
At this time, there are no tax incentives for developers or homeowners. Though this may change in the near future. If you are a developer, there are a couple of subsidy options open to you in Washington, D.C. The D.C. Department of the Environment has some grant funds available to help subsidize greenroofs on large-scale public-use developments. However, these are competitive and not available to all.

However, developers who can show that Low-Impact Development strategies such as greenroofing and bioretention can take up the first half inch of storm water on the site can apply to use the LID in lieu of conventional stormwater management practices. Because greenroofs and bioretention usually cost significantly less than sand filters and other conventional management techniques, developers can save a lot of money on a project by thoughtfully incorporating a greenroof into their building design.