Green roofs offer a way to improve energy efficiency, manage stormwater, and add visual interest to a home, but they require careful planning and specialized installation. Unlike traditional roofing, these systems add significant weight and depend on layered components that must work together to support plant life and protect your home’s structure below.
This project overview shows how a modular green roof system is installed on a small residential addition, along with the key structural, drainage, and installation details that make it work.
Project Overview
This green roof was installed on a 140-square-foot flat-roof addition to a 1925 Colonial Revival home in Teaneck, New Jersey. Homeowners Sarah Jack and Scott Harris chose a modular system planted with low-growing sedums to help manage rainwater runoff and improve energy efficiency.
The LiveRoof system used here consists of plastic trays filled with a lightweight, soilless growing medium and pre-grown vegetation designed to withstand both heavy rain and drought conditions. Each tray arrives nearly fully covered, allowing the roof to look established soon after installation.
When saturated, the system adds significant weight to the roof, which is why it must be installed on a structure designed to handle the additional load. Installed by R&S Landscaping, the system helps regulate temperature inside the home, keeping the space below warmer in winter and up to 6 to 8 degrees cooler in summer, while also reducing stormwater runoff and filtering pollutants.
In addition to its performance benefits, the planted roof adds visual appeal. “The roof looks so lively and colorful,” says Sarah. “It’s definitely improved our view.”
Shown: Hardy sedums are most popular for green roofs because they thrive in a soilless growing medium and are drought tolerant. They just need a yearly trim and occasional fertilizer to look their best. Homeowners can choose from a palette of suitable plants. Sedum spurium ‘Tricolor’ is one of the most ornamental.
Is a Green Roof Right for Your Home?
Green roofs can offer energy savings, stormwater management, and added visual appeal, but they aren’t suitable for every home. The added weight often requires structural reinforcement, which can limit where these systems can be installed.
They are most practical on flat or low-slope roofs with easy access, such as additions or garages, where the structure can be designed or modified to handle the load.
Before You Start
Green roofs require more planning and expertise than standard roofing projects. As This Old House Magazine notes, these systems often require contractors with specialized experience, and some must get installed by manufacturer-certified professionals. In many cases, that means working with a commercial roofer rather than a typical residential contractor.
This requirement can affect both the cost and feasibility of adding a green roof, especially on an existing structure.
How a Green Roof Is Installed
Installing a green roof involves more than simply placing plants on a building. The system relies on a layered approach that includes structural support, waterproofing, drainage, and a growing medium designed to sustain vegetation.
Because of the added weight and complexity, green roofs are typically installed using specialized systems designed to manage water, protect the structure, and support plant life.
The steps below show how a modular system like this one is assembled, from preparing the roof to placing and securing the planted trays.
Step 1
Prep the Roof
Even a flat roof should have some pitch so that it sheds water toward gutters. For a green roof installation, that pitch should be at least ¼ inch per foot of run to prevent standing water.
Before installation begins, a structural engineer should confirm that the roof can support the added load of the growing medium and plants. This Old House general contractor Tom Silva says, “Get a structural engineer to tell you what your existing roof needs to support the weight of a green roof. You might have to add more joists, strengthen existing ones, or add bracing.”
This added load is one of the biggest differences between a green roof and a standard roofing system, and it often determines whether an existing roof can be retrofitted.
Here, green-roof installer Rob Gaffney adds a second layer of rubber roofing membrane as a root barrier to prevent roots from damaging the roof.
Step 2
Add Planted Trays
The 1-by-2-foot black polypropylene trays, planted four months earlier and nurtured at a nursery, arrive by truck 95 percent covered with vegetation. They come stacked in racks wrapped in plastic to secure them for shipping. After the trays are unloaded, the plastic is removed to keep the plants from overheating, and the trays are moved by forklift, in this case, or crane to the rooftop.
Before installation begins, decorative L-shaped aluminum edging is temporarily weighted down around the roof’s perimeter, where it will cover the sides of the trays. Starting in a corner and working right to left, installers place the 40-pound trays in rows on the roof and inside the edging. They begin at the lowest end of the roof, then work across and up the slope.
Step 3
Connect the Trays
With a modular system like the one used here, any tray can be removed for maintenance without disturbing the others. In this system, plastic lips on the front and right sides overlap the left and back sides of neighboring trays for stability, creating a continuous surface that eliminates seams and helps reduce temperature fluctuations.
Feet built into the underside of the trays keep them elevated ½ inch above the roof’s surface, allowing runoff to flow underneath toward the gutters.
Step 4
Cut Trays to Fit
Trays often need to be cut to fit between the last full tray in a row and the edging. An installer measures each end piece, then transfers the mark to a full tray with a wax pencil. Because the trays are filled with a growing medium made of tough aggregate, a gas-powered concrete saw fitted with a masonry blade is used to make the cuts.
The cut end butts against the last full tray in the row. On this job, one installer laid the rows of trays, another took measurements for the end pieces, and a third made cuts on the ground before sending several rows’ worth of end pieces up to the roof.
Step 5
Remove Soil Elevators
Each tray arrives fitted with a 4-inch-wide band of white plastic, called a soil elevator, that allows the growing medium to rise 1 inch above the tray. Along with the curves built into the sides of each tray, this added height allows soil-to-soil contact so water, nutrients, and beneficial organisms can move laterally among the trays.
Sharing available water helps reduce the chance of dry or overly saturated spots. After a couple of rows are in place, the installer pulls out the soil elevator at an angle to avoid removing the growing medium. This helps the roof blend together almost instantly and reduces air space between the trays, increasing the system’s insulating R-value.
Step 6
Fit the Last Modules
While the planted sedums can withstand some foot traffic, installers follow a pattern that allows them to work their way off the roof without stepping excessively on the plants. From a ladder, they then secure the edging to the sides of the modules with exterior-grade screws.
Step 7
Water Well
Once the trays are installed, they are watered to help settle the soil. Green roof plants like these sedums typically don’t require irrigation unless there is a four-week period of high heat with less than 1 inch of rainfall.
During dry spells, the growing medium can shrink, leaving it vulnerable to weeds. Sarah and Scott can irrigate this roof with a self-coiling garden hose threaded into the second-floor bathroom sink faucet. A yearly trim and occasional fertilizer, as indicated by a soil test, are all that’s needed to keep the roof in good condition.
