How to Create a Gorgeous Green Roof: Your Step-by-Step Guide

Maximize space, save money, and do good for the environment with a green roof.

Green roofs
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Overview
  • Total Time: 1 day
  • Skill Level: Advanced
  • Estimated Cost: $13 to $45/sq. ft.

Green roofs are roofs that are covered entirely or in part with un-potted vegetation growing over waterproof layers of various materials. They are low-maintenance ways to maximize unused space, save money, and do good for the environment in more ways than one. Whether it's a private home, garage, apartment building, office complex, or a shed, you can take advantage of any flat or sloped roof by turning it into a garden.

There are two types of green roofs: extensive (soil layer of 6 inches or less) and intensive (soil layer of more than 6 inches). An extensive roof is better suited for a DIY project than an intensive one.

Benefits of a Green Roof

In case you're on the fence about getting a green roof, check out these unique advantages you may have not yet considered:

  • Your roof acts as insulation, keeping a house cooler in summer and warmer in winter, allowing you to save an estimated $5,000 per year on heating and cooling your home.
  • Green roofs create a sound barrier in high-traffic areas and can muffle noises from pool equipment or a pump house.
  • Your green roof will provide a habitat for wildlife and a food source for pollinators.
  • Green roofs help manage stormwater runoff, saving municipalities money. In turn, many of them offer “cool roof” rebates and low-interest loans.
  • Buildings account for 38% of global energy-related CO2 emissions, but since plants absorb carbon dioxide from the atmosphere, a green roof lowers a building's carbon footprint.
  • Green roofs have been shown to reduce the intensity of heat islands in urban settings.
  • A green roof may help your home qualify for LEED certification, which can your decrease homeowner's insurance, qualify your home for tax breaks, and increase its resale value.

Before Getting Started

Installing a green roof is a larger endeavor than just painting your roof green. Before you do anything else, you'll need to assess the suitability of your roof to support a garden.

Flat or Sloping?

Green roofs are easiest to install and maintain on a flat roof. They can be installed on low-sloping roofs with a rise of up to three feet for every 12 feet of length without the need for stabilization. Steeper slopes may require a reinforcing system such as a tray planting or terracing system to hold soil in place. It shouldn't take a degree in physics to figure out that a flat roof retains water better than a slanted one, but it is something you should consider in your planning.

Considerations About Weight

The ideal depth for the soil layer of a green roof without requiring special structural design is four to six inches. A green roof can add up to 30 pounds of load per square foot, so consult a structural engineer to see if you need to add reinforcements to your roof. Special consideration for extra bracing may be necessary in seismic zones depending on the height of the roof. In all cases, follow local building codes.

A green roof on top of an outdoor structure
Make sure that your roof can support a garden.

beekeepx/Getty Images

Treehugger Tip

Timing is everything. As you would with any garden, early spring or early fall is the best time to install a roof garden so that plants have time to establish themselves before the extremes of summer or winter.

Insulation

Insulation is only required when it is necessary to increase the R-value for the roof over conditioned spaces. If the space below the green roof is not conditioned—over a garden pavilion or shed—there’s no reason to provide anything more than some protection for the waterproof membrane, such as thin foam ‘fan board’ insulation or perhaps a layer of building felt.

Edging

For creating an edging, intermediate angle supports may be needed to keep the vertical edging sturdy. The horizontal leg of the supports can be slipped under the drainage mat and weighted with the topping soil to keep them from overturning. It is best to devise supports so they don’t penetrate the waterproof membrane surface to prevent leaks.

If using trays, only the membrane and 6-mil plastic sheet in the steps below are required. Be sure to create a roof that will accommodate the width and length of the trays without leaving gaps. The trays can be planted with the same plants as a roof without containers or purchased pre-grown commercially by a nursery. With a tray system, edging is not necessary.

Soil Mix

Homeowners can easily make a lightweight soil mix by blending an aggregate such as expanded shale, slate, or volcanic rock with organic potting soil. A blend of 85% aggregate to 15% potting soil has proven to be effective. Lightweight aggregate is available from landscaper shops in bags or by bulk, pre-blended, or by itself. The advantages of using an expanded shale-blended soil mixture are that it absorbs water, drains well, doesn’t compact, and is light. Commercial soil mixtures for green roofs are great if you can find an outlet, but find a mix with locally available materials to avoid unnecessary shipping costs.

Treehugger Tip

Another option for green roofs is to purchase vegetated mats. These are mats with succulents or other plants and are grown on the ground in the same manner that some nurseries grow sod.

Choosing Plants

Ideal plants to survive summer heat are those that can handle climate extremes and pests. Rooftop temperatures can reach 150 degrees F or higher. Fortunately, a wide range of plants do quite well in rooftop gardens. Those include many regional natives, herbaceous perennials, ground covers, succulents, sedums, herbs, and some edibles. Use only plants in these groups that are low-maintenance, shallow-rooted, and drought-resistant. Some grasses can be included but will require winter deadheading.

Plants that stay green year-round are good choices because the roof will remain green in winter when many plants are dormant. Check with local nurseries or other trusted sources for the suitability of plant material in your location.

What You'll Need

Tools

  • tape measure
  • shovel
  • garden rake or hoe
  • drill
  • garden trowel

Materials

  • bitumen membrane or pond liner
  • 6-millimeter sheet of plastic
  • plant trays (optional)
  • 3/4 inch foam insulation
  • soil
  • drainage mat
  • mesh gutter guards, wood, or other edging
  • plants
  • water

Instructions

Green roofs can be installed using a series of planting trays or by creating an edged rooftop landscape area. These instructions are for installing a landscape area on a flat surface using a protective, multi-layered waterproof barrier between the soil and the roof decking, which could be plywood, for example. The various layers, readily available from landscapers or box stores, will not only prevent water from soaking into and rotting the decking but will also prevent roots from reaching into the decking and weakening or rotting it.

With the decking in place, here are the steps to installing a green roof:

  1. Install Waterproof Membrane

    Install a monolithic type waterproof membrane (rubber or plastic) on top of the roof decking.

  2. Put Down Plastic Sheet

    Place a 6 millimeter sheet of plastic on the waterproof membrane (this will serve as a root barrier).

  3. Add Foam Installation

    Top the first two layers with one or more thin sheets of three-quarter-inch foam insulation suited for contact with damp soil.

  4. Set Drainage Mat

    Set a drainage mat (also called a dimple mat) with capillary spaces on top of the insulation. To keep the soil from clogging the mat, place the mat so the felt side faces up.

  5. Frame the Sides

    Frame the sides for the roof with mesh gutter guards, wood or other edging that will permit drainage to hold soil in place.

  6. Add Soil

    Add soil and distribute evenly throughout the garden space.

  7. Place Plants

    Set plants in place. Place taller plants in back to allow sun to reach shorter ones in front.

  8. Water

    Water to settle soil around plants.

Frequently Asked Questions
  • Should you anticipate any problems with a green roof?

    Drainage issues are the most common type of green roof "failure." If your roof is holding too much water, it could drown out plants or, at worst, cause structural issues with your house. To avoid this, drains need to be maintained so the vegetation doesn't choke them out.

  • What should you plant on your green roof?

    Common plants for green roofs include perennial succulents like stonecrop and hens and chicks or sedges and groundcovers.

  • How much maintenance does a green roof require?

    Green roofs typically need to be weeded two or three times a year to avoid overgrown vegetation clogging up drains. It may also need to be watered throughout dry periods.

  • Are green roofs feasible for cold climates?

    Plants on green roofs undergo the same cycles as plants growing on the ground. Your vegetated roof may not look vibrant through harsh winter months, but if you're growing perennials that can withstand that climate, they should return come spring.

View Article Sources
  1. "Green Roof Benefits." National Park Service.

  2. "2020 Global Status Report for Buildings and Construction." United Nations Environment Programme, 2020.

  3. "Using Green Roofs to Reduce Heat Islands." Environmental Protection Agency.