Here’s an eco-friendly project that can be built with readily available materials. As a bonus, it reduces home energy costs and stormwater runoff.
By Tom Oder
ROOFTOP GARDENING: A green roof of sedums, phlox, herbs, wild strawberries and other plants reduces stormwater runoff and energy costs for this renovated Atlanta home. (Photo: Tom Oder)
Is the flat roof beside your elevated porch an eyesore and costly heat island? Does the sight of pumps and filters ruin the view of your pool and garden from your second-story window? Is stormwater running off your roof causing problems? Installing a green roof is a do-it-yourself project that will enhance the view, reduce home energy costs and provide many eco-friendly benefits. Here’s a guide on how to do it:
What is a green roof?
Green roofs are roofs that are covered entirely or in part with vegetation growing over waterproof layers of various materials. A roof with plants growing in containers is not a green roof. There are two types of green roofs: extensive (soil layer of six inches or less) and intensive (soil layer of more than six inches).
Choosing a location
An ideal location for a green roof is below a higher roof, with sightlines from the house and access from a porch or window for maintenance. Strategically placing the green roof below the higher roof will allow it to slow and filter rainfall runoff from the higher roof as well as from rain that falls on the green roof. Catching the runoff can be accomplished by hanging a “rain chain” from a gutter down to the green roof. Rain chains are available in a variety of designs and can be purchased from online sites and some garden centers. Other placement considerations are to site a green roof where it will hide systems equipment such as pool pumps and filters.
What about weight?
The ideal depth for the soil layer of a green roof without requiring special structural design is four to six inches. When fully saturated, four inches of a blended soil using the expanded shale formula below weighs about 27 pounds per square foot, according to Meg Needle, a commercial architect with Lord, Aeck & Sargent Architecture in Atlanta. A gut check for sturdiness, she said, is to walk on the roof. There should be no bounce or wobble, she advised. If the roof structure is in doubt, consult someone with trusted structural expertise to suggest options for reinforcement. 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.
Flat or sloping roof?
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.
How to install a green roof
Green roofs can be installed using a series of planting trays or by creating an edged rooftop landscape area.
Here are the steps to install 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:
Install a monolithic type waterproof membrane (rubber or plastic) on top of the roof decking.
Place a 6 milimeter sheet of plastic on the waterproof membrane (this will serve as a root barrier).
Top the first two layers with one or more thin sheets of three-quarter-inch foam insulation suited for contact with damp soil. (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.)
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.
Frame the sides for the roof with mesh gutter guards, wood or other edging that will permit drainage to hold soil in place. 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.
Set plants in place.
Water to settle soil around plants.
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.
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. In areas of the country where summers are hot and dry, fall installation of matted plants is recommended because plants acclimate and establish best when the climate is mild, said Robin Andrews of the Greenbuild division of Saul Nurseries and ItSaul Plants in Alpharetta, Ga. Only a thin layer of soil is required with a mat system since most of the soil comes with the mat.
Use a lightweight soil
Homeowners can easily make a lightweight soil mix by blending an aggregate such as expanded shale, slate or volcanic rock with an organic potting soil. A blend of 85 percent aggregate to 15 percent potting soil has proven to be effective. Lightweight aggregate is available from landscaper shops in bags or by bulk; pre-blended or by itself. If you don’t have a truck or access to one, it may be easiest to have the aggregate delivered. 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.
Remember, rooftop temperatures can reach 150 degrees F or higher. Ideal plants to survive summer heat are those that are “bulletproof” against climate extremes, pests and diseases. 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.
Other than creating a pleasing sight and a conversation piece, green roofs placed over living spaces make home insulation more efficient because they reduce heating and cooling costs. They also reduce stormwater runoff, create a sound barrier in high-traffic areas and provide a habitat for wildlife. Installing a green roof over pool equipment or a pump house buffers the noise the equipment makes.
Plant list for an extensive green roof
An extensive green roof has a soil depth of three to six inches. This list features a selection of plants that are available in many parts of the country. It is not meant to be a complete list. Check with local nurseries or other trusted sources for suitability of plant material in your location.
Allium schoenoprasum *
Allium senescens glaucum
Delosperma basuticum ‘Gold Nugget’
Delosperma cooperi *
Delosperma keladis ‘Mesa Verde’
Delosperma nubigenum *
Phedimus takesimensis ‘Golden Carpet’ syn. Sedum takesimense *
Sedum acre ‘Aurem’
Sedum aizoon ‘Sweet & Sour’
Sedum album ‘Coral Carpet’
Sedum album ‘France’ *
Sedum album ‘Green Ice’ syn. Sedum album micranthum var. micranthum
Sedum album ‘Jelly Bean’ *
Sedum album ’Murale’ *
Sedum cauticola ‘Bertram Anderson’
Sedum emarginatum ‘Eco-Mt.Emei’
Sedum kamtschaticum *
Sedum kamtschaticum ellacombianum *
Sedum kamtschaticum ellacombianum ‘Akebono’
Sedum kamtschaticum var. floriferium ‘Weihenstephaner Gold’ *
Sedum makinoi ‘Limelight’
Sedum mexicanum ‘Lemon Ball’
Sedum reflexum ‘Blue Spruce’ *
Sedum rupestre ‘Angelina’ *
Sedum sexangulare *
Sedum sieboldii ‘October Daphne’
Sedum spurium ‘Album Superbum’ syn. ‘White Form’
Sedum spurium ‘Fuldaglut’ *
Sedum spurium ‘John Creech’ *
Sedum spurium ‘Voodoo’
Sedum ternatum ‘Chestatee
Talinum calycinum *
Achillea, assorted cultivars **
Agastache, assorted cultivars **
Aster, assorted species/cultivars *
Coreopsis, assorted species/cultivars **
Dianthus deltoides ‘Brilliant’ **
Echinacea, assorted cultivars **
Gaillardia grandiflora, assorted cultivars **
Heuchera villosa, assorted. cultivars **
Liatris, assorted species/cultivars **
Monarda, assorted species/cultivars **
Penstemon digitalis **
Phlox, assorted species/cultivars **
Rudbeckia, assorted species **
Sysrinchium angustifolium, assorted cultivars
Thymus, assorted species
Verbena tennuisecta **
Bouteloua, assorted species **
Carex, assorted species **
Schizachyrium scoparium ‘The Blues’ **
Sporobolus heterolepis **
Stipa tennuisima **
* These plants are considered “bulletproof,” meaning they likely will survive climate extremes anywhere in the United States.
** These plants require a minimum soil depth of six inches.
Source for plant list: Robin Andrews, Greenbuild division of Saul Nurseries and ItSaul Plants in Alpharetta, Ga.
Have other tips for how to build a green roof? Leave us a note in the comments below.