Green Roofs: A Guide to the Options

Environmentally friendly roof choices can save you money on heating and cooling costs and benefit your entire neighborhood.

A garden roof is covered with plants growing in a thin layer of soil. Image: LiveRoof/FloraSource Ltd.

Replacing the roofing of an average 2,450-square-foot, two-story house has a national median cost of $7,600, according to the “2015 Remodeling Impact Report” from the NATIONAL ASSOCIATION OF REALTORS®. When you sell, you can expect a handsome 105% return on your investment.

Plus, you’ll be glad you did. Homeowners polled for the “Report” gave their roofing replacement a Joy Score of 9.7 — a rating based on those who said they were happy or satisfied with their remodeling, with 10 being the highest rating and 1 the lowest.

But you can get back even more by incorporating features that make your home more environmentally sensitive, particularly if they are also energy upgrades. Appraisal Journal has estimated that every $100 cut from your annual energy bill adds $2,000 to your home’s value.

Here’s a guide to some of the energy-saving features you might want to consider. Whether they make sense for you depends on the climate where you live, the style of your roof, and how much you’re willing to invest in a green upgrade.

Lower Energy Costs with a Cool Roof

On a sunny summer day, a typical roof can get 100 degrees hotter than the outside air. But a cool roof — one that reflects sunlight and doesn’t hold onto heat — will stay just a little above air temperature. That can lower your air conditioning costs by up to 15%, or an average of $56 a year, according to the U.S. Department of Energy. It also keeps your neighborhood cooler, allowing others to save, too.

If you have a sloped roof, your options include metal roofing or composition shingles that meet Energy Star standards. These come in many colors and, while they may cost a little more than conventional versions, can lower roof surface temperatures by as much as 50 degrees.

If you’re replacing a flat roof, you can cool it off simply by covering the worn-out material with a reflective plastic membrane made of TPO or PVC.

Note that cool roofs don’t make sense everywhere. If heating costs dominate where you live, a reflective roof might actually increase your overall energy costs, because you’ll lose the sun’s warming effect in winter.

Bring in Natural Light Through the Roof

If you keep lights on all the time to brighten a dark room or hallway, consider adding a tubular skylight, which is simpler and less costly to install than a conventional roof window. With one of these, light comes in through a clear dome on the roof, bounces down through a reflective tube in the attic, and comes out through a diffuser that looks like a ceiling light fixture in the room below.

A light tube costs $150 to $400, but you’ll make some of that back in energy savings. Tests performed at the Alberta Research Council found that a 13-inch tubular skylight had the same light output as a 700-watt incandescent bulb, even in December. At standard electric rates, that’s the equivalent of $350 worth of light a year. Your savings will be more modest, but if the skylight lets you switch off just two 100-watt incandescents for six hours a day, you’ll pocket around $36 a year.

Generate Power or Make Hot Water on the Roof

Solar power and hot-water systems are pricey upgrades: A typical solar water-heating system costs $4,000 to $6,000, while a solar electric system might cost $25,000 to $50,000. But generous tax incentives and rebates make them more feasible than ever. If you live in Oregon, for example, cash incentives and state and federal energy tax credits can offset up to 80% of the cost of solar electric equipment.

The best time to add one of these systems is when you’re installing a new roof, since that’s where the solar collectors or photovoltaic panels are usually placed. If you don’t like the look of solar panels, consider using less obtrusive power-generating shingles.

Plant a Garden on the Roof

A green roof, also called a garden roof, is one covered with plants growing in a thin layer of soil over a waterproofing layer. Popular in Europe, such roofs are still a boutique choice here, especially on houses.

Planting your roof is an expensive proposition, but it has many benefits. Green roofs insulate well, lower air-conditioning costs, help prevent floods by reducing storm runoff, and last a long time. New modular systems make installation relatively easy. Plastic grids fit together, keeping soil and plants in place even on sloped roofs. The materials cost about $10 a square foot, or $15 to $30 a square foot if you order them pre-loaded with soil and plants.

Weight is an issue, so you might also need to invest in strengthening your roof structure. The lightest systems, with just enough soil for low-growing sedum, a kind of succulent, start around 11 pounds a square foot when saturated with rain, compared with 2 1/2 to 3 pounds for composition shingles and 8 pounds for slate.