On a hot, sunny day, a traditional black roof can reach temperatures exceeding 150 degrees Fahrenheit (65 degrees Celsius).
This absorbed heat raises inside temperatures, which leads to higher air conditioning use, steeper energy costs, and general discomfort.
A simple solution to reduce heat is using a cool roof, which is relatively low-tech, low-cost, and increasingly easy to implement.
A cool roof is designed to maintain a lower surface temperature in bright sunshine, more than 50 degrees F (28 degrees C) cooler than a traditional roof. The surface of a cool roof reflects more sunlight and releases more heat than a dark roof. Cool roofs can save money, conserve energy and cool the planet.
Here are ten methods to consider when looking to cool your roof:
1. Modified Bitumen Roofs
One of the most common cool roof options for a flat or low-sloped roof is a modified bitumen roof. Modified bitumen roofing is a membrane made up of sheets of plasticized or rubberized asphalt held together with rolled reinforcing fabric and fastened to the roof deck with hot asphalt or cool adhesive.
Traditional modified bitumen roofs are black or gray with a low solar reflectance, meaning that they absorb, rather than reflect, most of the sun’s light.
However, applying a white coating over the surface of the material at the factory can raise the solar reflectance to acceptable cool roof standards.
2. Single-Ply Membrane
Another type of cool roofing material used for low-sloping roofs is a single-ply membrane. Single-ply membranes are prefabricated plastic or vinyl sheets containing solar reflective coatings or materials.
The sheets are rolled onto the roof deck, then attached to the structure with some combination of chemical adhesives, mechanical fasteners, or ballast, such as gravel or pavers.
Single-ply membranes don’t require any additional coatings or surfacing because the cool roof properties are integrated within the product itself.
The three most common types of single-ply membrane are polyvinyl chloride (PVC), ethylene propylene diene monomer (EPDM) and thermoplastic polyolefin (TPO).
3. Built-Up Roofing
Built-up roofing is a term for the standard tar-and-gravel roof. Because it’s inexpensive, easy to apply, and easy to repair, it’s among the most commonly used roofing systems for flat and low-sloped roofs.
Built-up roofing involves layering a base sheet of asphalt or bitumen with fabric, followed by a protective layer of gravel, mineral granules, or some sort of aggregate rock.
Traditionally, the asphalt and surface gravel in a built-up roof have been black or dark gray, trapping much more light and heat than they release.
There are several ways to make a built-up roof cooler:
- Replacing the surface layer with white gravel or reflective marble chips can increase its solar reflectance.
- Applying a mineral surfaced cap sheet, which incorporates reflective granules or a reflective coating into the roofing material itself.
- Special coatings or pigments can sometimes be added to an existing asphalt surface to convert a standard roof to a cool roof. This typically requires one coat of a reflective material and another of an emissive material that releases heat.
4. Cool Roof Coatings
Cool roof coatings are best applied to low-sloped roofs in good condition. They are divided into two categories: cementitious coatings, which contain concrete or ceramic particles, and elastomeric coatings, which contain added polymers that make them less brittle and more adhesive to building surfaces.
Both cementitious and elastomeric coatings improve the solar reflectance of the roof surface. Elastomeric coatings act as a watertight membrane, while cementitious coatings must be applied over roofing materials that have already been thoroughly waterproofed.
Cool roof coatings are sprayed, rolled, or brushed onto an existing flat or low-sloping rooftop to create a bright white, paint-like reflective surface. These coatings are more commonly used in re-roofing projects than in new construction.
5. Cool Asphalt Shingles
For the steep-sloped roofs found on most U.S. homes, standard asphalt shingles can be replaced with cool asphalt shingles containing specially coated reflective granules to help them better reflect sunlight.
Shingles made from wood, polymer, and metal can also be coated to improve their solar reflectance. Shingles are generally not as reflective as any of the cool roof options available for low-sloping roofs, but their familiar appearance and traditional style make them a popular choice for residential construction.
6. Metal Roofs
Unpainted metal roofs are naturally reflective (high solar reflectance), but they tend to hold heat in (low emissivity), so they are generally a better choice for steep roofs than for flat or low-sloping roofs, which receive more sunlight throughout the course of a day.
A factory-applied cool roof coating can improve the heat releasing properties of a metal roof, enough so that some coated metal roofs are able to qualify as cool roofs.
A standard unpainted metal roof can be a cost-effective solution for some steep-sloped applications, since it is already reflective and requires no additional technologies or materials to achieve cool roof status.
7. Tiles
While they aren’t as widely used as shingles, both traditional and “cool-colored” tiles can be good choices for cooling a steep-sloped roof.
Some traditional tiles have naturally reflective properties, depending on their color and the type of the clay they contain.
Cool-colored tiles are created with pigments that reflect solar energy in the infrared spectrum. Cool-colored tiles are typically produced in earth tones such as brown, green, and terracotta.
8. Roof Mist Cooling Systems
If you’d rather enjoy the benefits of a cool roof without the addition of new surface materials or coatings, consider a roof mist cooling system, also called an evaporative cooling system.
A roof misting system lowers surface temperatures by spraying an extremely small amount of water across the roof, allowing the water to cool the roof as it evaporates.
At its most basic, a roof misting system can be little more than a garden hose set to the mist setting and aimed across the roof.
More sophisticated roof mist cooling systems typically feature a misting pump with a built-in thermostat, timer, and 360-degree misting nozzles that are strategically placed so the water hits every part of the roof. These systems are available for both industrial and residential use.
9. Green Roofs
While a rooftop garden isn’t technically a cool roof, it achieves many of the same benefits as qualified cool roof products and reduces stormwater runoff.
A green roof, or rooftop garden, is a vegetative layer grown on a rooftop. Green roofs shade the buildings below from direct sunlight and reduce surface and surrounding air temperatures through evaporation and transpiration, the processes by which plants remove moisture from the soil and emit it through their leaves. Rooftop gardens are so effective, that they can lower a city’s ambient temperatures.
10. Ballasted Roofing Systems
Ballasted roofing systems are roofs in which a membrane is laid on the roof deck, then held in place by the weight of concrete pavers or natural stones.
These stones also provide an insulating barrier against solar heat, much like an adobe building in the desert or an old stone farmhouse that stays cool inside, thanks to its thick stone walls.
