Green, blue, and cool roofs solve three different problems. A cool roof reflects sunlight so the roof and the space under it stay cooler. A blue roof holds rainwater on the roof during a storm and lets it drain out slowly, which keeps storm sewers from being overwhelmed. A green roof grows plants in an engineered growing layer that soaks up rain and shields the membrane.
Choosing usually comes down to two questions. Does your site have a stormwater requirement, and can the structure carry the weight of water or growing media? If the answer to both is no and you don’t need planted space on the roof, a cool roof is usually the practical choice, because it adds almost no weight and can go on when the roof is due anyway.
GSM Roofing has been a family-owned commercial roofing contractor in Ephrata, PA, since 1946, and we’ve built all three types. Our own headquarters has a 7,000-square-foot green roof, and the projects below show what each type took.
What Makes a Commercial Roofing System Sustainable?
On a commercial building, most of a roof’s environmental value comes from how long it lasts. A membrane that’s installed and maintained well keeps the materials already on the building in service for more years, and restoring a sound roof keeps the old membrane and insulation out of a landfill. After that come enough insulation, a reflective or vegetated surface where it suits the building, and drainage that meets local stormwater rules. Cool, blue, and green roofs each cover part of that list, but none of them does much good over wet insulation or failing seams, so we look at the roof’s condition before we talk about color or plants.
How Do Green, Blue, and Cool Roofs Compare?
Cool roofs lower roof temperature and cooling load with almost no added weight. Blue roofs detain stormwater and add about 5.2 pounds per square foot for every inch of water they hold. Green roofs absorb rainfall and protect the membrane but add the most weight and upkeep.
| Factor | Cool roof | Blue roof | Green roof |
|---|---|---|---|
| Main job | Reflects sunlight to lower roof temperature and cooling load | Detains rainwater and releases it slowly | Absorbs rainfall, protects the membrane, adds green space |
| How it works | Reflective membrane, coating, or surfacing | Flow-control drains hold water on a waterproofed roof | Waterproofing, root barrier, drainage, growing media, and plants |
| Stormwater effect | None | Detains and meters runoff | Absorbs and slows runoff |
| Cooling effect | Lowers roof surface temperature | Minimal | Shade and evaporation lower membrane and roof temperatures |
| Added weight | Little to none | About 5.2 lb per sq ft per inch of water, plus the system | 10 to 35 lb per sq ft saturated (extensive); 35 to 300 (intensive) |
| Best fit | Sunny roofs over air-conditioned space | Sites with stormwater detention requirements | Sites that need stormwater management, planted space, or green building credits |
| Upkeep | Periodic cleaning to help keep it reflective | Keep drains and flow controls clear; check after storms and freezes | Plant care, weeding, and drain checks, most of all in the first two growing seasons |
| On an existing roof? | Often, with a coating if the roof is dry and sound | Needs structural review and drainage changes | Needs structural review; usually done at replacement |
What Is a Cool Roof System?
A cool roof system is a membrane, coating, or surfacing with high solar reflectance and high thermal emittance, so it reflects more sunlight and sheds the heat it does absorb. The U.S. Department of Energy measures both properties on a scale of 0 to 1, and the higher the numbers, the cooler the roof. The Solar Reflectance Index (SRI) combines the two into one number, and it’s the value the LEED heat island credit uses.
Cool Roofing Materials for Low-Slope Commercial Roofs
On a flat or low-slope commercial roof, a cool roof usually takes one of four forms:
- A white or light-colored single-ply membrane such as TPO or PVC
- A reflective roof coating applied over an existing membrane or metal roof
- A light-colored fluid-applied membrane
- A modified bitumen or built-up roof surfaced with reflective granules or light-colored aggregate
Benefits of a Cool Roof System
The roof surface runs cooler, so less heat comes down into the space below. In an air-conditioned building that means a lighter cooling load and lower peak electricity demand on hot afternoons, and in spaces without air conditioning, like warehouses and production floors, it means a more tolerable summer. A cooler membrane also goes through smaller daily temperature swings, which can slow its aging. Across a city, reflective roofs help keep summer air temperatures down.
How to Compare Cool Roof Products
Compare products in the Cool Roof Rating Council’s Rated Products Directory using their three-year aged solar reflectance, thermal emittance, and SRI, not just the initial values. The directory became the main independent reference after ENERGY STAR stopped certifying roof products in June 2022, and it lists every rated product both new and after three years of weathering. Dirt, pollution, and weathering lower reflectance over time, and periodic cleaning, done the way the manufacturer specifies, helps recover some of that loss.
Do Cool Roofs Make Sense in a Mid-Atlantic Winter?
For air-conditioned buildings in Pennsylvania, New Jersey, Delaware, and Maryland, a cool roof’s summer savings usually outweigh its winter heating penalty. The U.S. EPA notes that the penalty shrinks because the winter sun is low and the days are short, and roofs that hold snow lose even less. Good insulation dulls the effect of surface color in both seasons. So a cool roof earns its keep on offices, retail stores, and outpatient healthcare buildings that run air conditioning all summer. A school that sits mostly empty in July, or a heated warehouse with little cooling, gets less from it.
In the colder parts of our region, a white membrane also runs colder in winter, so the roof’s air and vapor control deserve a closer look during design. It’s a design check, and it doesn’t rule out a cool roof. To estimate savings for a specific building, Oak Ridge National Laboratory’s Cool Roof Calculator takes your city, insulation level, and roof reflectance, and our guide to white vs. black commercial roofs walks through the tradeoff in more detail.
When Is the Best Time to Add a Cool Roof?
The best time is during a commercial roof replacement. The DOE notes that tearing off a sound roof just to make it more reflective is rarely economical, but when the roof is coming off anyway, choosing a reflective membrane is a small decision inside a project you’re already paying for. Replacement is also when the insulation gets settled. Under the IECC, which the energy codes in all four of our states are based on, new insulation on a replaced roof generally has to meet current requirements, with limited exceptions where parapets, curbs, or door thresholds make the added thickness impractical.
TE Connectivity’s Building 108 in Landisville, PA, is a good example. The old roof was ballasted EPDM over 2.5 inches of EPS insulation, about R-10. With the ballast and insulation coming off anyway, we installed 5.2 inches of polyiso, about R-30, under a new 60-mil white TPO membrane, so the plant got a reflective roof and three times the rated R-value in a single tear-off. Doing it later would have meant removing a brand-new membrane to get at the insulation. The new system carries a 20-year warranty.
Are Roof Coatings Eco-Friendly?
Roof coatings are often an eco-friendly roofing option because they restore a sound roof in place, keeping the old membrane and insulation out of the landfill, and a white coating turns a dark roof into a cool roof in the same project. When the coating wears, it can usually be cleaned and recoated instead of torn off. Products vary in odor and VOC content, which matters on occupied buildings, so ask for the product data before you choose one.
The mistake we’d warn owners about is coating a roof before anyone has checked what’s under the membrane. A coating seals in whatever moisture is already there, so we start with a moisture survey using infrared or impedance scanning, confirmed with core cuts, through our commercial roof inspections. Wet areas get cut out and replaced, the coating chemistry has to suit the roof, especially where water ponds, and if the survey shows widespread membrane failure, we’ll tell you the roof needs replacing instead.
At the Roxana Cannon Arsht Surgicenter in Wilmington, Delaware, part of the Christiana Care Health System, the crew first scanned the aging EPDM for wet areas and made repairs before applying a white Tremco AlphaGuard coating. A tear-off is hard to schedule around surgeries, and because this coating is odorless, we could work during regular business hours without disrupting the healthcare facility. Power washing was done after hours so its vibration wouldn’t disturb surgeries, and the two-part coating had about a 20-minute working window, so the crew mixed and applied it in small sections. Restoring the 20-year-old EPDM instead of tearing it off kept tons of debris out of the landfill and turned a dark roof white, and the restored roof carries a 30-year warranty.
What Is a Blue Roof?
A blue roof, also called a controlled-flow roof drainage system, stores rainwater on the roof during a storm and releases it slowly through flow-control drains. It won’t lower your energy use. What it does is hold back the surge of runoff that would otherwise hit storm sewers and combined sewer systems all at once, which reduces peak runoff and helps prevent street flooding and sewer overflows into rivers and streams. That’s why blue roofs show up most in cities with stormwater detention requirements.
A stormwater blue roof isn’t the same thing as the temporary blue tarps the U.S. Army Corps of Engineers installs on storm-damaged homes through Operation Blue Roof. It’s a permanent, engineered roof assembly.
Holding water on a roof on purpose changes how the roof has to be designed:
- Structure. Every inch of standing water adds about 5.2 pounds per square foot, plus the weight of the system itself. A structural engineer has to confirm the roof can carry the design water depth and the extra water that builds up if the drains clog and water rises to the overflow level. The Philadelphia Water Department’s stormwater manual, for example, caps ponding at 4 to 6 inches, depending on what the roof can carry, and requires stored water to drain within 72 hours after the design storm.
- Waterproofing. The membrane sits under water for hours or days, so best practice is a welded-seam thermoplastic membrane or a monolithic fluid-applied membrane, tested for leaks before anything is installed on top of it.
- Overflow protection. Secondary drains or scuppers still have to protect the roof when a storm exceeds the design.
- Penetrations and equipment. Pipes and curbs should be flashed at least 6 inches above the peak ponding level, and rooftop equipment should sit on curbs.
- Winter. In the Mid-Atlantic, ice can form at flow-control drains and overflows, so they need checking after freezes.
On the American Hydrotech blue/garden roof in Lancaster, PA, the original design put an expansion joint inside the water storage area, where it would have sat under stored water. We redesigned that detail and replaced it with a wall. The roof covers about 1,008 square feet and, at the time, was only the third of its kind in Pennsylvania and the first completed on a historic building. Before any blue or garden components went down, the membrane was checked with electronic field vector mapping (EFVM) leak detection and a flood test. Lightweight HydroDrain Max and needled rock wool took the place of heavier traditional garden roof components to keep the load down, the system carries a 20-year manufacturer’s warranty, and we fabricated a 22-gauge stainless steel vegetation-free zone around the American Hydrotech inspection chambers so the drains stay accessible.
How Is a Green Roof Built, and How Much Does It Weigh?
A green roof grows plants on top of a waterproofed roof, built in layers: the waterproofing membrane, a protection course and root barrier, often insulation, a drainage layer, filter fabric, engineered growing media (not ordinary soil), and the plants, which on commercial roofs are usually hardy sedums. The plants and growing media absorb rainfall and slow runoff, and they shield the membrane from sun and temperature swings. For the full list of advantages, see our post on the benefits of a green roof.
Green roofs come in two main types, and the difference matters for your structure. According to the Minnesota Pollution Control Agency’s stormwater manual:
- Extensive green roofs use 6 inches of growing media or less and weigh roughly 10 to 35 pounds per square foot when fully saturated. They’re planted with low-growing, drought-tolerant plants and need less maintenance.
- Intensive green roofs use deeper growing media and weigh roughly 35 to 300 pounds per square foot saturated. They can support shrubs, trees, and walking areas, but need more structure and more care.
Either type needs ongoing attention, and the first two growing seasons matter most, while the plants fill in. Budget for that care when you budget for the roof, because a green roof that isn’t weeded and watered while it establishes can end up with bare patches. That’s why we maintained the garden at Millmont Elementary School in Reading, PA, for two years after installation. There we installed an extensive American Hydrotech Garden Roof assembly on two roof elevations totaling 12,000 square feet, part of the Reading School District’s $40 million campus expansion. A fluid-applied rubberized asphalt membrane went down over the metal deck, followed by drain mat insulation, filter fabric, growing media, and thousands of plants in four sedum varieties. Our green roof maintenance guide covers what that care involves, and our green roof installation page explains how we build them.
The green roof on our Ephrata headquarters, with its vegetative garden, outdoor seating, and custom-made rain barrels, received the 2013 RoofPoint Excellence in Design Award for Excellence in Water Management.
Blue Roof vs. Green Roof: Which Do You Need, and Can You Have Both?
Both manage stormwater, but differently. A blue roof detains water and meters it out through flow-control drains, with no plants. A green roof absorbs part of the rain in its plants and growing media and slows the rest, with more weight and ongoing plant care. If detention volume is the requirement, a blue roof is the more direct tool.
You can also combine them in a blue-green roof, which stores stormwater in a detention layer beneath the growing media and meters it out through flow-control drains. Some systems use capillary wicking to move part of the stored water up into the growing media, while the rest drains out within the required time. GSM’s blue/garden roof in Lancaster pairs a water storage layer with 6 inches of engineered growing media and a sedum carpet. A blue-green roof takes the structural review and leak testing of a blue roof plus the plant care of a green roof, so it’s usually chosen when a site needs both stormwater detention and planted space.
Which Sustainable Roofing System Is Right for Your Building?
If cooling bills are the concern, look at a cool roof. If your municipality requires stormwater detention, a blue roof is the most direct way to handle it on the roof, and a green or blue-green roof makes sense when you also want planted space or LEED credit. From there, a few practical questions narrow it down:
- What can the structure carry? A cool roof adds almost no weight. Blue and green roofs need a structural engineer’s review before design.
- What do codes and local rules require? Commercial energy codes based on the IECC require reflective roofs only in climate zones 0 through 3. Pennsylvania, New Jersey, Delaware, and Maryland fall in zones 4 through 6, so the statewide energy codes here don’t require one. Some cities add their own rules; Philadelphia, for example, adopted a reflective roofing requirement for new construction in 2010. Stormwater detention rules and green building programs can apply too. If you’re pursuing LEED, its Heat Island Reduction credit recognizes vegetated roofs and high-reflectance roofs, which for low-slope roofs means an initial SRI of 82 or an aged SRI of 64.
- When is the roof due? Blue and green roofs are most practical when the roof is being replaced. A reflective coating is the exception, because it’s meant for a roof that’s still sound.
- What drives the cost? For a cool roof, the biggest factor is whether the existing roof can be coated or has to be replaced. For blue and green roofs, it’s the structural work, drainage changes, leak testing, and, for green roofs, the plants and early care. None of that can be priced without looking at the roof and the structure under it.
- Who will maintain it? A green roof needs the most care in its first couple of years, and a blue roof’s drains need checking after storms and freezes, so decide early whether your staff or a contractor will handle it.
On blue and green roofs, we work alongside your architect and structural engineer on the load and drainage details, and we work with owners on the LEED credits these roofs can earn. To talk about your building, call 800-532-ROOF or contact our Ephrata, PA or Elkton, MD office. We’ll start with the roof’s age and condition and, for a blue or green roof, what the structure under it can carry.
Green, Blue, and Cool Roof FAQs
What is the difference between green, blue, and cool roofs?
A cool roof reflects sunlight to keep the roof and building cooler. A blue roof temporarily stores rainwater on the roof and releases it slowly through flow-control drains. A green roof grows plants in engineered growing media that absorb rainfall and protect the membrane.
What is a cool roof system?
A cool roof system is a roof membrane, coating, or surfacing with high solar reflectance and thermal emittance. On low-slope commercial roofs, that usually means a white TPO or PVC membrane, a reflective coating, or a light-colored fluid-applied or granule-surfaced roof.
Do cool roofs make sense in cold climates?
For air-conditioned buildings, usually yes. The U.S. EPA notes that the winter heating penalty shrinks because the winter sun is low and the days are short, so in Pennsylvania, New Jersey, Delaware, and Maryland the summer cooling savings usually outweigh it.
Are roof coatings eco-friendly?
Roof coatings are often eco-friendly because they restore a sound roof in place, keeping the old roofing out of the landfill, and a white coating turns a dark roof into a cool roof. The roof has to be dry and structurally sound first.
Is a blue roof or a green roof better?
Neither is better overall, because they do different jobs. A blue roof is the more direct tool for detaining stormwater. A green roof also manages rainfall but adds plants, membrane protection, and potential green space, with more weight and more maintenance.
What does a blue roof protect you from?
A blue roof protects storm sewers, combined sewer systems, and local waterways from being overwhelmed during heavy rain. By holding water on the roof and releasing it slowly, it reduces peak runoff and helps prevent street flooding and sewer overflows.
What is a blue-green roof?
A blue-green roof combines a blue roof and a green roof. Stormwater is stored in a detention layer beneath the growing media and released slowly through flow-control drains, so the roof detains runoff and supports plants.
How much weight does a green or blue roof add?
A blue roof adds about 5.2 pounds per square foot for every inch of water it holds, plus the weight of the system itself. An extensive green roof weighs roughly 10 to 35 pounds per square foot when saturated, and an intensive green roof roughly 35 to 300. A structural engineer should confirm the roof can carry the load before either is designed.
Can I add a sustainable roof to my existing building?
In many cases. If the roof is dry and structurally sound, a reflective coating can turn it into a cool roof without a tear-off. Blue and green roofs usually need a structural review and are most practical when the roof is being replaced.
What is the most eco-friendly roofing for a commercial building?
For most commercial buildings, it’s the roof that lasts longest and sends the least material to a landfill. If the existing roof is dry and sound, a reflective coating restores it without a tear-off and makes it a cool roof. If the site has to detain stormwater, you’ll need a blue or green roof, because a cool roof doesn’t do that job.