When choosing a flat roof for your home, you can pick from three types: built-up roofing, modified bitumen roofing, or rubber membrane roofing. Flat roofs cost less than traditional roofs and create space for an outdoor patio or rooftop garden.
In this guide, we’ll cover popular types of flat roofs, how they’re constructed, and the pros and cons of each material. We’ll also share advice from This Old House general contractor Tom Silva to help you make the best decision for your home.
Understanding Flat Roof Systems
Despite the name, flat roofs aren’t perfectly level. They’re built with a slight slope that directs water toward drains, scuppers, or the roof’s edge so it can fall off rather than collect on the surface.
Tom Silva, a general contractor for This Old House, explains in the Season 46 episode “Septic, Stairs and Subs” how drainage works on flat roofs. “A flat roof really isn’t flat—and it shouldn’t be,” Silva says. “You want to move water from the center of the roof out to the edges so it drains properly and doesn’t sit and cause problems.”
Roofers typically create this slope using tapered insulation or subtle framing adjustments that guide water toward drains, scuppers, or the outer edge of the roof. Even a small pitch can make a big difference in preventing standing water and extending the life of the roofing system.
The three main types of flat roofs—each commonly referred to by an acronym—have different materials, installation methods, and performance characteristics.
Built-Up Roof (BUR)
Built-up roofs, often referred to as BURs, have been used in roofing for more than a century. They’re sometimes called “tar-and-gravel roofs” because the system consists of alternating layers of bitumen (asphalt) and reinforcing fabrics topped with a protective gravel layer.
These multiple layers create a thick, durable membrane that provides strong protection against water and ultraviolet (UV) light exposure. Because of their weight and layered construction, BUR systems are commonly installed on commercial buildings but can also work well on residential flat roofs that require extra durability.
Advantages of Built-Up Roofs
- Excellent fire resistance thanks to the gravel top layer
- Durable, multi-layer construction that resists water penetration
- Attractive appearance for buildings with windows or decks overlooking the roof
- Proven track record of long-term performance
Drawbacks of Built-Up Roofs
- Heavier than other flat roof systems, sometimes requiring additional structural support
- Installation can be messy and produce strong odors from heated asphalt
- Multiple layers can make leaks harder to locate if they occur
Modified Bitumen Roof (MBR)
Modified bitumen roofing or MBR is an evolution of traditional asphalt roofing designed specifically for flat and low-slope roofs. Manufacturers enhance asphalt with rubber or plastic polymers, improving flexibility and resistance to temperature changes.
Like built-up roofing, modified bitumen systems consist of several layers, including a base sheet, one or more reinforcing plies, and a protective cap sheet. These materials can be installed using torch-applied, cold-applied, or peel-and-stick methods, making them easier to install than traditional BUR systems.
Because it combines durability with relatively straightforward installation, modified bitumen is a popular option for residential flat roofs, additions, and garages.
Advantages of Modified Bitumen Roofing
- Easier installation than traditional BUR systems
- Flexible material that performs well during temperature swings
- Reflective cap sheets are available to improve energy efficiency
- Durable surface that can handle moderate foot traffic
Drawbacks of Modified Bitumen Roofs
- Torch-down application methods can pose a fire risk if not handled properly
- Less resistant to scuffs and tears than some rubber membranes
- May require more maintenance than single-ply membrane systems
Rubber Membrane Roof (EPDM)
Ethylene propylene diene monomer (EPDM) roofing is a synthetic rubber membrane widely used on residential and commercial flat roofs. It’s typically installed in large sheets, which helps reduce the number of seams where leaks can develop.
The material is designed to withstand prolonged sun exposure, temperature fluctuations, and moisture. EPDM membranes are usually black, though white versions are available to improve reflectivity in warmer climates.
Because the material is lightweight and relatively simple to install, EPDM is one of the most common flat roof systems for homes, garages, and small additions.
Advantages of EPDM Roofing
- Lightweight material that rarely requires structural reinforcement
- Highly resistant to scuffs, tears, and punctures
- Seams can be sealed with adhesive or tape for reliable waterproofing
- Long lifespan, often lasting 20–30 years or more with proper maintenance
Drawbacks of EPDM Roofing
- Standard black membranes absorb heat, which may increase cooling costs in hot climates
- Seams rely on adhesives that may degrade over time if not maintained
- Sharp tools or debris can puncture the membrane if the roof is frequently walked on
Comparing Flat Roof Costs
Flat roof replacement costs vary depending on the material, roof size, and installation complexity. Most flat roofing materials cost between $7 and $13 per square foot. The table below shows how common flat roof materials compare based on price.
| Flat Roof Material | Cost Per Square Foot |
| Built-up roof (BUR) | $7–$9 |
| Modified bitumen | $7–$9 |
| EPDM rubber membrane | $9–$13 |
Note: The cost data featured in this guide is based on estimates from Homewyse, an online database with home improvement project costs, unless otherwise noted.
Lifespan and Warranty Considerations for Flat Roofs
Flat roofs usually come with a 10–20 year warranty but can last up to 25 years with proper installation and maintenance. EPDM roofs often have the longest lifespan at up to 50 years, followed by BUR and modified bitumen at 20–30 years. Before choosing a roofing material, take time to understand what the warranty covers and for how long, as well as any maintenance requirements to keep the warranty valid.
Maintenance Requirements for Different Flat Roof Types
Regular roof maintenance helps prevent small issues from turning into expensive repairs. In addition to removing debris and checking for standing water after storms, inspect your flat roof at least twice a year, typically in the spring and fall, and after major weather events.
While all flat roofs require routine cleaning and drainage checks, each material has specific maintenance considerations.
BUR Maintenance
Built-up roofs rely on their protective gravel layer to shield the underlying asphalt from sun damage.
- Check that the gravel remains evenly distributed across the surface
- Reapply gravel in areas where it has shifted or worn away
- Inspect the surface for cracks, blisters, or exposed asphalt
- Keep roof drains and scuppers clear to prevent ponding water
Modified Bitumen Maintenance
Modified bitumen roofs are durable but rely on strong seams and protective coatings.
- Look for seam separation or lifted edges along the membrane
- Check the cap sheet for signs of surface wear or cracking
- Apply reflective coatings periodically to maintain UV protection
- Inspect areas around flashing, vents, and drains for early signs of leaks
EPDM Maintenance
EPDM membranes are flexible and easy to repair, but they can be punctured by sharp objects.
- Inspect seams and flashing for loose adhesive or separation
- Repair punctures or tears promptly with compatible patch kits
- Remove branches or debris that could damage the membrane
- Avoid dragging tools or ladders across the rubber surface
Climate Considerations When Choosing a Flat Roof
Your local climate plays a major role in how well a flat roof performs over time. Heat, rainfall, snow, and seasonal temperature changes can all affect roofing materials and drainage.
When choosing a flat roof system, consider how different materials perform in these conditions:
- Hot, sunny climates: Light-colored modified bitumen membranes can help reduce heat absorption and keep roof surfaces cooler, potentially lowering indoor cooling costs.
- Cold climates with freeze-thaw cycles: Flexible materials such as EPDM and modified bitumen handle expansion and contraction better than more rigid systems, helping prevent cracks during temperature swings.
- Areas with heavy rainfall: BUR systems provide extra protection against leaks thanks to their multiple waterproof layers.
- Regions with large temperature swings: EPDM rubber membranes remain flexible in both hot and cold conditions, reducing the risk of splitting or seam failure as the roof expands and contracts throughout the year.
Professional Flat Roof Installation vs. DIY
While some flat roofing materials, particularly peel-and-stick modified bitumen or certain EPDM systems, can be do-it-yourself (DIY)-friendly, we recommend professional installation for most flat roofs. Roofing contractors have the experience, tools, and knowledge to ensure proper drainage, seam sealing, and flashing installation.
Improper installation can also make the roof more vulnerable to wind damage. “When wind blows across a roof, it creates negative pressure that tries to lift the roofing membrane,” Silva says. “Proper fastening keeps the roofing system secured so it cannot flutter or tear.”
Professional roofers understand how to secure membranes, fasten insulation, and reinforce edges so the roofing system can withstand wind and weather over time. While DIY installation may work for small projects or repairs, hiring a professional helps ensure your flat roof is installed safely and performs as intended.
