Flat Roofing Systems: Pros and Cons

Before making the decision to have a flat roof installed, you should think about which type of flat roof system is right for your home. Below, you will find information about the three most popular types of flat roofs that are being installed today: Built up Roofs or BUR, Modified Bitumen Roofs, and Rubber Membrane Roofs.

Built Up Roofs (BURs)

A BUR, also sometimes referred to as a tar and gravel roof, is one of the most affordable flat roofing systems on the market today. They are usually made of three of more layers of waterproof ply sheets sandwiched between layers of hot tar. For reliability and durability, these roofing layers are then backfilled with a layer of smooth gravel or river rock.

Pros

  • BURs use multiple layers (plies) of bitumen and reinforcing felt. The redundancy means if one layer is damaged, others continue to protect the building. Properly installed BUR systems can last 20–30+ years with routine maintenance.
  • The combination of hot asphalt and layered membranes creates a monolithic, seamless waterproof barrier. It’s one of the most reliable systems for preventing ponding-water penetration on low-slope roofs.
  • The top layer of gravel or aggregate shields the membrane from sunlight, hail, and temperature swings. This minimizes cracking, blistering, and other age-related wear.
  • BUR systems with a gravel surface have inherent fire resistance, making them safer for many commercial applications.
  • Because the system is robust, BURs don’t require constant touch-ups. Annual inspections and small repairs are usually enough to keep them performing well.
  • If the building has HVAC units or regular foot traffic, BURs handle it better than most single-ply systems. The surface is tough, stable, and less prone to punctures.
  • A BUR with a light-coloured or reflective cap sheet helps reduce heat absorption, lowering cooling costs in the summer.

Cons

  • BUR is a time-tested technology, it has been around for more than 100 years, and the same installation process is still followed, the only difference is the advancement of the waterproofing membranes used.
  • Gravel is a great fire retardant
  • has a nice, finished look from windows or decks that overlook
  • BURs are very heavy and sometimes require that joists be strengthened to ensure stability in the roof
  • Tar and gravel systems smell bad and can be a very messy installation process
  • BURs are not recommended for homes and are mostly installed on garages

Modified Bitumen Roofs

Modified Bitumen (Mod-bit) roofing is an asphalt-based roofing system that is similar to BUR. They are made of asphalt rolls that are typically 3 feet wide, but a modified bitumen system only uses two layers, a base sheet and a cap sheet. Mod-bit can be installed in four ways: applied with hot tar, cold applied solvent based adhesives, torch down with an open flame to melt the sheets together, and peel and stick base sheets that self adhere when a release tape is removed.

Modified Bitumen Roof Breakdown

Pros

  • Modified Bitumen (Mod-Bit) roofing is one of the most widely used low-slope roofing systems today. It combines the reliability of traditional BURs with modern polymer technology. The result is a tough, flexible membrane that performs exceptionally well in Ontario’s climate.
  • Mod-bit membranes (SBS or APP) expand and contract without cracking, even in deep cold or high heat. This makes them a strong choice for Canadian winters and buildings with thermal movement.
  • The sheets are reinforced and installed with heat-welded or cold-applied seams. Once bonded, the seams are highly resistant to leaks, outperforming many single-ply systems in this area.
  • Because mod-bit is thicker than most membranes, it handles foot traffic, tools, and rooftop equipment better. It’s well-suited for commercial roofs where durability is non-negotiable.
  • These roofs can usually be repaired with heat welding or patching, making maintenance straightforward. Technicians can identify and fix issues quickly, reducing long-term ownership costs.
  • Like BURs, modified bitumen can be installed in two-ply or multi-ply configurations, creating a built-in redundancy that extends lifespan and improves resilience.
  • Cap sheets are available in cool-roof, granulated, or smooth finishes. Light-coloured or reflective surfaces help reduce heat absorption and lower cooling costs in the summer.
  • Mod-bit roofs consistently deliver 20–30 years of performance when installed and maintained correctly. They’ve been industry standards for decades because they work.

Cons

  • Peel and Stick materials are very user friendly and can even be a DIY project, given the right directions
  • It is a light-coloured surface (usually grey or white) that reflects heat and can cut energy costs
  • Torch-down application methods are a fire hazard and not always recommended
  • It is not as tough as a BUR system would be and is less resistant to tears

Rubber Membrane Roofs

Rubber membrane roofs are easy to spot and are generally installed in one of two ways. Either backfilled or weighed down with larger river rocks or adhered/glued down. Rubber roofs that are glued or adhered are typically black in colour and stretched over an insulation or fibre board. Backfilled rubber roofs, like BURs, are covered in rock, however the stones are usually larger and not adhered to the roof with tar. The rubber membrane can be exposed by gently moving the stones around.

Pros

  • A properly installed EPDM membrane can last 25–30+ years. The material doesn’t become brittle in the sun and holds up well under harsh weather.
  • EPDM is naturally UV-stable. Unlike some membranes, it doesn’t require extra coatings to protect against sunlight. This makes it ideal for roofs with full-day sun exposure.
  • Rubber membrane stays pliable even in deep cold, which allows it to expand and contract with the building without cracking. This is a major advantage in Ontario’s freeze-thaw cycles.
  • EPDM sheets are large and lightweight, which reduces installation time and labour costs. Fewer seams mean fewer potential leak points.
  • When seams are properly adhered, EPDM provides a continuous waterproof barrier that performs extremely well, even on roofs with minimal slope.
  • The thicker options (60–90 mil) offer strong puncture resistance. They handle light foot traffic and maintenance activity better than many other single-ply systems.
  • EPDM is generally one of the most affordable low-slope roofing systems per square foot. Combined with its long lifespan, it delivers strong lifetime value.
  • Annual inspections and minor seam touch-ups are usually all that’s required. The membrane ages slowly and doesn’t peel or blister easily.
  • EPDM is easy to flash around vents, HVAC units, pipes, and skylights. Repairs are straightforward and inexpensive.

Cons

  • Relatively easy installation process
  • Very resistant to cuts and tears while remaining light
  • Leaks are easy to identify
  • Vulnerable to punctures
  • Standard black colour absorbs heat, however lighter coatings can cost approximately 30% more