Flat roofs have long had a reputation of being unsuitable for the Pacific Northwest. Traditional built-up asphalt roofing (BUR) commonly used in flat roof construction may be well suited for desert climates, but does not hold up well to our rainy climate and is prone to leaks. The modern flat roof is another story, though. Advances in technology and the development of new roofing materials have expanded the options and extended the life span for residential and commercial flat roofs.

There are six common types of flat roofs, which we break down for you here. Of the six, we recommend two flat roof types for commercial or residential buildings here in the Willamette Valley. 

Common Types of Flat Roof Materials

  1. Built-up roofing (BUR) consists of alternating layers of reinforced fiberglass fabric and asphalt or tar, finished with a top layer of round rock or cap sheet.
    Pros: Fire-resistant, waterproof, excellent ultra-violet protection.
    Cons: Susceptible to wind and water damage, loses elasticity quickly, leaks can be difficult to find, not a good option for the Pacific Northwest climate.
  2. Modified bitumen roofing is similar to BUR and is made of asphalt combined with polymerized rubber or plastic, reinforced with fiberglass to create a flexible and durable waterproof membrane.
    Pros: Versatile, flexible, durable, waterproof, ideal for the rainy and temperate Pacific Northwest climate. Can be very fire resistant with the proper coating.
    Cons: Absorbs heat in the summer.
  3. Ethylene propylene diene monomer (EPDM) roofing is an extremely durable and waterproof synthetic rubber membrane.
    Pros: Extremely durable, lightweight, flexible, fire-resistant, easy to repair.
    Cons: Absorbs heat in the summer, seams tend to break down as the rubber expands and contracts, prone to leaks if not installed properly and often isn’t.
  4. Spray polyurethane foam (SPF) roofing consists of applying a liquid sprayed onto an existing roof, which expands into foam, creating a solid layer across the roof surface, then a white latex or acrylic coating to waterproof the foam.
    Pros: Good insulation, energy-efficient, easy to install and maintain, waterproof, versatile, and relatively low-cost.
    Cons: Requires warm, dry weather and clean surface conditions for proper installation, highly susceptible to damage from wind-blown debris, requires frequent care, too short of a season in the NW to complete.
  5. Thermoplastic Polyolefin (TPO) roofing is a single-ply white synthetic membrane with a fabric reinforcing scrim that stabilizes and strengthens the membrane.
    Pros: Durable, UV, water-resistant, versatile, light-weight, and flexible, available in a few colors, mid-level maintenance, least expensive of the membranes.
    Cons: Highly flammable like most other single-ply membranes, does not perform well in high heat, seam edges need to be resealed with edge sealant every so many years, 90% carbon and can’t be welded after a couple of years of UV exposure.
  6. Poly Vinyl Chloride (PVC) roofing is a single-ply membrane consisting of two layers of membrane with a fabric reinforcing scrim in the center that stabilizes and strengthens the membrane.
    Pros: Durable, UV, and water-resistant, versatile, light-weight, and flexible, available in a variety of colors, relatively low maintenance, fire-resistant with self-extinguishing properties, chemical resistant, holds up well in strong winds, can be welded for decades if needed. Lasts for decades.
    Cons: It’s more costly than TPO.

Our Top Flat and Low-Pitch Roof Recommendations

As roofing contractors with more than 40 years of roofing experience in the Portland metro area, we have come to understand which roofing materials perform well over the long-term in our climate. The Willamette Valley experiences wide fluctuations in temperature and moisture and seasons can range from searing heat and drought-like conditions to perpetual sogginess or even deep freeze. High wind is also common at certain times, and fire is a growing concern. 

These seasonal temperature variances and weather events can wreak havoc on flat roofs and certain materials will fare much better than others. Our top recommendations for residential or commercial flat roofs in Portland and the Pacific Northwest are 3-ply, cold process modified bitumen roofing and single-ply polyvinyl chloride (PVC) roofing.

  • 3-Ply, Cold Process Modified Bitumen Roofing

Modified bitumen is one of the most highly reliable and long-lasting options for flat roofs. It is extremely durable and tear-resistant making it an excellent choice for commercial buildings with high roof-top foot traffic, and when properly installed, it offers nearly fail-safe waterproof protection. Modified bitumen roofing is also a very flexible material that can withstand thermal expansions and contractions–even deep freezes–without losing its shape, becoming brittle, or cracking. This is also a relatively low-maintenance roofing material that is easily repaired if it is ever damaged. Repairs usually only amount to simply putting another sheet or patch of bitumen roofing over the existing sheet. With proper care and maintenance modified bitumen has a life expectancy of 20 years.

An added benefit we appreciate is the cold process our roofing installers can employ for installing this 3-ply roofing material. This method does not require the application of hot tar or the use of hot torches, which is a nice bonus, especially in the heat of summer.

  • Single-Ply, Polyvinyl Chloride (PVC) Roofing

Single-ply polyvinyl chloride (PVC) roofing is another excellent choice for flat roofs in the Pacific Northwest. Like modified bitumen roofing, PVC roofing is a highly durable, waterproof, low maintenance, and long-lasting material. One of the top attributes of PVC though is its superb fire performance. The PVC material is self-extinguishing, once the source of the flame is removed, and radiates less heat during combustion which helps prevent the spread of fire to other materials. It also generates less smoke, as compared to other common building materials. 

Another reason we like PVC roofing is the flexible nature of the material and how well it stands up to temperature fluctuations, as well as wind. It comes in different colors, including white, charcoal, and light gray which can offer better UV reflection and less heat absorption in the summer.  PVC roofs also have a long life span of up to 30 years requiring relatively low maintenance

All of these characteristics–whether we are talking about 3-ply modified bitumen roofing or single-ply PVC roofing–add up to a very appealing flat roofing system for residential and commercial properties. They are both highly durable, long-lasting, low maintenance, roofing materials. As roofing contractors, we appreciate being able to give our customers this peace of mind. 

As with any roof installation, the quality of the materials and the roofing contractor’s workmanship play an equal role in how well your roof will perform and how long it will last. An improperly installed roof will be prone to problems from day one, regardless of the materials used. Different materials and different types of roofs also require different expertise, technical knowledge, and specific skill sets. This is why it is important to make sure the roofing contractor you hire has the requisite experience.

Pacific West Roofing has been installing flat roofing systems for both residential and commercial properties throughout the greater Portland metro area for more than forty years. Our roofing installers, technicians, and repair crews are experienced with installing and maintaining both 3-Ply, Cold Process Modified Bitumen Roofing and Single-Ply, Polyvinyl Chloride (PVC) Roofing. 

If you have a flat roof in need of inspection, repair, or replacement, contact us today. Our friendly office staff will be happy to set up a time for us to come out, inspect your roof and give you an honest recommendation and cost estimate.

Spring Roof MaintenanceSpring Roof Maintenance

Spring Roof Maintenance Checklist


4 Signs You Need a New Roof

Portland Roofing Contractor Since 1980

CCB# 169414