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Why Apartment Buildings Typically Have Flat Roofs

Although apartment buildings in California vary in size, design, and architectural style, a common feature among many is their flat roofs. While these roofs may enhance the overall aesthetic appeal and design of apartment buildings, you may wonder why they are so common. Learn why most apartment building roofs in California are flat, what materials they are made of, and what benefits they offer.

Are Flat Roofs on Apartments Truly Flat?

While the term “flat roof” is commonly used to refer to apartment roofing, they do not have a truly flat roof angle. The California Building Energy Efficiency Standards categorize flat roofs as low-sloped roofs. They are legally defined as having a rise-to-run ratio of less than 2:12, corresponding to a slope angle of 9.5° or less. Any roof with a steeper slope angle is considered steep-sloped instead.

Legally, flat roofs do not need to have a flat or near-flat slope angle, such as 0°. Virtually no buildings in California feature perfectly flat roofs to ensure they function correctly under all seasonal weather conditions.

One of the primary purposes of an apartment roof is to protect its occupants from rainwater. A slight roofing angle creates a drainage effect, allowing water to flow off the roof’s surface into the gutters and flashing, preventing the formation of standing water. A perfectly flat roof would accumulate standing water and degrade quickly.

Most apartment buildings in California have HVAC units for the warmer months. The slight angling of flat roofing keeps rainwater from pooling in or around the outdoor units, preventing potential damage. Flat apartment roofing is also walkable, providing easier access to the roof’s surface for inspections, repairs, and HVAC maintenance.

Types of Flat Roofing Materials

Flat roofs are typically less expensive to construct and maintain than conventional, angled roofs, such as those found on single-family homes. This is because they do not use traditional roofing materials, such as tiles or shingles.

Instead, buildings with low-sloped roofing typically feature materials and surfaces unique to flat roofs. Many of these materials are designed for long-term longevity and durability. Common flat roofing materials include:

  • Standing seam metal panels. This type of roofing material uses specially designed interlocking panels made of metals. Common metals for this roofing type include zinc-coated galvanized steel, galvalume steel, aluminum, and copper. These materials are naturally durable and designed to be long-lasting. Metal panel roofing can last up to 50 years with proper maintenance. Some premium materials, such as zinc coatings, give your roof panels self-healing properties.
    Metal panels contract and expand as temperatures change; standing seam panels are designed to accommodate these thermal movements, ensuring the roof retains its structural integrity. They also reflect sunlight, allowing them to conform to California Cool Roof regulations.
  • Polyvinyl chloride (PVC). Polyvinyl chloride is a type of synthetic plastic polymer. PVC roofing is made of a single-ply PVC membrane that can be produced in a wide range of colors, making it an excellent choice for branding or aesthetic appeal.
    PVC is naturally resistant to ultraviolet (UV) rays, fire, water, and chemicals. It can also reflect sunlight efficiently and requires relatively little maintenance. A properly maintained PVC membrane roof can last up to 50 years, making it an excellent choice for a California apartment building.
  • Thermoplastic Polyolefin (TPO). TPO is a blend of polypropylene plastic and ethylene-propylene rubber. The resulting material combines the advantages of plastic and rubber, including puncture, tear, and impact resistance, making it suitable for use as a single-ply roofing membrane.
    TPO is also heat-reflective, highly flexible, durable, and resistant to UV rays, chemicals, and bacterial growth. A well-maintained TPO roof can last up to 30 years. It is also 100% recyclable, making it a more environmentally friendly choice than PVC. However, most TPO membranes are only available in white, limiting design and aesthetic considerations.
  • Ethylene-propylene-diene Monomer (EPDM). EPDM is a synthetic rubber material with high durability, impact resistance, and waterproof characteristics. It can be used as a standalone roofing material or as a supplementary membrane to improve a roof’s durability.
    However, EPDM membranes tend to absorb heat instead of reflecting it, which may reduce their suitability for California apartment buildings. The heat absorption issue can also reduce the average lifespan of an EPDM membrane. You may need to replace them every 10 to 15 years, even with proper maintenance.

Benefits of Flat Roofs on Apartment Buildings

Low-sloped roof angles on apartment buildings in California offer many benefits for building owners, homeowners association (HOA) members, and property managers. The following is a breakdown of their main benefits and why they are worth considering:

  • Initial cost-effectiveness. Roofs featuring lower slope angles require fewer materials and less time to complete than equivalent roofs with higher slope angles. This, consequently, reduces the total initial costs, including labor.
  • Easier maintenance. Once a flat roof has been constructed, it features a naturally walkable angle, making it easy to access for repairs and maintenance. The ease of access also helps cut down the cost of roof maintenance, as it doesn’t require as much safety equipment to traverse as a steeper-angled roof.
  • Better solar panel support. Although apartment buildings taller than 3 stories do not have to meet California Title 24 requirements, flat roofs are well suited for solar panels. They offer a wide, flat, unobstructed surface with frequent exposure to sunlight. Installing solar panels can be an excellent way to make an apartment building more energy efficient.
  • More usable space for HVAC. Besides installing solar panels, flat roofs are better suited for the installation of heating, ventilation, and air conditioning (HVAC) equipment. Flat roofing provides excellent space to support outdoor units eliminating the need to install them on the sides of the building or move them indoors. Placing HVAC systems on the roof also ensures better airflow, reduced noise, and less vibration, even for occupants located near the top of the building.
  • Regulatory compliance. Most materials used to construct flat apartment roofs have two essential features for California: high solar reflectivity and efficient insulation. These materials can help make your apartment California Cool Roof compliant, improving its energy efficiency and thermal regulation year-round.

Get Your Flat Apartment Roof Professionally Inspected Today

While flat roofs are easier to maintain than traditional, steep-sloped roofs, regular inspections and periodic assessments are still critical. Having your flat roof professionally inspected and maintained extends its lifespan, prevents the formation of leaks, and reduces the risk of small issues from worsening and turning into major damage.

AJ Reyes Roofing is a state-licensed professional roofing contractor with almost 30 years of experience serving the Southern California community. We are based in the San Fernando Valley and have experience working on all types of flat roofs, from short buildings to the tallest apartment blocks.

Our team can provide your roof with the care and maintenance it needs to last as long as possible, from inspections and cleaning to full repairs, membrane replacements, and roof renovations. Contact us today for a free estimate.

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