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Flat Roof Drainage: How Commercial Roof Drains Work

Flat roofs, characterized by their horizontal or nearly horizontal orientation, are a common sight on commercial buildings and modern residential designs. Their sleek appearance and space-saving potential make them a popular choice for architects and homeowners alike. However, despite their aesthetic appeal and practicality, flat roofs come with their own set of challenges, particularly when it comes to drainage.

In this blog, we’ll take you deep into the intricacies of flat roof drainage, exploring:

  • Why these roofs are prone to drainage issues
  • How to identify problems
  • Methods for improving drainage
  • When to seek professional help
  • The costs involved
  • The risks associated with neglecting drainage issues

Understanding Flat Roofs

long flat roof

Before delving into drainage issues, it’s essential to understand what constitutes a flat roof. Unlike traditional pitched roofs, which have a noticeable slope to facilitate water runoff, flat roofs have a slight pitch or are entirely level. They are typically constructed using materials like bitumen, EPDM rubber, PVC membrane, or built-up roofing (BUR) systems.

Flat roofs offer several advantages, such as ease of installation, cost-effectiveness, and additional outdoor space for recreational purposes or equipment installation. However, their design makes them more susceptible to drainage problems, as water can accumulate and pool if not properly managed.

Why Flat Roofs Are Prone to Drainage Issues

Several factors contribute to the drainage challenges commonly associated with flat roofs:

  • Lack of Natural Slope: Unlike pitched roofs, flat roofs lack a significant slope to encourage water runoff. Even roofs with a slight pitch can struggle to drain water efficiently.
  • Debris Accumulation: Leaves, branches, and other debris can accumulate on flat roofs, obstructing drainage paths and impeding water flow.
  • Poor Installation: Improper installation of roofing materials or insufficiently sealed seams can create areas where water can penetrate and accumulate.
  • Weather Conditions: Heavy rainfall, snow, or prolonged exposure to UV radiation can accelerate roof deterioration and exacerbate drainage issues over time.

Spotting Drainage Issues

old commercial roof

Recognizing drainage problems early can help prevent extensive damage to your flat roof and the underlying structure. Here are some signs to watch out for:

  • Pooling Water: Puddles or standing water that persists long after rainfall indicates poor drainage.
  • Sagging or Ponding: Areas of the roof that appear sunken or depressed may be prone to water accumulation.
  • Water Stains: Interior water stains on ceilings or walls can indicate leaks caused by drainage issues.
  • Sagging Ceiling: In severe cases, prolonged water infiltration can cause the ceiling to sag or show signs of water damage.

5 Methods to Improve Flat Roof Drainage

Addressing drainage issues promptly is crucial to maintaining the integrity of your flat roof. Here are several strategies to enhance drainage:

1) Install Roof Drains:

Roof drains, such as scuppers or internal drains, are designed to channel water away from the roof surface and into a drainage system.

2) Improve Roof Pitch:

In some cases, modifying the roof’s pitch slightly can improve water runoff and reduce pooling.

3) Clear Debris Regularly:

Schedule routine maintenance to remove debris from the roof surface and ensure unobstructed drainage paths.

4) Consider Tapered Insulation Systems:

Tapered insulation can be installed beneath roofing materials to create a slope that facilitates water drainage.

5) Apply Waterproof Coatings:

Waterproof coatings can help seal seams and protect the roof surface from water infiltration.

When to Call a Professional Contractor

While minor drainage issues may be addressed by diligent homeowners, certain circumstances warrant the expertise of a professional roofing contractor:

  • Extensive Damage: If your flat roof has sustained significant damage or deterioration, it’s best to seek professional assistance to assess the extent of the problem and recommend appropriate solutions.
  • Complex Repairs: Repairing leaks or installing drainage systems may require specialized tools and expertise beyond the capabilities of the average homeowner.
  • Safety Concerns: Working on a flat roof can be hazardous, especially for those without proper training or safety equipment. Professional contractors have the experience and resources to work safely at heights.

Cost to Repair Drainage Issues

The cost of repairing drainage issues on a flat roof can vary depending on the extent of the damage, the type of repairs needed, and the contractor’s rates. Here are some factors to consider:

  • Materials: The cost of materials, such as roofing membrane, insulation, and drainage components, will contribute to the overall expense.
  • Labor: Labor costs will depend on the complexity of the repairs and the contractor’s hourly rate.
  • Extent of Damage: Minor repairs, such as patching a leak or clearing debris, will generally be less expensive than extensive repairs or roof replacement.
  • Accessibility: If access to the roof is difficult or requires specialized equipment, it may increase the labor costs.

Dangers of Untreated Drainage Issues

Ignoring drainage problems on your flat roof can lead to a host of costly and potentially dangerous consequences:

Structural Damage:

Prolonged exposure to water can compromise the structural integrity of your roof deck, leading to rot, mold growth, and even collapse in extreme cases.

Interior Damage:

Water infiltration through the roof can cause damage to ceilings, walls, insulation, and electrical systems, resulting in costly repairs and health hazards from mold growth.

Energy Loss:

Poorly draining roofs can contribute to energy inefficiency by allowing water to penetrate insulation, reducing its effectiveness and increasing heating and cooling costs.

Safety Hazards:

Standing water on flat roofs can pose slip and fall hazards for occupants and maintenance personnel, increasing the risk of accidents and liability issues.

Install a Flat Roof Drainage System with Greentek!

Maintaining proper drainage on your flat roof is essential for preserving its longevity and protecting your property from water damage. By understanding the unique challenges of flat roof drainage, spotting signs of trouble early, and implementing effective drainage solutions, you can safeguard your investment and enjoy peace of mind knowing your roof is watertight.

If faced with complex drainage issues or significant damage, don’t hesitate to enlist the expertise of a qualified roofing contractor to ensure the job is done safely and effectively. Remember, proactive maintenance today can save you from costly repairs tomorrow.

Don’t know who to call? Greentek Roofing is here to help you create a drainage system that protects you from all seasons.


“GreenTek was awesome to work with. My rep James was easy to get in contact with and answered all my questions and made the process quick and smooth. The roof came out great and everything was finished on time and they handled everything with my insurance company so I didn’t have to. It made the process stress free for me.”

-Dylan O.

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