(316) 927-2233Whether you are buying a building, or evaluating the condition of your existing roof, it’s worth understanding common failure points of flat roofing systems to be able to make informed decisions about maintenance, budgeting, or the potential need for repairs. We have assembled a resource on what our team finds to be the most common failure points and leak sites for low-slope roofs, which works as a checklist for inspection items in evaluating your property’s roofing system.
We find that roof inspections fall into one of two categories: 1) condition assessment, or 2) diagnostic. Hopefully, you’re in the first category – evaluating the condition of a working roof system. However, this resource is also a starting point for figuring out the next steps if you have an active leak and are trying to identify, or diagnose, the source.
How do Commercial Roofs Fail?
We collected our own team’s experience by discussing specific causes of failure points across hundreds of flat roofing systems over the years. When we define “failure points” – we’re talking about water intrusion – as the purpose of a roofing system is to keep outside elements outside. This list is not comprehensive, but categorizes the most common problems. This also includes failure points that can be fixed by repairs as well as total system failures that require full replacement or complete re-covering.
- Open seams or flashings
- Drainage failures (slope changes, cracking, or debris clogging)
- UV radiation and material deterioration
- Wind uplift damage
- Ice and snow damage
- Hail strikes
- Falling tree limbs
- Critter damage (termites, raccoons, birds, mice, etc.)
- Human-generated Punctures (often caused by technicians dropping tools, screws, or heavy equipment)
We’ve come up with a miniseries that works as an inspection guide for commercial and low-slope roofs, split into three categories by problem source:
- Part 1: Improper installation (improper flashing, seam welding, and lack of pipe boots)
- Part 2: Poor quality roofing materials and how they respond to hazards on low slope roofs
- Part 3: Poor drainage design that leads to ponding water
In this part (Part 3), we’ll dive in to ponding water — how poor drainage design causes it and how well-designed roofs enable adequate water drainage.
Severe, critical ponding on a low slope commercial EPDM roof. Ponding this severe compounds as wood decking becomes saturated, bowing under the weight of the water. In this case, the roof had negative slope that prevented water runoff.
Part 3 – Poor Drainage Design
Drainage is one of the most critical design elements in a commercial roof system. When water is unable to shed properly, the long-term performance of the roof is compromised no matter how well the rest of the roof system is built. The roofing contractor you select can be the difference between dealing with more roofing issues in a handful of years or enjoying the peace of mind that comes with a roof capable of exceeding a 20-year warranty with proper maintenance. In this article, we’ll explore why drainage design demands careful consideration and what reputable roofing contractors look for.
Why Proper Drainage Design Is Vital for Low-Slope Commercial Roofs
Poor drainage design that results in ponding water causes water infiltration. The danger lies in how quietly this damage can accumulate: by the time you see signs of leaks, the substrate may already be compromised. Membrane punctures or cold, loose welds at seams will gradually invite ponding water, resulting in costly repairs by the time symptoms appear. Proper drainage design is especially critical in commercial roofing because most commercial roofs have minimal slope, under 2:12 pitch. Despite the term “flat roof,” all commercial roof systems should be built with some positive slope, which is required for water to properly drain off toward designated channels and away from the building.
This makes structural slope, which is slope provided by structural members and the building’s design, a critical consideration when installing a commercial roof.
Understanding Structural Slope and Its Role in Roof Drainage
Ideally, an architect, builder, and/or civil engineer has already designed a well-draining roof during your building’s initial construction. This begins with structural slope – the intentional angling of the roof deck through structural members to create positive drainage, typically with a slope of at least 2 percent (or 0.24:12). A building’s structural slope is intended to direct water toward drainage points, such as interior drains or scuppers.
Building owners can’t reasonably alter structural slope after the fact; however, it’s the foundation of an effective drainage system. It dictates how many drains are required, where they’re placed, and how effectively water runs to drains and scuppers. If slope is insufficient or poorly designed, water will naturally pond in low areas.
An example of a custom-designed tapered insulation system that creates slope.
Tapered Insulation Systems for Correcting Flat Roof Drainage
Low areas can be addressed retroactively by installing tapered insulation systems. Tapered insulation is a sloped foam panel system engineered to channel water toward interior drains or scuppers on low slope roofs. A well-designed tapered insulation system directs water toward drainage areas just as effectively as structural slope. These systems are designed to accommodate the roof’s drainage needs, with panel configurations that drain efficiently. For more information, see our full guide on tapered roofing insulation design and benefits.
Keep Roof Drains, Collector Heads & Scuppers Clear to Prevent Ponding Water
After a building is constructed, responsibility shifts to maintenance personnel, or a roofing contractor, to ensure that all drain components are functioning properly. Leaves, trash, and other debris obstructing water’s path off the roof are among the most avoidable causes of ponding.
If the scuppers, roof drains, and backup roof drains are clogged with leaves, trash, or debris, they can fail to facilitate water drainage during heavy rainfall, and it can accumulate on your roof like water in a bathtub. Low slope roofs are designed to withstand ponding water for 48 hours, which should offer enough time for any rooftop water to either evaporate or drain from the roof. Substantial amounts of standing water can remain on your roof for weeks, until it eventually weakens the roofing seams, membranes, flashing details, and sealants and finds its way inside your building.
Even without a punctured membrane or vulnerable seams, ponding water prematurely ages the membrane which increases the likelihood of water infiltration in the future.
Clearing debris enables water to drain from the roof as it was intended to.
A typical commercial roof may have multiple HVAC units, electrical conduits, and gas lines – all of which rely on exposed metal for waterproofing. These systems are designed for weather exposure, not prolonged submersion in standing water. Water that collects around these metal components accelerates rust, corrosion, and eventual failure.
Alternate Causes of Ponding: Hail & Incidental Roof Damage
While debris and a lack of positive slope in critical areas are the two most common causes of ponding, various damages can also prevent water from draining from your low slope roof.
- Hail damage: Hail impacts can compress the underlying insulation, creating small divots that interfere with the intended drainage path. These divots collect water, further increasing the small depression’s load. The more obvious consequence of hail impacts is membrane tears, which can invite water into the substrate and result in the opposite problem: swelling. Either way, the intended drainage path can be interrupted by hail damage.
- Incidental damage: Common sources of incidental damage include dropped tools, HVAC panel corners scraping the membrane, or standard foot traffic. Water seeps into those punctures and saturates the substrate. Wet insulation loses its rigidity, so this foot traffic creates craters where water can pond.
Reducing Roof Load: How to Minimize Standing Water Weight
Just one inch of standing water weighs approximately 5 pounds per square foot. On a commercial roof with a ponding area of 600 square feet, or 20 by 30 feet, that’s more than 3,000 pounds of additional weight. If water accumulates to two or three inches, that strain can triple or quadruple, exceeding the load capacity of the structure. This kind of weight can cause sagging, cracking, or even roof collapse in severe cases.
That’s why routine cleaning and maintenance from a reputable roofing contractor is essential to protect your building.
Prevent Mold & Algae Growth Caused by Ponding on Commercial Roofs
Stagnant water creates a humid environment where mold, algae, and bacteria thrive. These growths will discolor the roof and can slowly degrade roofing materials, as well as introduce health hazards for occupants. Routine roof inspections will help prevent this type of growth from causing significant damage to your roofing system.
How Ponding Water Amplifies UV Damage on Commercial Roofs
Ponding water on your commercial roof can act like a magnifying glass, concentrating the sun’s harsh UV rays, accelerating the deterioration of materials directly beneath the water. This phenomenon is called the “ponding prism effect,” and over time, it can cause membranes to blister or crack. This is why it is so important to get rainwater off and away from your building as quickly as possible.
Choosing the Right Commercial Roofing Contractor for Drainage Maintenance & Design
When choosing a roofing contractor to perform yearly maintenance checks, you will want to choose one that has years of experience evaluating and maintaining commercial low slope roofs. An experienced, reputable roofing contractor will know exactly what to look for when searching for potential leaks and weak points in your roofing system. If these problem areas are addressed early, it could potentially save you thousands of dollars in avoidable damage to your roof, building, and business operations.
Summary and Key Takeaways:
- 3 Common Causes of Leaks: This series of articles covers the 3 most common causes of premature roof failure. This article (Part 3) focuses on the ramifications of poor drainage design. Click on the links provided to learn more.
3) Poor drainage design
- Proper Roof Drainage: Because most commercial roofs have low slopes and are nearly flat, it’s critically important that they have at least 2% slope throughout to ensure that rainwater quickly sheds off and away from your building.
- Keep Drains and Scuppers Clear: It’s important to keep scuppers, roof drains, and backup roof drains free of leaves, trash, or debris. If these drains get clogged and ponding water remains on your roof for too long, it will eventually find its way inside your roofing system.
- The Ponding Prism Effect: Ponding water on your commercial roof can act like a magnifying glass, concentrating the sun’s harsh UV rays. This will eventually cause premature aging and failure of your roofing material, eventually causing leaks and water damage.
- Roof Maintenance Is Critical: Even the best-designed commercial roofs require upkeep to perform as intended. Regular inspections and maintenance extends the service life of your roofing system and allows you to catch minor issues before they become expensive repairs.
