Common Failure Points for Low Slope Commercial Roofs: Part 2 – Low-Quality Materials

Whether 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 2), we’ll examine how material choice affects your roof’s performance against the hazards every roof faces.

Part 2 – Poor Quality Roofing Materials

Water doesn’t care how new your roof is. If the materials can’t stand the test of time or the elements, they will be the source of roof failure. High-quality roofing products are the foundation of roof performance over its entire lifespan. If a material can’t withstand hail, UV exposure, or wind uplift, even quality workmanship can’t preserve the roof system. Quality materials paired with expertise is what separates short-term fixes from long-term reliability. This article focuses on the inevitable hazards low slope roofs face, and how material quality affects the outcomes.

Why are high-quality roofing materials important?

The use of inferior-grade flashing and waterproofing materials is a leading cause of commercial roof leaks. Budget roofers often cut their biggest corners with flashing, relying on low-cost pipe boots, flashing materials, membranes, and sealants to keep their bids competitive. While this will reduce upfront costs, it also shortens the lifespan of the roof and significantly increases the likelihood of premature leaks.

Experienced commercial roofers have seen time and time again how these shortcuts invariably lead to premature failures. That’s why the most reputable roofing contractors prioritize long-term value by using durable, tested materials that can extend the roof’s life beyond its warranty.

An example of a core sample on a low slope roof.
In this example, the depth-to-deck is approximately 7″ which suggests that at least two roof layers are present.

Before vetting existing materials or deciding on future materials, a core sample should be taken. Coring down to the deck reveals how many roof layers are on your roof, what the slope of the roof is, the condition of the roof’s substrate, and what type of roof decking is present. Knowing each of these is a prerequisite that will help you and your roofing contractor determine which materials are needed for your roof.

How can Hail Affect Poor Quality Roofing Materials?

In the lower Midwest, hail is a priority consideration when designing a roof system. Cover board is effective at muting its effects on low slope roof membranes by adding rigidity, reducing the risk of punctures. The industry standard of 60-mil TPO & PVC exists to provide adequate membrane thickness, further reducing the risk of punctures from hail impacts while enabling a 20-year material warranty. Flashing with low-quality material, however, can nullify precautions like cover board and 60-mil TPO and expose your roof to hail damage.

The granules on a modified bitumen (Mod Bit) roof, along with its layered composition (i.e., a base sheet with fiberglass matting on the cover board, followed by the cap sheet with granules) provides excellent impact resistance. We’ve seen Mod Bit cap sheets installed directly to a roof’s insulation, skipping the base sheet entirely and crippling the roof in the process. Generally, all Mod Bit membranes that use base and cap sheets perform well against hail.

Membrane Flashing

While standard field membrane is reinforced with scrim and ideally cover board, detail membrane lacks the same rigidity. Unlike field membrane, detail membrane isn’t scrim-reinforced. This gives it the flexibility to conform to irregular shapes like penetrations, curbs, and terminations, but also creates a vulnerability: without at least 60-mil thickness, these areas may be punctured by hail impacts.

During hailstorms, detail membrane’s flexibility is both its strength and its source of risk. When hail strikes, rigid areas diffuse force across its structure. Flexible, scrimless materials can deform sharply like the area under your feet on a trampoline. This transfer of kinetic energy into a single point can produce localized ruptures unless the membrane is thick enough to resist puncture. 45-mil TPO and PVC is much less likely to withstand this localized impact, and its shorter 15-year maximum material warranty reflects that.

Mod bit doesn’t depend on thickness metrics like “mil” in the same way single-ply membranes do. Instead, its thickness is sourced from its layers. Thinner mod bit systems without proper cap or base sheets can make it more vulnerable to hail, but it still generally outperforms 45 mil TPO or PVC membranes.

Metal Flashing

Cheap, thin metal flashing strips can easily get dented by hail strikes or even incidental contact from a technician on the roof. Thin-gauge flashing will bend after hail impacts, and the metal will pull away from the surface it’s meant to protect. Galvanized steel or coated metal that’s custom fabricated to serve its purpose, whether it’s as coping caps over parapet walls or drip edge metals, is much more resistant to hail impacts.

Pipe Boots

A hail hit on a pre-formed or molded membrane boot is unlikely to cause punctures due to their molded shape and the material’s integrity. The mold makes installation easier and more consistent, but weld points that transition between the boot and the field membrane are still vulnerable. Reputable low slope commercial roofers often pair pre-formed boots with reinforced target patches at the weld point to 1) further support the seam, and 2) insulate the potentially vulnerable seam below from the elements.

How does UV Radiation Affect Poor Quality Roofing Materials?

If water is the enemy of a roof, UV radiation is its henchman. UV radiation’s resulting heat causes thermal expansion that materials must be equipped to handle, along with the potential for UV to degrade chemical bonds in membranes, sealants, and plastics in low quality materials. On a low slope roof, durability isn’t a luxury or an add-on: it’s a requirement.

A molded TPO pipe boot with a metal clamp ring and Water Block sealant. 

Sealants & Pipe Boots

• Sealant: Roofing-grade water sealants are like caulk that’s graded for UV and water exposure. Sealants that are incompatible with the roofing membrane, or that lack UV endurance, will deteriorate prematurely. Sealant is used as a redundancy in many low slope roofs’ most vulnerable areas, including termination bars, curb flashings, flanges for interior drains, cut-edge sealant near perimeter edges, and sometimes coping caps. Using low quality or membrane incompatible sealant that degrades with UV exposure can allow gradual water infiltration that can go unnoticed while it saturates the substrate. Low-grade sealants start to dry-shrink and crack, pulling away within a year or two if they are consistently exposed to UV radiation.
  • • Whether sealant serves as a redundancy or a critical waterproofing method, using the right materials isn’t optional – it’s essential. Cutting corners on material quality just to land a job may lower the bid, but it risks future failure that the building owner ends up paying for. Durable, roofing-grade sealants should be standard on every roof, not an upgrade. Roofing contractors should set building owners up for success, not expensive repairs.
• Pipe Boots & Pitch Pockets: Some components rely on sealants as their primary waterproofing method. Pitch pockets rely on high quality, chemically compatible sealants. To learn about pitch pockets, read our comprehensive guide to pitch pockets here. Pipe boots use water block to create a continuous seal near the clamp ring. If corners are cut here, it can produce immediate and costly leaks.

Membrane

• TPO: 45-mil membranes lose tensile strength more quickly than thicker 60 mil and 80 mil membranes. TPO’s UV inhibitors are in the membrane, not the scrim backing. The thinner the membrane, the more quickly this protective top layer erodes – there’s too little of it to offer long-lasting protection against harsh UV radiation. The same thinness that makes it vulnerable to hail also accelerates UV degradation. In both cases, the membrane simply wears out faster.
• PVC: Polyvinyl chloride is inherently more UV resistant than thermoplastic polyolefin (TPO), but PVC membranes are still imbued with UV stabilizers. Consequently, thinner 45-mil membranes are degraded more quickly by UV radiation than standard 60-mil membranes.
• Mod Bit: Mod bit and asphalt shingles on steep slope roofs respond similar to UV radiation. Over time, cracking or alligatoring can occur. This is endemic in low-quality mod bit cap sheets which generally have worse granule adhesion, lower granule density, and thinner backing. Using high quality cap sheets provides a maintainable material that’s better-equipped to handle UV.

How can Wind Uplift Affect Poor Quality Roofing Materials?

In the lower Midwest, which is replete with high winds and lacks natural windbreaks, wind has the potential to cause premature roof failure. Uplift pressure isn’t evenly distributed. Corners and perimeter edges experience up to 3x more uplift than the field of the roof. That’s why the details of detail membranes and attachment methods matter: if one part fails, it compromises the entire system.

Effects of Wind Uplift on the Membrane

Membranes are mechanically fastened or adhered in specified patterns that mitigate negative pressure generated by wind across the roof. The perimeter and corners are most vulnerable to wind uplift and require additional fasteners or adhesives. Choosing the correct materials to install the membrane with can be the difference between a 30-year roof that outlasts its warranty and a 10-year roof that matches its installation price in repair costs.

• Fasteners: The fasteners used on a mechanically attached membrane must penetrate deep enough to secure into the roof deck, not just the insulation or cover board. This is one reason why a core sample is critical: it provides you with the depth to deck. If your roof requires a layover, knowing the depth to deck informs you of the necessary fastener length to secure through the roof deck.
  • • Some decks, like concrete, make mechanical fastening prohibitively difficult and expensive. Mechanically attached membranes on concrete decks are consistently 20 percent more expensive than fully adhered securement.
    • Metal decks, however, must be tested to ensure proper securement. The thicker the metal roof’s gauge, the better securement it will offer. Any metal decks less than 22-gauge thick should be assessed with a pull test to determine the efficacy of mechanical attachment. By fastening to the top ridge of a metal deck, installers can use shorter screws, which reduces material costs while maintaining proper securement. If the roof deck is thinner than 22-gauge, purlin attachment – fastening through the building’s structural members – might be necessary to maintain a strong hold.
    • Using the correct fasteners and applying them to meet manufacturers’ specifications is a prerequisite for the best warranties on low slope roofs. If you want a low slope roof that outlasts the warranty, mitigating wind uplift with proper materials is the starting point – not a luxury.
    • Mod Bit Securement: Mod bit roofs are usually fully adhered or torched down, neither of which requires mechanical fasteners. The correct fasteners are still required for mechanically attached insulation and cover board, however. Rolls of mod bit are also much smaller than TPO or PVC rolls which produces approximately 2.5x to 3x more seams.
• Perimeter & Corners: The perimeter of a low slope roof faces wind uplift forces that are three times higher than the field of the roof. This requires significantly stronger securement for both TPO and PVC membranes.
  • • For a 10 ft x 10 ft square of membrane on the field of a roof, fasteners are generally spaced every 12” along seams. This results in approximately 20 seam fasteners per 100 sq. ft. on a mechanically attached roof. In corner zones, fasteners are used every 6” or less, resulting in 60 fasteners along the seams in the same 10 ft x 10 ft square. Securement standards are affected by the roof’s exposure to wind (e.g., trees that mitigate wind and neighboring buildings), building height (taller buildings are further from natural windbreaks like the ground and trees), and geographical location. A well-designed roof system with the right materials is like a well-written instruction manual: when it’s of high quality, it’s invisible; when it’s of low quality, it’s all you can see. While a poorly written instruction manual can be an inconvenience, a roof with poor quality materials will be a catastrophic expense.
    • Piranha Plates: Mule-Hide recommends piranha plates in many places, particularly at roof-to-wall transitions or other significant slope changes. Piranha plates, or barbed plates, are barbed or high-grip plates that help transfer wind uplift force from the membrane to the substrate. This reduces the risk of membrane tears by distributing these pressures. Seams that experience significant wind uplift are extremely vulnerable without piranha plates to distribute the pressure. Reputable and experienced low slope commercial roofers often use piranha plates as a redundancy even on fully adhered roof systems.

Effects of Wind Uplift on Edge Metals

Edge metals are a low slope roof’s front line of defense against wind uplift. Negative pressure generated by wind regularly buffets these metals, whether it’s coping caps over parapet walls, drip edge on open edges, or termination bars. Metals that are improperly secured, undersized, or fabricated using metal that’s too thin can turn your front line of defense into your first source of leaks.

Unlike the roof membrane or substrate, edge metals aren’t protected from the elements: they are the protection, making high-quality materials especially important. If the edge metal fails, the membrane fails right behind it.

Although these coping caps on commercial TPO parapet walls are adequately thick, they were exclusively face-fastened, omitting continuous cleats.
Previous repair attempts are visible between the TPO membrane and trim board.
This suggests that previous leaks have been sourced to the parapet walls.

 

• Coping Caps: Coping caps cover parapet walls and usually secure the membrane termination beneath them. Because they often occupy a low slope roof’s highest point on its perimeter, wind uplift resistance is critical. Systems that rely solely on face-fastened screws lack the integrity to endure years of wind uplift, especially in the lower Midwest. That’s why proper installation involves continuous cleats. Using continuous cleats distributes the uplift forces, preventing acute stress on the membrane and metal. Thin metals, usually 26-gauge or lighter, are more likely to flutter, deform, or outright detach. For taller parapets or areas with high wind exposure, using at least 22-gauge metal can mitigate these potential issues. Ultimately, a coping system is only as strong as its securement. Skipping cleats or exclusively using face fasteners is a material shortcut that compromises the entire edge.
• Drip Edge: Drip edge can be installed on open edges to combat wind uplift, water intrusion, and wind-driven rain. Similar to coping caps, drip edge can serve as the termination point for the membrane along the roof’s perimeter and best-practice deploys continuous cleats. Unlike coping caps, drip edges are fastened with ring shank nails on top, enhancing resistance to wind uplift. Consequently, detail membrane is laid over the top of the drip edge to prevent water from rusting the fasteners or flowing around them. Failing to either 1) use coated drip edge metal, or 2) prime uncoated drip edge metal will result in that protective detail membrane failing to secure and potentially blowing in the wind.The proper materials for drip edge on low slope roofs – continuous cleats, ring shank nails, detail membrane, and coated or primed edge metal – won’t matter if the metal comprises a low-quality alloy. Because using the correct materials can reliably secure a low slope roof’s perimeter, bids from budget roofers that reduce material costs at the roof’s perimeter aren’t a gamble: they’re a nearly guaranteed future leak.

How Important Is Material Compatibility in Commercial Roofing?

We briefly cited this in Part 1 of the Commercial Failure Points series, but it’s important to note that commercial roofing isn’t Legos – each piece doesn’t fit with every other piece. Using incompatible materials introduces avoidable failure points that shorten a roof’s lifespan and void potential warranties. To illustrate the importance of using materials that are compatible with each other, below is a list of some common material selection errors we see.

• A TPO membrane requires TPO pipe boots. We often find residential or metal roof pipe boots on single-ply membrane roofs, which will invariably produce a leak in time.
• Adhesives, especially two-part systems, require very specific use temperatures. Many layovers on roofs with strong wind uplift demand strong adhesives. Two-part adhesive systems combine two separate containers to produce a chemical reaction when mixed and sprayed. Attempting to use many adhesive systems below 50 degrees Fahrenheit won’t be effective. For fully adhered systems, the right materials require the right temperature and installation. If your roof is in a perpetually cold climate, consider mechanical fastening or an adhesive that functions in cold weather.
• Low slope roofs require low slope sealant and caulking. There are many reasons for this, but the most obvious reason is that low slope roofing materials are waterproof, while steep slope roofing materials are water shedding. Material designed for a roof that sheds water can’t withstand 48 hours of standing water like low slope roofing materials are designed to.

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 2) focuses on the ramifications of using inferior-quality roofing materials. Click on the links provided to learn more.

1) Improper installation

2) Inferior roofing materials

3) Poor drainage design

• Material quality produces system durability: No amount of skill can compensate for cheap flashing, sealants, or membranes. Using high-quality components – from pipe boots to drip edge – protects against UV, hail and wind.
• Experience matters: Quality materials still require quality installation to be effective. The best roofing contractors know what needs reinforced, what to prime and with which primer, and which fasteners are needed for your roof deck.
• Cheap bids cut corners: Low-budget roofers can cut corners and offer lower bids because they use low quality and cost flashing materials, waterproofing membranes, and sealants. Yes, they might be able to cover your roof for less upfront cost, but you will likely end up paying more in the long run when your roof begins to leak in 5 to 10 years. Choosing a knowledgeable and reputable roofing contractor today will save you time, money, and headaches in the long run.