Water ponding on flat roofs is a serious issue that can lead to significant damage, even if the roof is protected with a torch-on or other waterproofing systems. While these systems are designed to resist water penetration, prolonged exposure to standing water accelerates deterioration and increases the risk of leaks, structural failure, and costly repairs. Below is a detailed breakdown of the detrimental effects of water ponding on flat roofs:
1. Accelerated Degradation of the Waterproofing System
• UV Degradation: Even though torch-on membranes have UV-resistant properties, prolonged water exposure intensifies the degradation process by magnifying UV rays, especially in high-heat climates like South Africa.
• Thermal Expansion and Contraction: Water ponding creates localized temperature fluctuations, causing the membrane to expand and contract more than other areas. Over time, this leads to cracks, blisters, and delamination.
• Chemical Breakdown: Stagnant water can interact with pollutants, acids, or chemicals in the atmosphere, leading to the gradual breakdown of the waterproofing system.
2. Increased Risk of Water Ingress
• Seam Failures: Torch-on membranes are typically installed in sheets with welded seams. Water ponding increases hydrostatic pressure, forcing water into any weak points or poorly sealed seams.
• Capillary Action: Water can travel through small cracks or defects in the membrane via capillary action, reaching underlying layers.
• Permeability Issues: While waterproofing systems provide a strong barrier, continuous water exposure may increase permeability over time, allowing moisture to seep in.
3. Structural Damage
• Increased Load on the Roof: Water is heavy—each square meter of 25mm-deep ponding water adds approximately 25 kg of additional weight. Over time, this extra load can:
• Cause deflection and sagging of the roof structure.
• Exacerbate ponding issues, creating a cycle of worsening damage.
• Lead to cracks in the concrete substrate or failure of timber/metal deck structures.
• Corrosion of Reinforcements: If water infiltrates through cracks in a concrete roof, it can corrode the steel reinforcements (rebar), reducing structural integrity.
• Rot in Timber Decks: Timber or plywood decks beneath the waterproofing system can start rotting if moisture reaches them, leading to structural instability.
4. Growth of Biological Contaminants
• Mold, Algae, and Fungi Growth: Ponded water encourages the growth of mold and algae, which can:
• Make the roof slippery and unsafe for maintenance crews.
• Create unsightly stains and bio-deterioration of the membrane.
• Weaken adhesion of the waterproofing system.
• Attracts Pests and Mosquitoes: Stagnant water is a breeding ground for mosquitoes and other pests, creating potential health hazards.
5. Reduced Lifespan of the Roof System
• Shortened Waterproofing Membrane Life: Even high-quality torch-on systems have a reduced lifespan when exposed to prolonged ponding, requiring premature replacement or costly repairs.
• Cracking and Splitting: As the waterproofing ages, exposure to ponding causes cracks and splits, making the roof more susceptible to leaks.
• Loss of Reflectivity (Cool Roof Systems): If the waterproofing system includes a reflective layer (such as an aluminized torch-on cap sheet), prolonged water exposure can cause dirt accumulation and deterioration, reducing its cooling effectiveness.
6. Damage to Interior Spaces and Insulation
• Leak Development: Once water penetrates through a damaged waterproofing system, it can enter the interior of the building, leading to:
• Ceiling and wall stains.
• Plasterboard or paint deterioration.
• Damage to electrical systems (fire hazard).
• Insulation Saturation: In insulated flat roofs, water infiltration can saturate the insulation material, leading to:
• Reduced thermal efficiency.
• Increased energy costs due to loss of insulation performance.
• Potential mold growth within insulation layers.
7. Compromised Adhesion of Overlays or Coatings
• If an additional coating (e.g., acrylic, polyurethane, or silicone) is applied over the torch-on membrane, water ponding can cause:
• Loss of adhesion, leading to peeling or delamination.
• Premature failure of the coating.
• Blistering or bubbling due to trapped moisture.
Preventative Measures
To mitigate the risks associated with ponding water, the following strategies should be implemented:
1. Ensure Proper Roof Drainage:
• Install additional drains, scuppers, or tapered insulation to facilitate water runoff.
• Regularly clean debris from drains and gutters to prevent blockages.
2. Slope Correction:
• If the roof has inadequate slope, install tapered insulation or lightweight screeds to improve drainage.
3. Use High-Quality Waterproofing Systems:
• Consider reinforced torch-on membranes with additional protection in ponding areas.
• Use a moisture-resistant primer before installing waterproofing layers.
4. Apply Ponding-Resistant Coatings:
• Use coatings specifically designed to withstand ponding water, such as polyurethane or silicone-based coatings.
5. Regular Maintenance and Inspection:
• Conduct routine inspections to identify early signs of ponding and waterproofing failure.
• Repair any minor cracks, blisters, or open seams before they develop into major leaks.
Conclusion
While torch-on and other waterproofing systems are designed to resist water, prolonged ponding significantly accelerates deterioration, leading to leaks, structural damage, biological growth, and costly repairs. Proper drainage, high-quality materials, and regular maintenance are essential to ensuring the long-term performance of a flat roof. If ponding is unavoidable, specialized coatings and reinforced membranes should be considered to enhance durability and resistance to standing water.
