Ponding water is a common but extremely detrimental roof condition that can lead to structural problems with the roof deck, damage to the roof surface, unwanted vegetative growth and accumulation of damaging contaminants. Even if your roof is in good shape, it’s important to know when water is a problem – and take action.
What is ponding water?
Ponding water is defined as an excessive accumulation of water, typically in a low-lying area, that remains on a roof for 48 hours or longer even when conditions are suitable for drying. Left unresolved, ponding water leads to accelerated erosion and deterioration of the membrane surface and results in premature failure of the roof system.
Even reasonably small amounts of moisture intrusion underneath the roof membrane may decrease the thermal resistance of the insulation. More importantly, moisture intrusion can cause serious damage to the deck, insulation, and building interior as well as the membrane.
Common effects of ponding water:
- Damage to the roof surface. Ponding water magnifies UV rays, which prematurely age the membrane causing splits, cracking, and mineral loss. Ice can also build up and, with temperature changes, move across the roof surface “scrubbing” the membrane and potentially causing considerable physical damage.
- Deformation of the deck structure. Ponding water can substantially increase the load on roof decks. As water accumulates, deck deflections can increase and result in additional ponding water that could compromise the structural integrity of the deck.
- Growth of algae and vegetation. When water stands for long periods of time, algae and vegetation growth will likely occur, and may cause damage to the roof membrane. Additionally, vegetation and other debris can clog drains and cause additional ponding.
- Accumulation of dirt, debris and other contaminants. These elements can affect and damage the membrane surface over time.
Preventing ponding water, and avoiding these headaches, begins with proper roof design. A roof designed with adequate slope built in can shed water quickly and effectively – a minimum positive slope of ¼:12 is required on asphalt roof systems and is a good rule of thumb.
Beyond roof design, prevent these damages with proper installation, use of high-quality roofing materials, and proactive maintenance.