Ponding Water

Ponding water is rainwater or meltwater that collects in low areas of a roof and fails to drain. In North American practice ponding is usually defined as water still present 48 hours after rainfall ends under drying conditions. Short-lived puddles that evaporate quickly are generally not considered ponding.

Causes

• too little slope or fall in the original design
• deck deflection, which creates a dip that holds water and then deflects further under the added weight
• construction tolerances, uneven decks and backfalls
• compressed or dented insulation under traffic or equipment loads
• drains set too high, too few outlets, or outlets blocked by leaves, dirt and algae
• interrupted falls behind rooflights, plant bases and upstands
• membrane laps running against the direction of flow

Effects

Ponding speeds up membrane ageing and surface erosion, encourages plant and fungal growth, and lets more water enter if the membrane is punctured. It adds dead load: 25 mm (1 in) of water over 20 m2 (215 sq ft) weighs about 500 kg (1,100 lb). Water left on a roof can freeze, creating slip hazards and stressing some membranes. On inverted roofs it also reduces thermal performance. In severe cases the cycle of deflection and accumulation, called ponding instability, can lead to structural failure, which is why US codes require a structural check for roofs designed with slopes below 1/4 in per 12 in.

Prevention and correction

• design adequate slope (see positive drainage)
• use tapered insulation, crickets and saddles
• add drains or overflow outlets where surveys show low spots
• recess outlets into sumps
• keep drains and gutters clear through routine inspections, typically in spring and autumn

Product ratings

Many coatings and some membranes are not warranted for ponding water. Acrylic coating manufacturers commonly exclude it, while some silicone and polyurethane coatings include it. Specifiers check product literature when ponding cannot be eliminated.