Roof drains and ponding water

A flat roof depends on its drains. When drains block or the roof surface sags, water ponds, and ponding water wears membranes faster, finds every weak seam and adds weight to the structure. Keeping drains clear and dealing with persistent ponding are among the simplest, most valuable things a building owner can do for a low-slope roof.

How low-slope roofs drain

A "flat" roof is built with a slight slope, through the structure, tapered insulation or crickets, that directs water to drainage points:

• Interior drains: openings in the roof connected to piping that runs down through the building to the storm system. They need strainers to keep debris out.
• Scuppers: openings through a parapet wall that let water run off the edge, often into a conductor head and downspout.
• Gutters: along roof edges without parapets.
• Overflow drains or scuppers: set slightly higher than the primary drains, to let water escape if the primary drains block.

Building and plumbing codes set requirements for roof drainage design, including secondary drainage in many cases. When a roof is altered, drainage should be reviewed by a qualified designer.

What counts as ponding, and why it matters

Some water standing on a low-slope roof right after rain is normal. Ponding is usually described as water that remains well after rain has stopped, commonly a couple of days in drying weather. Industry guidance on the exact definition varies, and roof warranties sometimes have their own wording, so check what applies to your roof.

Why ponding matters

• Faster wear: standing water, sun and temperature swings break down membranes and surfacing more quickly.
• Leaks: any open seam or puncture under a pond leaks continuously rather than briefly.
• Weight: water is heavy. Ponds can make a roof deflect more, which deepens the pond and adds more water. In extreme cases this cycle can lead to structural damage.
• Ice: in winter, ponds freeze, and ice can damage membranes and block drains.
• Debris and growth: ponds collect dirt and can support vegetation, which damages membranes.
• Warranties: some roof warranties exclude damage from ponding.

Common causes

1. Blocked drains or missing strainers, often from leaves, gravel or debris left by trades
2. Drains set too high relative to the surrounding roof
3. Deck deflection or settlement over time
4. Crushed or wet insulation creating low spots
5. Rooftop equipment or curbs that block water flow
6. Insufficient slope in the original design or after re-roofing

A drain maintenance routine

• Inspect drains, scuppers and gutters at least in spring and fall, and after major storms
• Remove debris from strainers and the area around each drain
• Check strainers are present and secured
• Look for damage to the membrane and flashing at each drain
• Check overflow drains and scuppers are clear and unobstructed
• Note any ponding areas and photograph them for comparison over time
• Ask contractors working on the roof to clean up after themselves
• In winter, check that drains are not frozen or buried in ice after thaws

Anyone going onto the roof needs safe access and fall protection appropriate to the roof's edges, skylights and openings. A planned building maintenance schedule makes sure this work isn't forgotten.

Fixing ponding

Fixes depend on the cause:

• Blocked drains: clean and, if needed, camera-inspect and clear the drain piping.
• Low spots: add tapered insulation or crickets during repairs or re-roofing to direct water to drains.
• Drain height: reset or add drains so they sit at the low points.
• Wet insulation: remove and replace wet sections, then repair the membrane.
• Structural deflection: have a structural engineer assess the deck and framing before adding any weight or making changes.

Ponding next to rooftop units and parapets also affects snow and ice in winter; see snow loads on roofs.

How ehZee can help

We inspect low-slope roofs with a focus on drainage: drains, scuppers, overflows, slope, ponding areas and the interior below. Our roof inspections identify the cause of ponding and recommend the right next step, including structural review where needed. Call 1-833-888-0091 or email contact@ehzee.ca.

Common questions