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.

Investigating a ponding problem

Ponding has several possible causes, and the fix depends on which one applies. A methodical investigation looks like this:

1. Map the ponds. After a rain, mark each pond on a roof plan with its approximate size, and note how long it takes to dry. Photographs from the same spots each time make comparisons easy.
2. Check every drain. Is the strainer present, is the drain clear, and does it sit at the low point or above the surrounding membrane?
3. Look for obstructions. Curbs, sleepers, equipment supports and old repairs can act as small dams.
4. Feel the surface. Soft or spongy areas around a pond often mean wet or crushed insulation underneath. A moisture survey can confirm it.
5. Look below. From inside, check the deck and framing under the ponds for sag, staining or cracking.
6. Review the history. Has the roof been re-covered, have units been added, has the pond grown over the years?

A pond that sits at a clean, low drain points to piping problems. A pond away from any drain points to slope or deflection. A pond that is growing year after year points to the structure, and calls for a structural engineer.

Common mistakes

• Removing strainers to make water drain faster. Debris then goes into the piping, where it is much harder to clear.
• Patching over a pond. A new membrane layer in a low spot still holds water and adds a little more weight.
• Forgetting the overflows. Overflows blocked by debris or ice give no protection when the primary drain fails.
• Adding heavy equipment to a roof with ponding. New loads on a deck that is already deflecting need engineering review first.

A typical case

A single-storey warehouse has one large pond that now takes most of a week to dry after rain. The two nearest drains are clear, but one sits slightly above the membrane around it. The pond itself is in the middle of a bay, away from any drain, and the membrane there feels soft underfoot. From inside, the steel deck in that bay shows slight sag. The inspection report recommends a structural review of that bay before any further work, then replacement of the wet insulation with tapered insulation to drain the area, and lowering the high drain at the same time.

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