Flat roofs are common on modern homes, extensions, garages and balconies around Canberra. They look clean and can make great use of space, but they have one well-known weakness: water doesn’t always run off as quickly as it should.
When water sits in puddles on a flat roof long after the rain has stopped, it’s known as ponding. Here’s why it happens, why it matters and what can be done about it.
What counts as ponding?
No roof is perfectly flat. So-called flat roofs are built with a slight fall so water drains towards gutters, outlets or scuppers. A few small puddles that disappear shortly after rain are not usually a concern.
Ponding is when water stays put for a long time after rain, often a day or two in good drying weather. If you see the same puddles in the same spots after every rain, it’s worth looking into.
Why ponding is a problem
Standing water puts extra stress on a flat roof in several ways:
- Membrane wear. Constant moisture and sun exposure can break down some waterproofing membranes faster.
- Leaks. If there’s a small defect in the membrane, standing water gives it more time to find its way through.
- Extra weight. Water is heavy. Large ponds add load to the roof structure, which can make sagging worse.
- Debris and growth. Ponds collect dirt and leaves and can encourage moss, algae and even small plants.
- Freeze-thaw. On frosty Canberra nights, water can freeze and expand in any gaps or seams.
Common causes
Not enough fall
The roof may have been built with too little slope, or with a slope that doesn’t lead clearly to an outlet.
Blocked outlets and gutters
Leaves, dirt and debris can block outlets, rainwater heads and gutters. Water then backs up across the roof.
Sagging structure
Over time, timber can deflect or sag, particularly if it has been exposed to moisture. This creates low spots where water collects, which then adds more weight and makes the sag worse.
Uneven repairs or additions
Patches, extra layers of membrane or equipment like air-conditioning units can create ridges that trap water.
Too few outlets
Some roofs simply don’t have enough drainage points for the area they cover, especially during heavy Canberra storms.
What you can check from the ground
Please don’t climb onto a flat roof to investigate, even if it seems easy to reach. Membranes can be slippery and edges are often unprotected.
Instead, you can:
- Look from an upstairs window or a safe vantage point after rain
- Check whether downpipes are flowing during rain
- Look for stains or damp patches on ceilings below the flat roof
- Note how long puddles last and where they are
Practical tip: Take a photo of the roof from the same spot a few hours after rain, then again the next day. This gives a roofer a clear picture of where water sits and for how long.
How ponding is fixed
The right fix depends on the cause. A roofer will usually start by inspecting the membrane, outlets and structure.
Clearing and upgrading drainage
Sometimes the solution is as simple as clearing blocked outlets. In other cases, adding extra outlets, larger rainwater heads or overflow points can help water leave the roof faster.
Re-grading low spots
Low areas can be built up with a suitable screed or tapered insulation to create a better fall towards outlets. A new membrane is then applied over the top.
Repairing or replacing the membrane
If the membrane has been damaged by long-term ponding, it may need targeted repairs or full replacement. Modern liquid-applied and sheet membranes each have their place, and your roofer can explain which suits your roof.
Addressing structural issues
Where sagging is the cause, the structure may need to be assessed and repaired before any waterproofing work. Fixing the membrane without fixing the sag usually means the problem returns.
Preventing future ponding
- Keep outlets and gutters clear, especially in autumn
- Avoid storing items or placing heavy pots on the roof
- Get the roof inspected regularly and after major storms
- Deal with small repairs promptly
If you’ve noticed water sitting on your flat roof, the Roofing team can take a look. Book a free inspection and we’ll explain the cause and your repair options.
