Resources · Valleys & gutters
Ponding on low-slope & flat additions
A lot of Geelong homes have a tiled main roof and a flat-roofed extension out the back — a sunroom, an alfresco, a carport, an added bedroom. Those low-slope sections are often where the leaks start, and the reason comes down to a single word: fall.
"Fall" is the gentle slope built into a roof so water runs off it. A pitched tile roof has plenty. A flat or low-slope roof has only a little — and it needs every bit of it. When that fall is inadequate, water doesn't drain away after rain. It sits. That standing water is called ponding, and it's the root of most low-slope leaks.
Why ponding causes leaks
Standing water finds weaknesses that running water never would. Given time to sit, it works into seams, laps and fixings, and the longer it pools the more pressure it puts on every join. Ponding also accelerates wear — sitting water breaks down coatings and membranes faster, and on metal it holds moisture against the surface and speeds up corrosion over time.
A pitched roof sheds water in seconds. A poorly drained flat roof can hold it for days — and water always wins a waiting game.
Why additions are so often the culprit
Extensions and outdoor-room conversions are frequently built to a budget, sometimes without the careful drainage design the main roof had. Fall gets compromised to keep ceiling heights or to tuck the new roof under an existing eave. Years later, the result is a section that ponds every time it rains. Because the leak is slow and the ceiling cavity is often closed in, the damage can be well advanced before anyone notices a stain.
The other warning signs
- Visible puddles or tide-mark stains on the roof surface that linger after rain stops.
- Sagging or a soft spot in the flat-roof ceiling below.
- Blistering, splitting or lifting in the roof membrane or coating.
- Rust streaks on a metal low-slope roof, especially along seams.
How low-slope roofs are repaired
The right fix depends on how bad the fall problem is. Sometimes drainage can be improved and the surface re-sealed or re-membraned. In worse cases the only durable answer is to re-establish proper fall so water actually runs to the outlet. Repairs use materials suited to sitting water — quality membranes or quality metal and sealants, rather than a coating that'll blister off again in a couple of summers. Patching the symptom without addressing the fall is why these leaks so often come back.
Flat sections also rely on their drainage outlets and gutters staying clear — our guide to leaf-clogged valleys and gutters is worth a read if yours sit under trees.
Flat extension giving you grief?
We'll assess the fall and the surface, and tell you what it'll take to stop the water sitting.