What is ponding on flat roofs?
Ponding is the standing water that remains on a flat roof 48 hours after rainfall. A well designed flat roof should shed water quickly. When it does not, the water finds the low spots and sits there. Over time that standing water stresses the waterproofing, adds load to the structure and encourages silt and plant growth.
Why ponding happens
Inadequate falls or deck deflection
Flat roofs are not truly flat. They need a gentle slope so water flows to the outlets. The industry rule of thumb is a finished fall of around 1:80, often designed at 1:40 to allow for construction tolerances. If the original falls were never built in, or if timber joists have deflected, shallow hollows form and water collects.
Blocked outlets and gutters
Leaves from Midhurst’s many oaks and beeches can block outlets, hopper heads and valley gutters. Silt washed from high points settles in the low areas and makes the pond deeper. Even a modest blockage can back water up across a wide area.
Poor detailing and overloading
Low parapet weirs, too few outlets, air conditioning plinths and walkway pads can all interrupt drainage paths. Heavy roof build ups and stored materials weigh the deck down and may increase deflection at mid span, which worsens ponding.
Risks to your membrane and building
Persistent ponds shorten the life of most membranes. UV light reflects off water and accelerates ageing. Freeze and thaw cycles open up small cracks. Silt and algae keep the surface damp which encourages blisters. Standing water finds weak laps, penetrations and poorly sealed trims, leading to leaks, stained ceilings and damp insulation. Extra water load also increases structural stress. If left untreated the problem usually grows as the pond deepens and the deck sags.
Fixes that restore drainage
Quick wins and maintenance
Start with a clean roof. Clear outlets, gutters and hopper heads, fit leaf guards and remove debris. Re cut small chases to guide water to outlets. Make sure plant stands and cables do not form dams. A twice yearly clean in spring and autumn suits Midhurst’s leafy surroundings and keeps small issues from becoming big ones.
Re shaping the roof
Where the deck shape is to blame, we introduce new falls. Tapered insulation is a popular solution that improves thermal performance and gently redirects water to sumps or outlets. On concrete substrates, lightweight screeds can form crickets to break up wide spans. On timber, localised strengthening or sistering joists can reduce deflection before new finishes go down.
Improving outlets and overflows
Adding extra outlets reduces the distance water must travel. Sumps formed in insulation speed up drainage at low points. On roofs with parapets, scuppers and emergency overflows stop water from rising during cloudbursts. Where downpipes are undersized we up rate the rainwater goods so the system can cope with peak downpours common along the South Downs.
Membrane upgrades
If the waterproofing is tired, we replace it with a system suited to your roof and budget. Options include high performance torch on felt, single ply, EPDM or cold applied liquid membranes that excel around complex details. We design the build up so laps sit out of low points and specify walk pads to protect traffic routes. Good detailing at upstands, rooflights and penetrations is key to long term performance.
How we diagnose ponding
Our surveys combine visual checks with level readings to map falls. We test outlets, review the deck for deflection and where necessary take core samples to confirm the existing build up. You receive a clear report with photographs, a drainage plan and costed options, so you can choose the right remedy with confidence.
Local know how in Midhurst
Midhurst has a mix of historic cottages, modern extensions and small commercial units. We work sympathetically on older properties and in conservation areas, coordinating with owners and managing agents to minimise disruption. From a small roof over a shop on Church Street to a large warm roof near the River Rother, we tailor the fix to the building and its use.
Book a ponding survey
If you have standing water after rain, damp ceilings or algae on your flat roof, we can help. Contact the team at Roofing Midhurst to arrange a friendly site visit. We will pinpoint the cause, advise on the best route to restore drainage and protect the membrane, then deliver a neat long lasting repair.