What is a polder roof?
A polder roof is a flat roof that deliberately holds rainwater in a storage layer and only then releases it slowly. The roof works like a small polder: store during a downpour, empty out gradually once the sewer has capacity again. In English-speaking practice the same idea is usually called a blue roof.
That is different from an ordinary flat roof, which is designed to shed water as fast as possible, and different again from a green roof, which retains water mainly in its substrate.
Build-up
Above the waterproofing sits a storage layer: usually plastic retention crates or a comparable open structure in which water stands between the ribs. Above that there may be a substrate layer with planting, or a paved or timber deck. Discharge is through a restrictor or a controlled valve.
Common crate depths are in the order of 85 to 150 mm; the storage capacity achieved depends on the system and lies roughly between 80 and 140 mm of water depth per square metre. For comparison: a heavy summer storm can easily deliver several tens of millimetres in a short time.
Static or dynamic
- Static (passive). A fixed restrictor opening lets water drain away at a constant, low rate. Simple, low maintenance and needing no power or sensors.
- Dynamic (smart). A control valve linked to the weather forecast empties the roof before a storm arrives and holds water back when it stays dry. The same storage layer is then far more effective: it is empty exactly when you need it.
| Static (passive) | A fixed restrictor opening drains water at a constant, low rate. Simple, low maintenance, no power or sensors needed. |
|---|---|
| Dynamic (smart) | A control valve linked to the weather forecast empties the roof before a storm arrives and holds water back when it stays dry. |
Why it is used
- Limiting flooding. In dense urban areas the sewer is the bottleneck during heavy rain. Water kept on the roof does not end up in the street.
- Meeting attenuation requirements. Many authorities set requirements for on-site rainwater attenuation for new build or major refurbishment, expressed in millimetres of storage per square metre of hard surface. On inner-city sites with no garden, the roof is often the only place this can go. Check the exact requirement with the local authority — it varies by location.
- Reducing heat stress. The stored water evaporates and cools the surroundings.
- Irrigation. A roof terrace or roof garden can be fed from its own store rather than from mains water.
What to watch
- Structure. Water weighs roughly 10 kg per m² per centimetre of depth. 100 mm of storage is therefore about 100 kg/m² of additional load the roof structure must be able to carry. Always have this checked by a structural engineer, certainly on an existing roof.
- Overflow. Essential. If the restrictor blocks, excess water must be able to escape safely and visibly — ideally through an overflow in the parapet.
- Waterproofing. The membrane stands permanently or for long periods under water. Choose a system and detailing designed for that, and keep leak detection possible.
- Maintenance. The restrictor opening and the filters need periodic checking.
Related terms
- Green roof
- Flat roof
- Rainwater drainage
- Roof covering
- Infiltration
Want to learn more about construction terms? Visit our knowledge base at fredsdiyplans.com.