A rainwater tank comes in three types: underground, rigid above-ground or flexible pillow tank. An underground tank is hidden but needs excavation. A rigid above-ground tank goes in quickly but holds little. A pillow tank sits on level ground with no building work, under a deck or in a crawl space: 1 to 10 m³ online, larger sizes made to measure. Installed in autumn, it will have filled up by summer.
Underground, rigid or flexible: which rainwater tank should you choose?
A pillow tank wins when you want to avoid digging, store several cubic metres or make use of a low space. An underground tank still makes sense when excavation work is already planned.
| Criterion | Underground tank | Rigid above-ground tank | Pillow tank |
|---|---|---|---|
| Work involved | Excavation, lifting equipment, backfill | Flat, stable base | Level, protected ground, no digging |
| Space | Hidden once backfilled | Tall and visible | Low: on the ground, under a deck, in a crawl space |
| Typical volume | A few m³ to a few dozen | A few hundred litres to a few m³ | 1 to 10 m³ online, up to 500 m³ made to measure |
| Installation time | Several days | A few hours | A few hours once the base is ready |
| Maintenance | Manhole access, pump required | Drain before frost | Visual check, drain at the low point |
| Warranty and lifespan | Depends on the model | Depends on the model | 5-year warranty, 10 to 15 years |
Our pillow tanks are made of UV-treated, polyester-reinforced PVC fabric; they are closed and opaque. Full comparison: pillow tank or rigid tank.
Our rainwater pillow tanks
The price shown on the product page excludes valves and accessories. Free delivery in mainland France and Europe.
What size of rainwater harvesting tank do you need for your roof and garden?
One millimetre of rain on one square metre of roof gives one litre of water, minus losses.
Cautious rule of thumb: a runoff coefficient of 0.8 for a pitched roof and 0.5 for a flat roof; a tank of about 6% of the annual volume, i.e. nearly three weeks’ reserve. Full method: sizing a pillow tank.
How much space does a 10,000-litre rainwater tank need?
Laid flat, our 10,000-litre pillow tank measures 4.32 × 4.23 m and weighs about 48 kg empty.
| Volume | Laid flat | Empty weight |
|---|---|---|
| 1 m³ | 2.40 × 1.45 m | about 10 kg |
| 2 m³ | 2.90 × 2.00 m | about 18 kg |
| 3 m³ | 4.22 × 2.21 m | about 22 kg |
| 4 m³ | see the product page | about 25 kg |
| 5 m³ | 3.47 × 2.86 m | see the product page |
| 6 m³ | 3.69 × 3.27 m | about 34 kg |
| 10 m³ | 4.32 × 4.23 m | about 48 kg |
| Over 10 m³ | to your dimensions | depends on volume |
An underground tank also involves excavation, lifting and backfilling, depending on the ground.
How do you install a rainwater pillow tank?
No slab, no digging: the tank sits on level, clean, compacted ground free of sharp stones, with a ground sheet underneath.
- Rainwater diverter on the downpipe, with a filter.
- Inlet at the top of the tank.
- Overflow to the outside, always fitted.
- Outlet at the bottom with a valve.
- Pump to the garden or the house.
For water under pressure, add a booster set. For the permitted uses of rainwater, check with your local authority.
Under a deck, in a crawl space or over 10 m³?
Our design office draws up a low tank to your dimensions, rectangular or round, overflow included, delivered in 6 to 9 weeks.
Orders of magnitude from projects studied by Flocea. At 3 m wide, a full tank stands 50 to 60 cm high. See our crawl space rainwater tanks.
An awkward space or more than 10 m³ to store?
Floor area, clear height and water use: our design office checks what volume will fit.
Store and use your rainwater