Retention tanks Roth RT 3500–10,000 L – 50% or 100% retention · throttled outflow · DN 600
Retention tanks hold rainwater back and release it into the sewer with a time delay and at a throttled rate – for compliance with the outflow quantities specified in the development plan and drainage statute. The Roth retention tank on a Twinbloc® basis is available in five sizes from 3500 to 10,000 litres, optionally with 50% or 100% retention volume, with an integrated filter basket, DN 600 shaft and a floating retention throttle.
What is a retention tank?
A retention tank collects rainwater from the roof or surface and releases it into the sewer with a time delay and at a regulated volume via a throttle. It prevents peak outflows during heavy rain that would overload the sewer network. Many municipalities and development plans require the retention of precipitation with defined outflow quantities – the retention tank meets this requirement.
Retention throttle – the core element
The floating retention throttle (standard: 1", flow rate 0.07–0.5 l/s) sits in the tank and limits the outflow quantity regardless of the fill level. The water drains at the prescribed maximum rate – even when the tank is full after heavy rain. Throttle to 2" or 3" on request. The exact setting depends on the requirements of the local drainage statute.
50% or 100% retention
With 50% retention, the tank is split in half: 50% usable volume (extraction for garden, toilet) and 50% retention volume (retention buffer). With 100% retention, the entire tank contents serve retention – no usable water is extracted, everything is released to the sewer in a throttled manner. Which option is required is determined by the development plan or the drainage permit.
Twinbloc® basis & double tank
The smaller models RT 3500, RT 5000 and RT 6000 are single tanks on a Twinbloc® basis – flat, ground-stable, car-driveable with an optional cover. The large models RT 7000 and RT 10000 each consist of two coupled Twinbloc® base tanks. All models have a filter basket (1 mm), DN 600 shaft and a walk-on plastic cover as standard.
Retention tank 3500L to 10,000L, with 50% or 100% retention capacity. 💧
Delivery time: 5-10 Tage
Planning & installing a retention tank – what you need to know
What do I need a retention tank for?
Many development plans and municipal drainage statutes require that precipitation water be retained on the property and discharged into the sewer only at a certain maximum rate (e.g. 2 l/s·ha). A retention tank meets this requirement: it buffers the peak outflow during heavy rain and releases the water evenly in a throttled manner – the sewer is relieved and the requirement is met.
How does the retention throttle work?
The floating retention throttle is a calibrated outflow valve in the tank that always lets the same amount of water pass per second, regardless of the fill level. Standard: 1" with 0.07–0.5 l/s. When the tank is full after heavy rain, the water still only drains at the set rate – the rest stays in the tank and drains off over hours or days. The throttle size is specified by the official drainage permit.
50% or 100% – what applies to me?
This depends on the requirements in your development plan or building permit: 100% retention: the entire tank volume serves retention – no usable-water extraction. The water is discharged entirely throttled into the sewer. 50% retention: only half the volume is a retention buffer, the other half can be extracted as usable water (garden, toilet). A combination of retention obligation and rainwater use.
Which volume do I need?
The required retention volume is calculated from: outflow yield (l/s·ha, specified), rainfall yield (per DIN 1986-100 or local), and connected area (roof + sealed surfaces). Rule of thumb: per 100 m² of sealed surface, about 1000–3000 litres of retention volume are needed, depending on precipitation intensity and outflow specification. The exact calculation is carried out by a specialist planner or your building authority.
Car-driveable – when does it make sense?
The Roth retention tank comes as standard with a walk-on plastic cover. For installation under car parking spaces, driveways or garage entrances, the optional Roth car-driveable shaft cover is required. This increases the permissible load and must be installed in a statically correct manner. For truck traffic, a special design is necessary – on request.
Filter basket – why important?
The integrated filter basket (mesh size 1 mm) holds back leaves, sediment and coarse dirt before the water reaches the retention volume. This prevents the throttle from becoming clogged by suspended matter and impairing the retention function. Regular cleaning of the filter basket (at least annually, more often after heavy-rain events) is mandatory for the lasting functional reliability of the system.
Frequently asked questions about retention tanks
The right retention tank for your construction project
Our specialist team advises you free of charge – on volume, throttle size, retention and the official requirements at your location.
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