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Valsir S.p.a
Waste and water
supply systems, underfloor heating system
and flush cisterns


Rainplus®, the ultimate solution in case of rain


Rainplus® represents the most modern technology in the constructions sector and in the drainage of rainfall from the roofs of medium and large size buildings. The system makes use of the building height as the driving force, which together with the effect of negative pressure, allows high speed flow rates to be reached thus maximising drainage efficiency.

Rainplus® allows the entire flow of water to be directed to any part of the building whatsoever, thus enabling the most modern rainwater harvesting systems to be installed as required by the Green Buildings Program.

The Rainplus® siphonic drainage system is made up of special outlets that are patented, designed and tested to American Standard ASME A112.6.9 and European Standard EN 1253 and connected to HDPE® high density polyethylene pipes. The outlets also incorporate special components that allow installation on any type of roof and waterproofing sheet.

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Characteristics of excellence

The advantages of Rainplus®


When compared to conventional systems, Rainplus® requires a lesser number of roof outlets and allows a substantial reduction in pipe diameters, in the number of fittings required and the number of downpipes: this results in savings of up to 80% on vertical pipes and from 20% to 30% across the entire system.

Space saving

The roof outlets are connected to single horizontal collector pipes that are fitted without fall and the downpipes are positioned anywhere along the building perimeter thus avoiding interference.

High performance

When in operation drainage pipes flow at 100% full over the entire system, draining velocity is therefore greater resulting in the self cleaning action of the pipework.

Eco sustainability

The ease in directing the pipes to storage tanks makes the collection of rainwater easier for reuse in irrigation systems, fire ponds and tanks for non-potable uses in general.

Time and labour saving

Construction programmes are accelerated due to reduced installation times and less groundwork is required due to the reduced number of embedded pipes.

Increased design flexibility

Complete control over downpipe location and absence of embedded pipes gives increased design flexibility of the siphonic system.

Ecosostenibilità rainplus


The Valsir Rainplus® system and HDPE are produced with completely recyclable materials which, at the end of their useful life can be recovered. The production processes employed are energy efficient and of reduced impact. Valsir has adopted the Green Building principles in terms of respect for the environment and conservation of resources, pipes and fittings are in fact certified by the prestigious Singapore Green Building.

Conventional systems VS. Rainplus system

There are two types of rainwater drainage systems: conventional systems that are incorrectly called gravity systems, and the Rainplus® siphonic system, which is also known as a vacuum system or full section system.

Both use the force of gravity but in a decidedly different manner, resulting in differences in performance, design and calculation.

conventional drainage system

A conventional drainage system can be designed for large surface areas but does not cut off air flow into the pipe. For this reason pipes are sized for filling ratios of 20% or 33% (depending on national or local standards and regulations) allowing considerable amounts of air into the pipes of 80% or 67% of the pipe section.

The Rainplus® siphonic drainage system®

The Rainplus® siphonic drainage system is made up of special outlets that incorporate an anti-vortex plate that prevents air entering into the pipes. The outlets are connected via short pipes of relatively small diameters to the horizontal water collector which is located just under the building roof.

Rainplus®, how does it work?

Rainplus® is defined as a siphonic rainwater drainage system because it is based on the same principle as a siphon. The siphon is in general a reversed U shaped pipe used to pour a fluid from one container to another located in a lower position.

Bernoulli’s principle

In fluid dynamics, Bernoulli’s principle states that for an inviscid flow, an increase in the speed of the fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid’s potential energy. Bernoulli’s principle is named after the Dutch-Swiss mathematician Daniel Bernoulli who published his principle in his book Hydrodynamica in 1738.

Rainplus®, integrated engineering

A high degree of understanding of the Rainplus® operating rules and system flow regime is required to design and construct correctly performing and safe siphonic systems.

Unlike conventional gravity systems, a high degree of engineering and expertise is necessary, both in the design and construction phase.

Flow stages


EN 1253 and ASME A112.6.9 technical standards are used to evaluate the performance of a siphonic system and, as they define the measurement method of the flow rates as a function of the water depth on the roof, they allow the flow regime phases to be analyzed.


Stage 1

With a moderate flow, of 10 or 15% of the design rainfall intensity value, the roof outlet works as it would in a conventional system and the flow is defined “gravity flow”, in that air content in the pipes is elevated.


Stage 2

When the water discharged from the roof is between 10 to 15% and 60% of the full bore flow condition, water flow is discontinuous and the system therefore fluctuates from a gravitational flow regime to a full siphonic action. At these rainfall values the water that accumulates on the roof fills the outlet, cutting off air flow into the pipe and triggering the siphonic action.

The velocity of water discharged therefore increases which results in falling water levels, allowing air to be drawn into the piping network and breaking the siphon; for this reason this stage is called “plug flow”.


Stage 3.

When the water discharged is between 60% and 95% of the design rainfall intensity value, the pipes are completely full of water although many air bubbles are still present. This stage is called “bubble flow” and features high flow velocities thanks to the siphonic effect.


Stage 4

When the flow is over 95% of the design rainfall intensity value, the siphonic effect operates at full capacity reaching maximum velocity with no air entering the pipes. This stage is called “full flow” and does not produce noise or vibrations.

System components

Outlets and accessories

One of the key elements of the system is the Rainplus® outlet that has been designed and manufactured to meet the requirements and testing criteria set by the international standards EN 1253 and ASME A112.6.9.

Kit Overflow Rainplus®

Local regulations or standards on the design of rainwater systems may require emergency systems (also known as overflow systems) to be provided that are capable of draining unexpectedly intense rainfall that exceeds the design rainfall level.

Bracketing systems

Valsir offers a wide range of bracketing systems and accessories for installing the entire drainage network. The Rainplus® bracketing system, composed of special clips together with support rails and relative accessories, is designed to resist the forces of heat contraction and expansion in the drainage network and is available in diameter 40 mm up to diameter 315 mm

Pipes and fittings

The Valsir HDPE product line for the creation of siphonic drainage systems is available in diameters 40 mm up to diameter 315 mm and is characterised by a wide range of fittings such as access pipes, elbows, reducers and reduced branches.

Types of installation

One system, different types of installation

Installation in a metallic gutter by welding

Installation in a metallic gutter by rivets

Installation with plastic waterproofing membrane

Installation with bituminous waterproofing membrane

A quality made system

The Rainplus® siphonic system is made up of roof outlets, pipe clamps and other clamping accessories, Valsir HDPE pipes and fittings, a software for 3D design and extremely professional computations.

Valsir HDPE pipes and fittings are manufactured to EN 1519 and have been approved to many standards worldwide (CSTB, IIP, KIWA, SKZ, ETA, Lloyd’s Register, etc.).

The light weight and wide range of pieces available make Valsir HDPE the ideal solution for the construction of siphonic drainage circuits. Valsir HDPE also offers great installation flexibility thanks to the various jointing methods, such as butt-welding and electrofusion.



To download the details of product approvals register to download area

Case history

Rainplus is an innovative system that was installed all over the world.


Sydney (Australia)


Novo Mesto (Slovenia)

Blue Route Mall

Capetown (South Africa)

General Electric Oil & Gas

Perth (Australia)


Valsir provides complete support both during the planning phase and on site, thanks to a first-class technical office made up of a team of highly experienced engineers, capable of dealing with the most complex system requirements.

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