From a flow point of view there are two main types of roof drainage system, gravity flow and full-bore flow (UV-) systems. In gravity flow systems downpipes are usually dimensioned with a fill-factor of 1/5 - 1/3. Lateral pipework may be designed to have a flow factor of 0.70 - 1.0. As a consequence the drainage capacity may be considered to be dependent upon the gradient of the laterally installed pipework.

In UV-Systems, all pipework has a flow factor of 1.0 at dimensional rainfall intensity and the pressure in the pipework differs from the prevailing atmospheric pressure. The full difference in height (head) between the roof outlets and the discharge point to the main drainage system at ground level is utilised when dimensioning the UV-System pipework. As a result considerably reduced pipe dimensions (when compared to gravity flow systems) are produced with consequent higher flow velocities in the pipework.

The basic principles established by Messrs. Ebeling & Sommerhein together with 3 decades experience of practical application are encapsulated in the UV-System Manual which is available in English, French, German and Swedish language versions.

National Standards bodies, Local Authorities and independent certifying agencies from various countries have examined and approved UV-System technology and outlets during the years since its invention. For example the British Board of Agrement awarded the original UV-System a BBA certificate in 1988. The current BBA certificate (95/3179) was completed in 1995.

Benefits of UV-Systems - In Brief

The key advantage of UV-Systems is the minimising of downpipe provision. Not only are downpipes reduced in number and diameter but the siting of drain points is totally flexible.

The elimination of air from pipework, both vertical and horizontal, is achieved firstly by the use of the extensively tested UV-System outlet which prevents air entrainment. Secondly UV-System dimensioning software utilises the height of the building as a hydraulic "head" balancing the friction and local losses in the pipe configuration and reducing the pipe diameters to the minimum necessary to transport the design volume of water. Hydraulic balance is achieved via computer by permutating the lateral and vertical pipe cross-sections.

The resultant full-bore flow or siphonic action allows the routing of an internal, high level, horizontally-installed collecting pipe system serving multiple roof outlet points. Ultimately each UV-System discharges via a vertical downpipe into the ventilated flow main drainage system.


Extensive experience of UV-System design and installation world-wide leads us to believe that although traditional roof drainage methods have been used for centuries, they are now being replaced by full-bore flow systems for the roof drainage of many contemporary building projects. Conventional gravity flow drainage systems cannot effectively dispose of the volumes of water precipitated on large and/or complex roofs without costly multiple discharge points inserted throughout the building. The sophistication of today’s buildings and the rigours of architectural and engineering criteria often make traditional roof drainage disposal impractical and out-of-keeping with modern advanced building technologies.