Publishing and copyright information

The BSI copyright notice displayed in this document indicates when the document was last issued.

The following BSI references relate to the work on this standard:

Committee reference B/505
Draft for comment 06/30150944 DC

Publication history

First published March 2007

Foreword

Publishing information

This British Standard is published by BSI and came into effect on 30 March 2007. It was prepared by Technical Committee B/505, Waste water engineering. A list of organizations represented on this committee can be obtained on request to its secretary.

Relationship with other publications

This British Standard should be read in conjunction with BS EN 12056‑3.

BS EN 12056‑3 deals with the layout and calculation of roof drainage. The provisions for non-siphonic systems are described in detail, but those for siphonic systems are limited to performance requirements. The standard applies to all materials used for roof drainage systems.

This British Standard gives information facilitating the use of BS EN 12056‑3 in the United Kingdom with regard to siphonic roof drainage systems; it does not alter any of the provisions of that bsfunded by the Department of Trade and Industry as part of its Partners in Innovation scheme [1].

The study involved extensive consultation with manufacturers and installers of siphonic roof drainage systems used in the UK and also with specifiers.

Use of this document

As a guide, this British Standard takes the form of guidance and recommendations. It should not be quoted as if it were a specification and particular care should be taken to ensure that claims of compliance are not misleading.

This British Standard has been prepared to give information about the majority of installations of siphonic systems but it is impractical to cover every eventuality. In some particular cases, it might not be possible to reflect all the provisions of this standard. In such cases it is advisable to discuss any departures with the client or the client’s appropriately qualified representative.

Any user claiming compliance with this British Standard is expected to be able to justify any course of action that deviates from its recommendations.

Presentational conventions

The provisions in this standard are presented in roman (i.e. upright) type. Its recommendations are expressed in sentences in which the principal auxiliary verb is “should”.

Commentary, explanation and general informative material is presented in italic type, and does not constitute a normative element.

Contractual and legal considerations

This publication does not purport to include all the necessary provisions of a contract. Users are responsible for its correct application.

Compliance with a British Standard cannot confer immunity from legal obligations.

Introduction

BS EN 12056‑3:2000, 6.2 specifies the performance requirements for siphonic systems as follows:

6.2.1 The system shall drain the runoff from the impermeable area served, calculated in accordance with clause 4, without taking gutter storage into consideration. It is recommended that the recurrence period method (see 4.2.1) should be used whenever possible.

6.2.2 For flat roof drainage, the system shall conform to clause 5.4. For gutter drainage, the system shall conform to clauses 5.1 and 5.2.

6.2.3 The siphonic effect shall commence quickly enough to prevent design water depths on the roof or in the gutter being exceeded.

6.2.4 The flow capacity of individual outlets shall be balanced to ensure the entire system performs as specified.

6.2.5 The siphonic system shall be designed to take into account any surcharging of the buried drainage.

6.2.6 Pipes and fittings shall withstand the maximum positive and negative pressures encountered under design conditions.

6.2.7 The minimum velocity in the system at the design rainfall shall be chosen to prevent deposition in pipework and to ensure rapid commencement of the siphonic effect.

6.2.8 Outlets shall be fitted with strainers to exclude solid material and prevent blockages. The effect of a strainer on water levels in gutters or on flat roofs shall be taken into account.

6.2.9 The minimum internal pipe diameter, d_i, shall be 32 mm.

6.2.10 The design method shall be validated by physical testing.

6.2.11 The lowest design pressure shall be chosen to prevent possible cavitation damage and collapse of pipes.

6.2.12 Reductions in diameter in the direction of flow are permitted in siphonic systems.

6.2.13 The system shall be installed in accordance with the design assumptions. The effect of any difference between the design and the system as installed shall be calculated and appropriate action taken.

1 Scope

This British Standard applies to roof drainage systems using circular-section pipes that operate under siphonic action. It does not apply to siphonic drainage systems using rectangular or trapezoidal section conduits for water conveyance.

It gives information and provides guidance on design parameters, installation, verification and maintenance needs of siphonic roof drainage systems conforming to the performance requirements specified in BS EN 12056‑3:2003, 6.2. Guidance is also given on carrying out checks of designs, for the use of specifiers and regulatory authorities.

Particular issues for site drainage systems that receive flows from siphonic roof drainage systems are also covered.

2 Normative references

3 Terms, definitions and symbols

3.1 Terms and definitions

3.2 Symbols

4 General

4.1

4.2

4.3

5 Performance

6 Design parameters

6.1 Design rainfall intensity and flow rates

6.2 Available head

6.3 Minimum pressure in pipes

6.4 Minimum velocity in pipes

6.5 Balancing of systems

6.6 Roof drainage

6.7 Speed of priming

7 Components of siphonic systems

7.1 General

7.2 Siphonic outlets

7.3 Pipework

8 Hydraulic design

8.1 General principles

8.2 Siphonic outlets

8.3 Roof drainage

8.4 Design of pipework

8.5 Tailpipes

8.6 Collector pipes

8.7 Downpipes

8.8 Priming

8.9 Primary and secondary systems

8.10 Discharge arrangements

8.11 Overflows

8.12 Site drainage

9 Validation of designs

9.1 Outline checks

9.2 Design software

10 Installation

10.1 Preventing blockages during construction

10.2 Pipework jointing

10.3 Fixings and support

10.4 Insulation/trace heating

10.5 Acoustic insulation

10.6 Tolerances

10.7 Changes from design

10.8 Health and safety

11 Testing and commissioning

11.1 Inspection

11.2 Flow testing

11.3 Positive pressure testing

11.4 Handover

12 Maintenance, inspection and cleaning

12.1

12.2

12.3

12.4

12.5

13 Information to be provided

13.1 By the specifier

13.2 By the system designer/installer

Bibliography

BS 8490:2007

Guide to siphonic roof drainage systems

Published:

30 Mar 2007

Withdrawn:

21 Feb 2025