Manual for the Design of Roof Drainage Systems A guide to the use of European Standard BS EN 12056 3 2000 R W P May Report SR 620 March 2003Manual for the Design of Roof Drainage Systems A guide to the use of European Standard BS EN 12056 3 2000 R W P May Report SR 620 March 2003 Address and Registered Office HR Wallingford Ltd Howbery Park Wallingford OXON OX10 8BA Tel 44 0 1491 835381 Fax 44 0 1491 832233 Registered in England No 2562099 HR Wallingford is a wholly owned subsidiary of HR Wallingford Group Ltd ii SR 620 10 03 03 iii SR 620 10 03 03 Contract Research This report was prepared as part of an in house project funded wholly by HR Wallingford The HR job number was DFT0141 and the work was carried out by Mr R W P May of the Water Industry Group Prepared by name Title Approved by name Title Authorised by name Title Date HR Wallingford Limited 2003 This report is a contribution to research generally and it would be imprudent for third parties to rely on it in specific applications without first checking its suitability Various sections of this report rely on data supplied by or drawn from third party sources HR Wallingford accepts no liability for loss or damage suffered by the client or third parties as a result of errors or inaccuracies in such third party data HR Wallingford will only accept responsibility for the use of its material in specific projects where it has been engaged to advise upon a specific commission and given the opportunity to express a view on the reliability of the material for the particular applications iv SR 620 10 03 03 v SR 620 10 03 03 Summary Manual for the design of roof drainage systems A guide to the use of European Standard BS EN 12056 3 2000 RWP May Report SR 620 March 2003 This manual provides comprehensive guidance on how roof drainage systems for buildings should be designed using information given in European Standard 12056 3 2000 Gravity drainage systems inside buildings Part 3 Roof drainage layout and calculation The manual is intended to be used in conjunction with BS EN 12056 3 and contains separate chapters dealing with the following topics Rainfall data Effective catchment area Design flow loads Capacity of freely discharging gutters Outlets from gutters Capacity of gutters with restricted discharge Drainage of flat roofs Overflow weirs Rainwater pipes and drains inside buildings Siphonic systems Each Chapter consists of two parts The first provides background information either explaining the basis of the recommendations in BS EN 12056 3 or in some cases giving additional data The second part describes in detail the steps that should be followed in the calculations A series of worked examples is also included to illustrate the design procedures vi SR 620 10 03 03 vii SR 620 10 03 03 Contents Title page i Contract iii Summary v Contents vii 1 Introduction 1 2 Layout and scope of Manual 3 3 Rainfall data 4 3 1 Background information 4 3 1 1 Rainfall characteristics 4 3 1 2 Design rainfall duration 4 3 1 3 Geographical location 5 3 1 4 Risk and return period 5 3 1 5 Determining the design rainfall intensity 7 3 2 Calculation steps 7 3 2 1 Category 1 7 3 2 2 Categories 2 3 and 4 8 4 Effective catchment area 9 4 1 Background information 9 4 1 1 Effect of wind 9 4 1 2 Calculation of effective areas 9 4 1 3 Vertical surfaces 10 4 1 4 Complex roof layouts 10 4 2 Calculation steps 10 5 Design flow loads 12 5 1 Background information 12 5 1 1 Basis of calculations 12 5 1 2 Division of flow load between outlets 12 5 2 Calculation steps 13 6 Gutters with free discharge 14 6 1 Background information 14 6 1 1 Definitions 14 6 1 2 Hydraulics of gutter flows 14 6 1 3 Free discharge from gutter 14 6 1 4 Differences between BS EN 12056 3 and BS 6367 15 6 1 5 Effect of gutter slope 16 6 1 6 Design method for eaves gutters 17 6 1 7 Design method for non eaves gutters 19 6 2 Calculation steps 21 6 2 1 Overall procedure 21 6 2 2 Eaves gutters 21 6 2 3 Non eaves gutters of rectangular or trapezoidal shape 22 6 2 4 Non eaves gutters of semi circular or similar shape 22 7 Outlets and box receivers 23 viii SR 620 10 03 03 Contents continued 7 1 Background information 23 7 1 1 Flow conditions at outlets 23 7 1 2 Formulae for outlet capacity 23 7 1 3 Outlets in gutters with non flat soles 25 7 1 4 Outlets in gutters with flat soles 25 7 1 5 Grated outlets 25 7 1 6 Side outlets 26 7 1 7 Box receivers 26 7 2 Calculation steps 26 7 2 1 Outlets in gutters with flat soles 26 7 2 2 Box receivers 27 8 Gutters with restricted discharge 29 8 1 Background information 29 8 2 Calculation steps 29 9 Flat roofs 31 9 1 Background information 31 9 1 1 Layout 31 9 1 2 Design flow conditions 31 9 1 3 Outlets and sumps 31 9 2 Calculation steps 32 9 2 1 Gutters and outlets 32 9 2 2 Chutes 32 9 2 3 Outlets 32 9 2 4 Sumps 33 10 Rainwater pipes and drains inside buildings 34 10 1 Background information 34 10 1 1 Vertical pipes 34 10 1 2 Rainwater pipes and drains with gradients flatter than 10 o 34 10 2 Calculation steps 35 11 Overflow weirs 37 11 1 Background information 37 11 1 1 Purpose of overflows 37 11 1 2 Performance of overflows 37 11 2 Calculation steps 37 12 Siphonic roof drainage systems 38 12 1 General description 38 12 2 Removal of air and negative pressures 38 12 3 Performance features of siphonic systems 39 12 4 Design of siphonic systems 40 13 References 42 Appendix Worked examples 1 SR 620 10 03 03 1 INTRODUCTION Some of the first recommendations on the hydraulic design of roof gutters to be widely adopted in the UK were contained in BRS Digests 116 First Series and 34 Second Series Refs 1 and 2 published by the Building Research Station now the Building Research Establishment BRE A metricated version of both documents appeared as BRS Digest 107 in 1969 Ref 3 In 1974 a new British Standard Code of Practice CP 308 Drainage of roofs and paved areas Ref 4 was produced and introduced a new design method for valley parapet and boundary wall gutters based on hydraulic theory developed by Beij Ref 5 In 1983 CP 308 was replaced by a completely revised British Standard Code of Practice BS 6367 Ref 6 with the same title as before In BS 6367 Beij s theory was applied consistently to all types of gutter and new design equations were given for the capacity of outlets in gutters and box receivers The Meteorological Office also provided new detailed information on the incidence of short period rainstorms in the UK BRE revised Digest 107 to maintain conformity with CP 308 and this was published as Digest 189 Ref 7 in 1976 In 2000 BS 6367 was replaced by European Standard BS EN 12056 3 Ref 8 which was prepared by CEN Task Group TC 165 WG 21 TG3 with active involvement by the UK BS EN 12056 deals with gravity drainage systems for buildings up to the point where the flows reach the external walls of the building Some of the information in the old BS 6367 that related to the drainage of paved areas is now contained in BS EN 752 4 Ref 9 BS EN 12056 consists of the following five parts Part 1 General and performance requirements Part 2 Sanitary pipework Layout and calculation Part 3 Roof drainage Layout and calculation Part 4 Waste water lifting plants Layout and calculation Part 5 Installation and testing instructions for operation maintenance and use The initial draft of Part 3 was prepared by the UK based on the same general design principles as BS 6367 but with different procedures for some of the calculations Account was taken of results of new data from tests carried out in Switzerland on the capacity of level and sloping gutters and of some differences in design practice in other European countries General performance criteria for siphonic roof drainage systems were also included However overall the basis of BS EN 12056 3 is equivalent to that of BS 6367 and in many cases the two documents give closely similar results The common European text of BS EN 12056 3 does not include national meteorological data and provides less background information than BS 6367 on the layout and design of roof drainage systems Also a majority of the European countries decided that they did not require a design procedure for gutters with restricted discharge due to their using different methods of gutter construction or types of outlet The UK therefore added six National Annexes NA to NF covering meteorological data and relevant design information from BS 6367 where this did not conflict with the common text of the European Standard These National Annexes are denoted as Informative as opposed to the main text and Annex A dealing with the testing of gutters and outlets which have a Normative status The use of Anexes NA to NF is therefore not mandatory in the UK although they can be cited in contracts if required BS EN 12056 3 has the status of an Approved Document in Part H of the UK Building Regulations Ref 10 2 SR 620 10 03 03 This manual deals only with Part 3 of BS EN 12056 and the UK National Annexes The objectives of the manual are to explain the basis of the information in EN 12056 3 relating to the design of roof drainage systems describe in a sequence of appropriate steps how the information should be applied to various types of design problem The manual does not cover straightforward factual information in EN 12056 3 relating to Materials and components for rainwater goods National Annex NA and Layout installation inspection testing and maintenance National Annex NE 3 SR 620 10 03 03 2 LAYOUT AND SCOPE OF MANUAL This manual is intended to be used in conjunction with BS EN 12056 3 but is not a replacement for it The manual therefore describes the various stages in the design of a roof drainage system but it is necessary to refer to BS EN 12056 3 for particular Figures or items of information The steps in the design process following the direction of flow through a rainwater system are described in the following Chapters of this manual Chapter Number Design step 3 4 5 6 7 8 9 10 11 12 Rainfall data Effective catchment area Design flow loads Capacity of freely discharging gutters Outlets from gutters Capacity of gutters with restricted discharge Drainage of flat roofs Overflow weirs Rainwater pipes and drains inside buildings Siphonic systems Each Chapter consists of two parts The first provides background information either explaining the basis of the recommendations in BS EN 12056 3 or in some cases giving additional data The second part describes in detail the steps that should be followed in the calculations Although BS EN 12056 3 gives some general criteria for the satisfactory performance of siphonic roof drainage systems it does not provide calculation procedures for sizing the pipework The detailed design of siphonic systems is normally carried out using proprietary software the principles of which are explained in Chapter 12 However siphonic systems normally need to cater for the same rainfall requirements as conventional systems in which the rainwater pipes are designed to flow only part full and the flow capacity of the gutters depends on the same factors whether the outlets are of conventional or siphonic type The information in Chapters 3 4 5 6 8 9 and 10 of this manual is therefore equally applicable to siphonic systems The various design procedures in BS EN 12056 3 are illustrated by five worked examples included in Appendix A 4 SR 620 10 03 03 3 RAINFALL DATA 3 1 Background information 3 1 1 Rainfall characteristics In BS EN 12056 3 the values of rainfall intensity r are expressed in units of l s per m 2 of effective catchment area on which the rain falls The value of r can be related to the corresponding rainfall intensity r O in mm h by the formula r r O 3600 3 1 The design rainfall intensity to be used for sizing a rainwater drainage system depends on three factors the duration of the rainfall event D in minutes the geographical location of the building the return period of the event T in years As an example an event with a return period of 50 years will occur on average once every 50 years However events of this magnitude will not normally occur regularly at 50 year intervals Thus there is a finite chance that one 50 year event could be quickly followed by another and then perhaps not be repeated for a much longer period than 50 years Statistical data for a particular location are obtained by scanning a rainfall record with a window of duration D minutes and counting how frequently a depth of rainfall M in mm falling in that time is exceeded by heavier storms These measurements can then be analysed to determine the return period T of that particular rainfall depth M the value is called the DminMT depth e g 2minM5 depth for a two minute storm event with a return period of 5 years The corresponding average rainfall intensity for the event is r O 60 M D in mm h or r M 60 D in l s per m 2 using the units in BS EN 12056 3 The word event is used here purposefully rather than the word storm This is because the design value of rainfall depth occurring in D minutes will usually form the most intense part of a longer storm in which the intensity varies continuously with time The design event used for sizing a roof drainage system will therefore not normally come out of a clear blue sky and then end equally suddenly but will have been preceded by a period of less intense rainfall Rainfall records from around the world all show that at any particular location the value of rainfall intensity r increases as the duration D of the event decreases and its return period T increases The general relationship between r D and T for any location in the UK can be established for D 2 10 minutes from Figures NB 6 and NB 7 in National Annex NB of BS EN 12056 3 as explained below in Section 3 2 2 3 1 2 Design rainfall duration As mentioned above the shorter the duration of the event considered the higher will be the value of the design rainfall intensity However it is also necessary to consider the time of concentration of the drainage system i e the time taken for rain falling on the most upstream part of the roof to reach the outlet from the roof or gutter If the time of concentration T C is greater than the design rainfall duration D the flow rate at the outlet will not reach the maximum value that could be produced by the rainfall intensity r if all the roof were contributing Thus for design purposes the worst case situation occurs when D is just equal to T C 5 SR 620 10 03 03 Note 1 The above description is a little simplified because in practice rainfall intensities vary continuously with time and alter somewhat the relationship between D and T C that gives rise to the worst case condition However for engineering purposes it is reasonable to assume that the design case occurs when D T C and this is the basis for the well established Rational Method for the design of piped drainage systems The value of T C depends upon the size and layout of the roof and the drainage system In the case of a building with gutters T C will usua