Section H3 Rainwater Drainage

  1. The Requirement
    1. Guidance
      1. Section 1: Gutters and rainwater pipes
        1. Section 2: Drainage of paved areas
          1. Section 3: Surface water drainage
            1. Appendix H3-A: Oil separators

    The Requirement

    Requirement

    Rainwater drainage
    H3. (1) Adequate provision shall be made for rainwater to be carried from the roof of the building.

    (2) Paved areas around the building shall be so constructed as to be adequately drained.

    (3) Rainwater from a system provided pursuant to sub- paragraphs (1) or (2) shall discharge to one of the following, listed in order of priority:

    (a) an adequate soakaway or some other adequate infiltration system; or, where that is not reasonably practicable,

    (b) a watercourse; or, where that is not reasonably practicable,

    (c) a sewer.

    Limits on application

    Requirement H3(2) applies only to paved areas:

    (a) which provide access to the building pursuant to requirement M1 (access and use of buildings other than dwellings), or requirement M2 (access to extensions to buildings other than dwellings), or requirement M4(1), (2) or (3) (access to and use of dwellings);

    (b) which provide access to or from a place of storage pursuant to requirement H6(2) (solid waste storage); or

    (c) in any passage giving access to the building, where this is intended to be used in common by the occupiers of one or more other buildings.

    Requirement H3(3) does not apply to the gathering of rainwater for re-use.

    Guidance

    Performance

    In the Secretary of State’s view the requirements of H3 will be met if:

    a. rainwater from roofs and paved areas is carried away from the surface either by a drainage system or by other means;

    b. a rainwater drainage system:

    i. carries the flow of rainwater from the roof to an outfall (a soakaway, a watercourse, a surface water or a combined sewer),

    ii. minimises the risk of blockage or leakage,

    iii. is accessible for clearing blockages.

    c. rainwater soaking into the ground is distributed sufficiently so that it does not damage the foundations of the proposed building or any adjacent structure.

    Introduction to provisions

    1.1 The provisions in this document in relation to the drainage of paved areas apply only to paved areas:

    a. within the curtilage of a building which are

    i. provided in accordance with requirements M2 and M4, to provide access to the principal entrance (see Approved Document M volume 2 section 1 for buildings other than dwellings and Approved Document M volume 1 sections 1A, 2A and 3A for dwellings);

    ii. provided in accordance with requirement H6 to give access from the building to the place for storing refuse, and from the place of storage to the collection point (see Approved Document H6);

    b. which are yards or other forms of access intended to be used in common by more than one building.

    The provisions of H3 only apply if these surfaces are paved.

    1.2 Methods of drainage other than connection to a public surface water sewer are encouraged where they are technically feasible.

    1.3 The capacity of the drainage system should be large enough to carry the expected flow at any point in the system.

    1.4 The flow depends on the area to be drained and the intensity of the rainfall.

    1.5 The capacity depends on the size and gradient of the gutters and pipes. Capacities and minimum sizes are given in the text.

    1.6 Rainwater or surface water should not be discharged to a cesspool or septic tank.

    Section 1: Gutters and rainwater pipes

    Design rainfall intensities

    1.1 For eaves, gutters the rainfall intensity should be obtained from Diagram 1.

    1.2 Where the design incorporates valley gutters, parapet gutters, siphonic or drainage systems from flat roofs, and where over-topping of these systems would have particularly high consequences such as water entering the building, wetting of insulation or other dampness the design should be carried out in accordance with BS EN 12056 (see paragraph 1.17).

    Gutters

    1.3 The flow into a gutter depends on the area of surface being drained and whether the surface is flat or pitched (and, if it is pitched, on the angle of pitch). Table 1 shows a way of allowing for the pitch by working out an effective area.

    Table 1 Calculation of drained area Part H

    Table 1 Calculation of drained area Part H3

    1.4 Table 2 shows the largest effective area which should be drained into the gutter sizes which are most often used. These sizes are for a gutter which is laid level, half round in section with a sharp edged outlet at only one end and where the distance from a stop end to the outlet is not more than 50 times the water depth. At greater distances the capacity of the gutter should be reduced. The Table shows the smallest size of outlet which should be used with the gutter.

    1.5 Where the outlet is not at the end, the gutter should be of the size appropriate to the larger of the areas draining into it. Where there are two end outlets they may be up to 100 times the depth of flow apart.

    Table 2 Gutter sizes and outlet sizes Part H Table 2 Gutter sizes and outlet sizes Part H3

    Diagram 1 Rainfall intensities for design of gutter and rainfall pipes (litres per second per square metre) Part H3
    Diagram 1 Rainfall intensities for design of gutter and rainfall pipes (litres per second per square metre) Part H3

    1.6 Gutters should be laid with any fall towards the nearest outlet. Where there is a fall or the gutter has a section which gives it larger capacity than a half-round gutter or the outlet is round edged it may be possible to reduce the size of the gutter and pipe.

    Paragraph 1.17 gives a reference to some detailed recommendations which make reductions possible.

    1.7 Gutters should also be laid so that any overflow in excess of the design capacity, caused by conditions such as above normal rainfall, will be discharged clear of the building, reducing the risk of overspilling of rainwater into the building or structural overload. On flat roofs, valley gutter, and parapet gutters, additional outlets may be necessary.

    Rainwater pipes

    1.8 Rainwater pipes should discharge into a drain or gully but may discharge to another gutter or onto another surface if it is drained. Any rainwater pipe which discharges into a combined system should do so through a trap (see Approved Document H1).

    1.9 Where a rainwater pipe discharges onto a lower roof or paved area, a pipe shoe should be fitted to divert water away from the building. Where rainwater from a roof with an effective area greater than 25m² discharges through a single downpipe onto a lower roof, a distributor pipe should be fitted to the shoe to ensure that the flow width at the receiving gutter is sufficient so that it does not over-top the gutter.

    1.10 The size of a rainwater pipe should be at least the size of the outlet from the gutter. A down pipe which serves more than one gutter should have an area at least as large as the largest of the contributing outlets and should be of sufficient size to take the flow from the whole contributing area.

    Siphonic roof drainage systems

    1.11 Siphonic roof drainage systems should be designed in accordance with BS EN 12056-3 (see paragraph 1.17) and should take particular account of the following:

    a. The need to take account of surcharge in the downstream drainage system as this can reduce the flow in the downpipe.

    b. For long gutters the time taken for the system to prime the siphonic action may be excessive. Overflow arrangements should be provided to prevent gutters from over-topping.

    1.12 Further information on the design of siphonic drainage systems can be found in Hydraulics Research Ltd Report SR 463 Performance of Syphonic Drainage Systems for Roof Gutters.

    Eaves drop systems

    1.13 Eaves drop systems allow rainwater from roofs to drop freely to the ground. Where these are used, they should be designed taking into account the following:

    a. the protection of the fabric of the building from ingress of water, caused by water splashing on the external walls;

    b. the need to prevent water from entering doorways and windows;

    c. the need to protect persons using doorways, etc. from falling water;

    d. the need to protect persons and the fabric of the building from rainwater as it hits the ground by splashing, for example by provision of a gravel layer or angled concrete apron deflecting the water away from the building;

    e. the protection of foundations from concentrated discharges such as those from valleys or valley gutters or from excessive flows due to large roofs (i.e. where the area of roof per unit length of eaves is high).

    Rainwater recovery systems

    1.14 Rainwater drainage systems used to collect water for re-use within the building (rainwater recovery systems) should take account of
    the following:

    a. storage tanks should comply with requirement H2 (see Approved Document H2 paragraphs 1.69 to 1.71);

    b. pipework, washouts and valves should be clearly identified on marker plates (see Water Regulations Advisory Scheme Information Guidance Note 09-02-05 Marking and Identification of Pipework for Reclaimed
    and Grey Water Systems).

    1.15 Further guidance on rainwater recovery systems can be found in the Water Regulations Advisory Scheme leaflet No. 09-02-04. Reclaimed Water Systems. Information about installing, modifying or maintaining reclaimed water systems.

    Materials for gutters, rainwater pipes and joints

    1.16 The materials used should be of adequate strength and durability, and

    a. all gutter joints should remain water tight under working conditions. Pipes inside a building should be capable of withstanding the air tightness test described in paragraph 1.32 of Approved Document H1, and

    b. pipework in siphonic roof drainage systems should be able to resist to negative pressures in accordance with the design, and

    c. gutters and rainwater pipes should be firmly supported without restricting thermal movement, and

    d. different metals should be separated by non-metallic material to prevent electrolytic corrosion.

    Alternative approach

    1.17 The performance can also be met by following the relevant recommendations of BS EN 12056 Gravity drainage systems inside buildings. The relevant clauses are in Part 3 Roof drainage layout and calculation, Clauses 3 to 7, annex A and National Annexes, and in Part 5 Installation, testing instructions for operation maintenance and use, Clauses 3, 4, 6 and 11. These standards contain additional detailed information about design and construction.

     

    Section 2: Drainage of paved areas

    2.1 This section gives guidance on the design of paved areas for rainwater drainage systems. It is applicable to the drainage of paved areas around buildings and small car parks up to 4,000m². For the design of systems serving larger catchments, reference should be made to BS EN 752-4 (see paragraph 2.19).

    2.2 Surface gradients should direct water draining from a paved area away from buildings. Where the levels would otherwise cause water to concentrate along the wall of a building, a reverse gradient should be created, for at least 500mm from the wall of the building, to divert the water away from the wall.

    Diagram 2 Rainfall intensities for design of drainage from paved areas and underground rainwater drainage (litres per second per square metre) Part H3
    Diagram 2 Rainfall intensities for design of drainage from paved areas and underground rainwater drainage (litres per second per square metre) Part H3

     2.3 Gradients on impervious surfaces should be designed to permit the water to drain quickly from the surface. A gradient of at least 1 in 60 is recommended. The gradient across a path should not exceed 1 in 40.

    Design rainfall intensities

    2.4 Design rainfall intensities of 0.014 litres/ second/m² may be assumed for normal situations. Where ponding of rainfall is undesirable rainfall intensities should be obtained from Diagram 2.

    2.5 For very high risk areas, where ponding would lead to flooding of buildings, the drainage should be designed in accordance with BS EN 752-4 (see paragraph 2.19).

    Freedraining surfaces

    2.6 Paths, driveways and other narrow areas of paving should be freedraining to a pervious area such as grassland, provided that:

    a. the water is not discharged adjacent to buildings where it could damage foundations; and

    b. the soakage capacity of the ground is not overloaded.

    2.7 Where water is to be drained onto the adjacent ground the edge of the paving should be finished above or flush with the surrounding ground to allow the water to runoff.

    2.8 Where the surrounding ground is not sufficiently permeable to accept the flow, filter drains may be provided (see paragraph 3.33).

    Pervious paving

    2.9 Pervious paving consists of a porous or permeable surface overlying a granular layer which acts as a storage reservoir, retaining peak flows while the water soaks into the underlying subsoil. They should be considered for larger paved areas where it is not possible to drain the rainwater to an adjacent pervious area. The design of the storage layer is undertaken on a similar basis to the design of the storage volume in soakaways (see paragraphs 3.24–3.28). Where infiltration is not possible (see paragraph 3.25), they may also be used with an impermeable barrier below the storage layer as a detention tank prior to flows discharging to a drainage system (see paragraph 3.35).

    2.10 For steeply sloping surfaces, a check should be made to ensure that the water level can rise sufficiently in the granular storage layer to allow the storage capacity to be mobilised. A check should also be made to ensure that the stored water will not accumulate around the foundations of the building. Where infiltration is not possible (see paragraph 3.25), they may also be used with an impermeable barrier below the storage layer as a detention tank prior to flows discharging to a drainage system (see paragraph 3.35).

    2.11 Pervious paving should not be used where excessive amounts of sediment are likely to enter the pavement and block the pores.

    2.12 Pervious paving should not be used in oil storage areas, or where runoff may be contaminated with pollutants. Surface water should not be allowed to soak into the ground where ground conditions are not suitable (see paragraph 3.25).

    2.13 Further information on pervious paving can be obtained from CIRIA report C522 – Sustainable urban drainage systems – design manual for England and Wales.

    Drainage systems

    2.14 Where it is not possible for surfaces to be freedraining, or to use pervious paving, impervious paving should be used with gullies or channels discharging to a drainage system.

    2.15 Gullies should be provided at low points where water would otherwise pond. Intermediate gullies should be provided at intervals to ensure that gullies are not overloaded and the depth of flow in channels is not excessive.

    2.16 Gully gratings should be set approximately 5mm below the level of the surrounding paved area in order to allow for settlement.

    2.17 Provision should be made to prevent silt and grit entering the system, either by provision of gully pots of suitable size or by catchpits.

    2.18 Drainage from large paved areas should be designed in accordance with BS EN 752-4 (see 2.19).

    Alternative approach

    2.19 The performance can also be met by following the relevant recommendations of BS EN 752-4:1998 Drain and sewer systems outside buildings, Part 4 Hydraulic design and environmental aspects. The relevant clauses are Clause 11 and National Annexes ND and NE.

     

    Section 3: Surface water drainage

    3.1 This section gives guidance on the design of surface water drainage systems. It is applicable to the drainage of small catchments with impervious areas up to 2 hectares. For the design of systems serving larger catchments, reference should be made to BS EN 752-4 (see paragraph 3.36).

    Outlets

    3.2 Surface water drainage should discharge to a soakaway or other infiltration system where practicable.

    3.3 Discharge to a watercourse may require a consent from the Environment Agency, who may limit the rate of discharge. Maximum flow rates can be limited by provision of detention basins (see paragraph 3.35).

    3.4 Where other forms of outlet are not practicable, discharge should be made to a sewer.

    Combined systems

    3.5 Some sewers carry both foul water and surface water (combined systems) in the same pipe. Where they do the sewerage undertaker can allow surface water to discharge into the system if the sewer has enough capacity to take the added flow (see Approved Document H1 paragraph 2.1). Some private sewers (drains serving more than one building that have not been adopted by the sewerage undertaker) also carry both foul water and surface water. If a sewer operated as a combined system does not have enough capacity, the surface water should be run in a separate system with its own outfall.

    3.6 In some circumstances, where a sewer is operated as a combined system and has sufficient capacity, separate drainage should still be provided (see Approved Document H5).

    3.7 Surface water drainage connected to combined sewers should have traps on all inlets.

    Design rainfall intensities

    3.8 Design rainfall intensities of 0.014 litres/ second/m² may be assumed for normal situations. Alternatively the rainfall intensity may be obtained from Diagram 2.

    3.9 Where low levels of surface flooding could cause flooding of buildings the rainfall intensities should be obtained from BS EN 752-4 (see paragraph 3.36).

    Design

    3.10 Where there is evidence of a liability to surcharging from sewers, or levels in the building or on the site make gravity connection impracticable, surface water lifting equipment will be needed (see Approved Document H1 paragraphs 2.8 to 2.12).

    Layout

    3.11 Refer to paragraphs 2.13 to 2.21 of Approved Document H1.

    Depth of pipes

    3.12 Refer to paragraphs 2.27 and 2.28 of Approved Document H1.

    Pipe gradients and sizes

    3.13 Drains should have enough capacity to carry the flow. The capacity depends on the size and gradients of the pipes.

    3.14 Drains should be at least 75mm diameter. Surface water sewers (serving more than one building) should have a minimum size of 100mm. Diagram 3 shows the capacities of drains of various sizes at different gradients. However the capacity can be increased by increasing the gradient, or by using larger pipes.

    3.15 75mm and 100mm rainwater drains should be laid at not less than 1:100. 150mm drains and sewers should be laid at gradients not less than 1:150 and 225mm drains should be laid at gradients not less than 1:225. For minimum gradients for larger pipes see BS EN 752-4 (see paragraph 3.36).

    Diagram 3 Discharge capacities of rainwater drains running full Part H3
    Diagram 3 Discharge capacities of rainwater drains running full Part H3

    Materials for pipes and jointing

    3.16 See paragraph 2.40 of Approved Document H1.

    Bedding and backfilling

    3.17 See paragraphs 2.41 to 2.45 of Approved Document H1.

    Clearance of blockages

    3.18 See paragraphs 2.46 to 2.54 of Approved Document H1.

    Workmanship

    3.19 See paragraphs 2.55 to 2.58 of Approved Document H1.

    Testing and inspection

    3.20 See paragraphs 2.59 to 2.62 of Approved Document H1.

    Contaminated runoff

    3.21 Where any materials which could cause pollution are stored or used, separate drainage systems should be provided. This should include an appropriate form of separator or treatment system or the flow should be discharged into a system suitable for receiving polluted effluent.
    3.22 On car parks, petrol filling stations or other areas where there is likely to be leakage or spillage of oil, drainage systems should be provided with oil interceptors (see Appendix H3-A).

    Soakaways and other infiltration drainage systems

    3.23 Infiltration devices include soakaways, swales, infiltration basins and filter drains.

    3.24 Further information on the design of infiltration drainage systems can be found in CIRIA Report 156 – Infiltration drainage – Manual of good practice.

    3.25 Infiltration drainage is not always possible. Infiltration devices should not be built:

    a. within 5m of a building or road or in areas of unstable land (see Planning Policy Guidance Note 14 Annex 1);

    b. in ground where the water table reaches the bottom of the device at any time of the year;

    c. sufficiently far from any drainage fields, drainage mounds or other soakaways so that the overall soakage capacity of the ground is not exceeded and the effectiveness of any drainage field is not impaired (see Approved Document H2);

    d. where the presence of any contamination in the runoff could result in pollution of a groundwater source or resource.

    3.26 Soakaways for areas less than 100m² are generally formed from square or circular pits, filled with rubble or lined with dry-jointed masonry or perforated ring units. Soakaways serving larger areas are generally lined pits or trench type soakaways.

    3.27 Soakaways should be designed to a return period of once in ten years. The design should be carried out with storms of differing durations to determine the duration which gives the largest storage volume. For small soakaways serving 25m² or less a design rainfall of 10mm in 5 minutes may be assumed to give the worst case. For soakaways serving larger areas reference should be made to the sources listed in paragraph 3.30. Where the ground is marginal overflow drains can be acceptable.

    3.28 Percolation tests should be carried out to determine the capacity of the soil (see Approved Document H2 paragraphs 1.34 to 1.38). Where the test is carried out in accordance with Approved Document H2, the soil infiltration rate (f) is related to the value Vp derived from the test by the equation:

    H - Percolation tests 3.28

    3.29 The storage volume should be calculated so that, over the duration the storm, it is sufficient to contain the difference between the inflow volume and the outflow volume. The inflow volume is calculated from the rainfall depth (see paragraph 3.26) and the area drained. The outflow volume (O) is calculated from the equation:

    H - Percolation tests 3.28 (2)

    The side of the storage volume when filled to 50% of its effective depth, and D is the duration of the storm in minutes.

    3.30 Soakaways serving larger areas should be designed in accordance with BS EN 752-4 (see paragraph 3.36), or BRE Digest 365 Soakaway design.

    Other types of infiltration system

    3.31 Swales are grass-lined channels which transport rainwater from a site as well as controlling flow and quality of surface runoff. Some of the flow infiltrates into the ground. There may be an overflow at the end into another form of infiltration device or a watercourse. They are particularly suitable for treatment of runoff from small residential developments, parking areas and roads.

    3.32 Infiltration basins are dry grass-lined basins designed to promote infiltration of surface water to the ground.

    3.33 Filter drains or french drains consist of the trench, lined with a geotextile membrane and filled with gravel. Much of the flow infiltrates into the ground. A perforated pipe is often laid through the gravel to assist drainage.

    3.34 Flow enters the top of the filter drain directly from runoff, or is discharged into it through drains.

    Detention ponds

    3.35 Detention ponds are used to attenuate the flow from a drainage system, to limit the peak rate of flow into a sewer system or watercourse. Further information on design may be found in the references given in paragraph 3.36 and in Sustainable Urban Drainage Systems – A Design Manual for England and Wales published by CIRIA.

    Alternative approach

    3.36 The requirement can also be met by following the relevant recommendations of BS EN 752-4 Drain and sewer systems outside buildings. The relevant clauses are in Part 4 Hydraulic design and environmental considerations Clauses 3 to 12 and National Annexes NA, NB and ND to NI. BS EN 752, together with BS EN 1295 and BS EN 1610, contains additional detailed information about design and construction.

    Appendix H3-A: Oil separators

    Legislation

    A.1 Under Section 85 (Offences of polluting controlled waters) of the Water Resources Act 1991, it is an offence to discharge any noxious or polluting material into a watercourse, coastal water or underground water. Most surface water sewers discharge to watercourses.

    A.2 Under Section 111 (Restrictions on use of public sewers) of the Water Industry Act 1991, it is an offence to discharge petrol into any drain or sewer connected to a public sewer.

    A.3 Premises keeping petrol must be licensed under the Petroleum (Consolidation) Act 1928. Conditions may be placed on licences.

    A.4 The Environment Agency issues guidance notes on the provision of oil separators.

    A.5 The Health and Safety Executive issues guidance notes on the storage of oil.
    Technical guidance

    A.6 For most paved areas around buildings or car parks where a separator is required, a by-pass separator should be provided which has a nominal size (NSB) equal to 0.0018 times the contributing area. In addition it should have a silt storage volume in litres equal to 100 times NSB.

    A.7 In fuel storage areas and other high risk areas full retention separators are required. These should have a nominal size (NS) equal to 0.018 times the contributing area. In addition it should have a silt storage volume in litres equal to 100 times NS.

    A.8 Separators discharging to infiltration devices or surface water sewers should be Class I.

    A.9 Separators should be leak tight. Inlet arrangements should not be direct to the water surface. Adequate ventilation should be provided.

    A.10 Separators should comply with the requirements of the Environment Agency and with BS EN 858-2002 A1 2004 and BS EN 858-2:2003. In addition, where the Petroleum Act applies, they should comply with the requirements of the licensing authority.

    A.11 Separators should be maintained regularly to ensure their continued effectiveness. Provision should be made for access for inspection and maintenance.

    A.12 Further information on provision of separators is available in Use and design of oil separators in surface drainage systems, Pollution Prevention Guideline No. 3. This can be obtained from the Environment Agency.