Drainage

When installing new bathrooms, shower rooms and W.C’s regardless of their location you will need to apply for Building Regulations consent. You do not need to apply if you are replacing appliances like for like. The Building Regulations will cover the drainage installation, ventilation of the space and in some circumstances the electrical installation.

Drainage systems are one of the most important aspects of a working home or property. These carry waste water from toilets, sinks, baths, dishwashers and more, away from the home and into the sewer system.

Building regulations part H refers to the drainage and waste disposal guidelines which must be met in the UK. It states that adequate drainage systems must be provided as well as highlighting the importance of pollution prevention, working sewage infrastructure and sewerage maintenance. it also cover some important technical design standards, such as rainwater drainage, cesspools and internal sanitary pipework.

It’s essential that your drainage plumbing is installed correctly for safety and hygiene reasons,

You can always seek the advice of your Local Authority Building Control but this is some  guidance  to the principles of installing a foul water drainage system in England & Wales following Building Regulations Part H. We hope it provides some useful information before your next drainage installation project.

The principles of underground drainage

Pre-planning

Installing underground drainage requires extensive and careful forward planning. You will need to consider the exact layout of your drainage system and how it will connect together. At the same time, you need to stay in compliance with Building Regulations Part H.

At this early stage, you must ensure that underground drainage is not laid lower than the foundations of any nearby building unless you plan to fill the trench with concrete. The exact requirements can be found in Building Regulations Part H but, to summarise, a trench dug below the foundation within 1 metre of the building must be filled with concrete to the same level as the lowest point of the foundation. more than a metre from the building must be filled with concrete up to a level below the foundation that is equal to the distance from the building less 150mm.

You should check for existing drains and sewerage systems in the area by contacting the local water authority or referring to the building deeds. To prevent expensive and dangerous problems arising later, existing drains may need to be diverted or otherwise protected. Proximity to public sewers may also force some adjustments to your plan. You may also need to seek written permission from the local sewerage authority before beginning the project.

Where there are obstructions — such as buildings, foundations or water features — the system may need to incorporate more bends after access chambers. Safety is paramount. You will need to make sure, before starting, that the project isn’t going to compromise the foundations of the given property or lot.

Also, it’s essential that your plans ensure an adequate level of hydraulic capacity for current and future demands on the system — especially if you anticipate future work happening. Failing to plan for increased use of the system can lead to blockages, floodage, and even structural failure. A detailed drainage design proposing precise workings of your project can be an invaluable asset to refer to throughout the planning permission and initial installation process.

Planning pipe gradients

By law, foul water drainage systems are required to be self-cleansing. That means that gravity, combined with the force of the water, should drive wastewater into sewer systems without any help.

To ensure this is the case, the recommended minimum gradient (or fall) for foul water drainage is:

75mm & 100mm pipeworks when no WC is connected or;

1:80 (12.5mm per metre) if a WC is connected to the system, 100/110mm pipework minimum when WC or trade effluent connected, or;

1:150 if a minimum of 5 WC are connected (150/160mm pipework only).

The water velocity within the pipe must not fall below 0.70 m/s as this ensures adequate self cleaning and are generally designed to run at a maximum of ¾ full bore. Failure to comply with the above can lead to a higher risk of blockages.

Planning for access

All underground drainage systems need an appropriate number of access points. This is to enable future testing, inspection and maintenance. In your project plan, you will need to plot the locations of inspection chambers, rodding chambers, manholes and other fittings.

Planning for excavation

Before digging trenches for your pipes, you should know that they must not be left open for a long period. Guidance suggests that trenches should be backfilled as soon as possible. Naturally, they must also be fully supported at the sides during the pipe-laying process. Sheet piling is typically used to provide earth retention and excavation support.

These are good reasons to have a solid plan in place before proceeding. To comply with Building Regulations, trenches should be as narrow as possible while allowing 300mm of space for sidefill material.

The design of above ground drainage is also governened by Part H of the Building Regulations: Drainage and waste disposal. Although this gives guidance on system design, it simply states that the system must be adequate. Some basic principles to consider

The principles of above ground drainage

Traps

In the UK they are based on the use of water-sealed traps, which prevent foul air from entering the building. These should be installed at all points of discharge into the system and need to retain a minimum seal of 25mm of water or equivalent. The table below gives minimum trap sizes for common sanitary appliances.

Traps come in various configurations, such as low-level bath and shower traps for fitting in shallow spaces, anti-siphon bottle traps, which can be used under washbasins with a vertical pipe or long run of pipe, and “S” or “P” tubular traps. They should be attached immediately below the outlet, or as close as possible, and should be designed so that they are easily accessible.

Appliance	Diameter of trap (mm)	Depth of seal (mm)
Washbasin, bidet	32	75
Bath, shower	40	50
Urinal bowl, sink	40	75
WC pan, outlet <80mm	75	50
WC pan, outlet >80mm	100	50

Pipework materials and joints

A variety of materials are used for soil and waste pipework. They are covered by a number of different standards, as shown in the table below:

Material	Standard
Soil pipework
Unplasticised polyvinyl chloride (PVCu) – 110mm
and 160mm diameter	BS EN 1329-1
PVCu – 82mm diameter	BS 4514*
High-density polyethylene (HDPE) – 40-315mm diameter	BS EN 1519-1
Modified polypropylene (PP-MD) 40-160mm
diameter	BS EN 1451 / 1411
Cast iron	BS EN 877
Waste pipework
Polypropylene (PP)	BS EN 1451-1
Acrylonitrile butadiene styrene (ABS)	BS EN 1455-1
Chlorinated polyvinyl chloride (PVCc)	BS EN 1566-1

Each material is jointed in a different way:

PVCu soil pipes can either have a ring seal, which means they push-fit together, or they can be solvent welded

HDPE soil pipes are jointed by electrofusion or butt-welding, which is quick and simple but requires the use of specialist equipment. Push-fit joints are also required to accommodate thermal movement

ABS and PVCc waste pipes are solvent-weld jointed

PP acoustic soil and waste pipes can only be jointed using the push-fit method

Polypropylene waste pipes are push-fit jointed.

Branch pipes serving a single appliance should have at least the same diameter as the appliance trap.

Discharge stacks

Branch pipes connect to a central stack which discharges to a drain. When designing the stack, a number of factors need to be considered.

Positive pressure. To eliminate positive pressure, the minimum distance from the base of the stack to the lowest branch connection varies depending on the height of the stack, as the table shows:

Application	Minimum height
Single dwelling up to three storeys	450mm
Up to five storeys	740mm
More than five storeys	One storey
More than 20 storeys	Two storeys

The stack base should use two 45˚ bends or a bend with a radius of 200mm or more.

Stub stacks. Branch pipes can also discharge into an unventilated “stub stack” as long as they are only used to connect ground-floor appliances. The vertical drop to the drain must not exceed 1.5m from a WC and 2.5m from a waste appliance. The head of the drain should be vented to the atmosphere. The maximum loading of a stub stack is 5 litres/second.

Ventilation

Discharge stacks need to be ventilated in order to prevent the water seal in the trap from being broken by a pressure build up in the system. A secondary ventilation stack is sometimes required to increase stack capacity due to the number of appliances connected. The table below shows the different options depending on the calculated flow rate of the system.

Stack size (mm)	Vent (mm)	Maximum capacity (litres/second)
Primary ventilated stack
82	n/a	2.6
110	n/a	5.2
160	n/a	12.4
Secondary ventilated stack
82	50	3.4
110	50	7.3
160	82	18.3

Another option is to specify an aerator, which eliminates the need for secondary venting by breaking the flow on each floor, thereby keeping the pressure difference well within the limit of 3mbar. A 110mm aerator can increase the stack capacity to 7.3l/s.

Branch pipes will not need separate ventilation as long as they do not exceed the length and gradient shown in the following table:

Appliance	Maximum run (m)	Gradient (%)	Number of bends	Maximum drop (m)
Washbasin or bidet with 32mm-diameter pipe	1.7	1.8-2.2	None	0
Washbasin or bidet with 40mm-diameter pipe	3	1.8-4.4	2	0
Bath or shower	3	1.8-9.0	No limit	1.5
Sink	3	1.8-9.0	No limit	1.5
Washing machine or dishwasher	3	1.8-4.4	No limit	1.5
WC	No limit	1.8 min	No limit	1.5
Sink disposer	3	13.5 min	No limit	1.5

If a branch pipe exceeds these limits, it should be connected via a branch ventilated pipe to external air or a ventilating stack, or an air admittance valve should be used.

An air admittance valve can be used to terminate primary vented discharge stacks, providing the air flow rate of the valve is at least eight times the peak flow rate on the stack.

Size of stack

 In order to work out the correct size of the stack for a specific system, the designer has to add up the number of discharge units and for each one apply a factor depending on the type of use – for example, a house, school or hospital. The procedure and calculation method are explained in Tables 2 and 3 and clause 6.3 of BS EN 12056-2.

Prevention of cross-flow

A branch pipe should not discharge into the stack in a way that could cause cross-flow into any other pipe. To prevent cross-flow from a large-diameter branch into a smaller connection, the latter should join the stack in one of the following ways:

Above the centre line of the larger branch

At right angles to the larger branch

At least 200mm below a restricted zone beneath the centre line of the larger branch. The depth of this zone is determined by the diameter of the stack: 90mm for an 82mm stack; 110mm for a 110m stack; 250mm for a 160mm stack.

Specially designed fittings can also alleviate this problem, such as the eight-way collar boss, which has four vertical and four horizontal connections. To link multiple WCs to a soil stack, a manifold system is used, varying the angle of the connector at each WC.

Offsets 

Due to the design of commercial buildings, discharge stacks often need to be offset. When calculating the required flow capacity, the stack cannot be sized as a secondary ventilated system unless vent connections are incorporated on every floor. No connection is permitted within 450mm of the offset.