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At Arup we run a series of surgeries aimed at small- to medium-sized practices to assist with projects where our full involvement is not deemed necessary. These surgeries allow architects with smaller projects to demonstrate their drainage design skills and present them in the form of a review.

The reviews frequently show that some designers try to incorporate as many BS EN clauses into their design as possible. Within a short time, the design has lost its purpose.

It is this experience that leads me to encourage designers to develop their systems in line with the performance of the regulations, and not as an exercise in complying with the code. It is foolish to expect the code to be broad enough to encompass every situation and, since it takes so long to revise, users cannot keep up with new demand from developers and the wealth of new products entering the sector.

Designers should remember it is the installation that must comply with the regulation, not the design, so the responsibility extends to site. It is critical that the system can be installed from the design information supplied:

the golden rule of drainage is 'keep it simple'.

The six criteria in the performance schedule are the main points of reference when checks are undertaken. Here we will discuss the four most relevant points. In the secretary of state's view, the requirement of H1 will be met if a foul water drainage system:

1. minimises the risk of blockage or leakage;

2. prevents foul air from the drainage system entering the building under working conditions;

3. is ventilated;

4. is accessible for clearing blockages.

1. MINIMISE THE RISK OF BLOCKAGE OR LEAKAGE The most common defects in new properties are blockages and leakage. The causes are often legacies from the design or installation process.

Designers should concentrate on the horizontal sections of the system, as this is where problems occur.

Gradients, slopes and falls Modern sustainable systems demand less water than before.

Currently, the revised guidance calls for an increase in falls of the drains to overcome the decrease in water, but this is a temporary solution because it actually increases the depth of the drain: the very thing we are trying to avoid. The future is in the use of smaller-diameter pipes (see diagram 3) but in the meantime, increasing the gradient of the drain will retain the depth of water in the pipe.

When a new regulation is introduced, it affects the standard mechanisms that have developed over time to become the norm. To keep abreast of these subtle changes, the accepted 1:80, 1:60 and 1:40 pipe gradients must be re-evaluated. A helpful reference document is Gravity Flow in Channels by Butler and Pinkerton, published by Thomas Telford. Its concise charts provide all the information needed in order to optimise horizontal pipe design and provide the best installation.

Golden rule: minimum velocity 0.75m/s. Note: 4' at 1:40 or 6' at 1:60 are not golden rules.

Bedding and surround This is an important feature of drain-laying. The correct selection of pipes, type of bed and surround can all determine the shallowest depth of excavation. Modern pipe systems are specially designed to be flexible in line with seasonal ground movements and the settlement of new buildings. These movements can affect the drain gradient even after the installation has been signed off. Penetrations of foundations where the drain leaves the building should always be detailed.

Golden rule: flexibility is important.

Encasing pipes in concrete is an outdated concept.

One of the most contentious issue raised by the codes is the different approach to access into drains. As an effect of its inclusion, the manhole has dictated many other aspects of a building. Its limitations have finally led the codes' authors to consider the rodding point system.

Leakage is normally associated with workmanship issues, but a well thought-out design can ensure leakage is minimised. All manufacturers' pipe and fittings systems will have proprietary seals and couplings, but what do you do when the design requires a specialist product from another manufacturer?

The specification should be sufficiently thorough to ensure specialist components are supplied with the appropriate adaptor. Away from city and large-town supply networks, installers will resort to the most unusual solutions to connect one component to another. This increases the risk of leakage.

Golden rule: interfaces of differing materials present problems, and drainage is no different.

Leakage can also be caused by damage from the installer.

Below-ground leakage is often ignored as it may not be noticeable immediately.

Making sure that the pipes are protected from surface loads will prevent leakage from fractures. Care should be taken with pipe systems that are susceptible to crushing.

Compacting plates are a principal method of providing protection to the drain, but their operation can also affect it adversely. Limiting machine compaction until the surround is 450mm deep will reduce the incidence of damage.

The advent of CCTV technology has shown the extent of damage that goes undetected until after warranties and retentions have expired. Stage-by-stage drain surveys are cost-effective and can be conclusive in disputes.

A good reference source is provided in the simplified bedding tables produced by the Clay Pipe Development Association.

Golden rule: conduct stage-by-stage testing and witnessing. Don't simply wait for the fi nal inspection by Building Control.

2. PREVENT FOUL AIR FROM THE DRAINAGE SYSTEM ENTERING THE BUILDING UNDER WORKING CONDITIONS This is another way of saying 'make sure that trap seals remain charged in sanitary appliances and be mindful where the vent pipes terminate'.

Trap-seal loss can be caused by leakage, or by unequal air pressure in the system, caused either by discharges or by evaporation.

Only two of these cases are applicable to operation or design issues. Leakage is described above. You should assume that if water can leak out during discharges, air can also leak out.

The most realistic way to protect trap seals in traditional systems is to ensure that the drainage system has sufficient venting to relieve air-pressure fluctuation when water is conveyed through the pipes. In small buildings this is achieved by venting the top of the soil pipe to the outside. In larger buildings, where sanitary fittings are located away from the soil pipe, BS 12056 Part 3 gives a schedule of maximum lengths for waste pipes.

Waterless trap seals (HepVo) can significantly reduce the problems associated with venting and waste-pipe lengths. Acceptance of these products ranges from reluctant to enthusiastic, although, in some cases, there is strenuous discouragement. There is nothing in the regulations that says they cannot be used.

Again, when faced with these challenges, it is advisable to resort to the performance schedules, because a code will state 'all sanitary fittings shall have a water trap seal'.

Termination of vent pipes can be an emotive issue for architects. Having pipes poking out of the roof is often unwelcome, particularly in larger schemes with multiple stacks. The roofscape can look disorganised if there are numerous ad hoc penetrations through the roof. Tradition plays a part in this approach and the Approved Document in Part H reflects a basic approach that can only give rise to an unappealing roof line. This dilemma can be resolved by referring to the schedule and then deciding to reject the Approved Document approach.

The regulation is met as long as the system is ventilated and the foul air from the system cannot enter the building. Why not investigate connecting all of the vents together? After all, all the pipes connect together at the bottom of the system.

Venting system through air bricks on gable walls is perfectly acceptable.

Golden rule: always ventilate.

3. VENTILATE This item is often confused with Item 2. It is the requirement to allow air to pass through the main sections of the drainage system even when it is not in use (see diagram 6). This is to ensure that noxious or explosive gases are not allowed to build up in the system and cause problems. The existence of relief venting mentioned in Item 2 helps keep air moving up through the system, but it should not be confused with the requirement of Item 3.

One of the prevailing acceptances of modern components in drainage design is the air-admittance valve.

The Approved Document permits their use with certain limitations, but the misuse of these devices is one of the most commonly misunderstood principles in drainage design.

Golden rule: do not be driven by convention; be creative.

4. MAKE ACCESSIBLE FOR CLEARING BLOCKAGES This is a requirement for points in the system at which drain-clearing equipment can be inserted into the pipeline.

Modern jetting and remote clearing technologies allow the number of access covers to be reduced while still allowing sufficient access into the drain.

As discussed, the rodding point system gives a greater degree of flexibility in setting out the drainage system and does not impose restrictions on the positioning of connections or require the siting of large covers inside the building.

Pipe systems contain proprietary access fittings for above- and below-ground installations. Where this type of fitting needs to be of a more aesthetic appearance, there are companies that can provide access door covers compatible to system pipe diameters.

Be aware that the table in the code refers to distances between access points based on the assumption that manual clearing techniques will be used.

Since most drain clearing is now undertaken by companies with specialist equipment, it is rational to extend the distances in the code to take advantage of modern technology.

David George is an associate public health engineer at Arup

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