The appropriate use of modern cast iron systems
Gerald Tobias of Gerald Tobias Associates, one of the country's most innovative drainage engineers.

Above ground drainage In a non-domestic building - an office, factory, school, hospital, supermarket etc, the cost of the above ground drainage systems would generally be a very small percentage of the overall cost of the building. That is likely to be true whatever material is chosen, so it makes sense to go for the most appropriate - but what is that? Clearly cost is a consideration, but so is durability, ease of installation, low maintenance, low noise transmission, and very importantly fire resistance.

There are 2 elements of an above ground drainage system that should be designed and specified to last the lifetime of the building.

1) the internal rainwater pipes

2) the soil discharge stacks

Even when a building is modernised every 15 or 20 years, these elements along with the structure will likely remain.

If the toilet or kitchen area is refurbished the branch discharge pipes will often be renewed and therefore it seems appropriate to specify plastics or copper for that element. But if the main stacks are to be specified to last the lifetime of a building, perhaps 50-70 years or more - an appropriate material is mechanical jointed cast iron - for it is one of the few materials you can reasonably fit and forget.

Apart from some conservation work where caulked lead joints are still occasionally specified, the material available to specifiers for the past 25 years has been mechanical jointed cast iron to BS416, such as Timesaver.

However, for the last few years in the UK there has been a more modern approach to quality installation with lighter, thinner walled cast iron systems such as SMU and Ensign. Since these materials obtained their BBA Certificates, most professional drainage engineers have experience of specifying them in numerous buildings with no reported troubles.

The logic of specifying plastic materials and then spending extra time and money on additional brackets and fire stopping to make them as safe as cast iron seems pointless.

So in assessing whether iron is appropriate, one has to weigh up the actual installed cost of the chosen material and finally in these litigious days, the consequences of getting it wrong.

Under building drainage

If elements of the above ground drainage should last the lifetime of the building it is most certainly true that all the underbuilding drainage should be designed and specified with that objective.

To decide which of the three main types of material for below building use, cast iron, vitrified clay and plastic is appropriate, it is necessary to carry out a risk analysis.
(See Table 1)

Most engineers would agree that the risk of settlement, sheer pressure and over zealous maintenance methods are potential problems more likely to take a clay or plastic system out of operation than a cast iron one. So you can reduce or remove the risk by using cast iron. It can bridge minor voids caused by settlement, resist sheer pressures and successfully take the internal knocks from the rodding.

TABLE 1. RISK ASSESSMENT - DAMAGE TO BURIED PIPE

The next stage of the risk analysis is to consider the consequences of a drain failure and its accessibility for repair.

(See Page 2 ).