Designing for Poor Workmanship

A designer needs to make reasonable provision for workmanship error and take into account the risk of imperfect work in site conditions.

This was affirmed in a decided case relating to defective tanking to a basement built into well-drained chalk.[1] The construction was in situ concrete and concrete masonry, tanked externally with a bonded sheet waterproofing. Following good practice, a land drain was placed externally but, contrary to best guidance, it was higher than the bottom of the waterproofing. A subcontractor carried out the work to designs provided to him.

The completed basement leaked after heavy rain. Internal water-resisting render was applied and money was withheld from the subcontractor. Law suits ensued.

Flawed Design?

The design was argued to be faulty for the following reasons:

1. Clause 3.3 of BS 8102, Code of Practice for the Protection of Structures Against Water from the Ground, states that the designer should:
   1. Consider the consequence of less than adequate workmanship,
   2. Consider the consequence of leaks, and
   3. Consider the form and feasibility of remedial work.
2. Clause 3.1.1 of BS 8102, Pre-Design Considerations, recommends that basements should include provision for resisting a pressure equivalent of at least 1m head of water.
3. By installing the land drain in the position shown, the designers created a head of water that would bear against the membrane. In these circumstances, any defect would constitute less-than-adequate workmanship, as the consequence of those defects would be flooding through the membrane into the basement.
4. It is not realistic or reasonable to expect a bonded sheet membrane to be applied without any defects at all.
5. The interpretation of the above was that a design team must anticipate that defects will occur in a membrane, and so must design a system in such a way that water pressure is removed before it comes to bear against the membrane. If they are unable to achieve this, it is implied that an alternative form of waterproofing must be used.
6. Furthermore, a bonded sheet membrane is only one element within an overall waterproofing system. The membrane, together with the drainage and the structure, all form part of the system and must be considered together. No one element should be considered in isolation

Flawed Workmanship?

1. The counter argument contained the following:
2. Clause 3.1.1 of BS 8102 says that the membrane alone must be capable of withstanding a head of water of at least 1m without leaking.
3. The installation of the land drain above the floor slab did not induce a water head in excess of 1m, and so the design complied with BS 8102.
4. In the absence of a design fault, the problem has to lie with the installation of the membrane, by default.

The Judgement

Unreserved judgement was given in the subcontractor’s favour. Despite the certainty that water must enter though flaws in the tanking, the existence of which must evidence errors in workmanship, the design was blamed for lacking provision for this eventuality and the workmen exonerated. This decision is directly contrary to the findings in some other cases dealing with comparable matters.

Avoiding Negligent Design

Good design addresses the ways in which the work can be done, the circumstances in which it is to be done and seeks to match the tasks it creates for trades people to the skills they can reasonably be expected to bring to it. Working in a muddy excavation, in exposed conditions with rolls of relatively delicate material, which has to be kept clean whilst it is stuck to site-formed surfaces, has its difficulties.

The manufacturers of sheet tanking membranes generally claim them to be capable of working as an effective water barrier. Designers have to place some reliance on manufacturers’ published guidance when using proprietary products. This does not excuse the reckless abandonment of tried-and-tested design solutions. Traditionally, where tanked construction was desired, it was considered wise to combine it with water-retaining concrete or engineering brick and land drainage to relieve water pressure before it builds up against the tanking. New waterproofing systems are available which, from the manufacturers’ literature, are capable of maintaining dry basements without reliance on anything but a supporting structure.

As with all aspects of building, designers must decide whether to draw on published guidance derived from long experience or to rely exclusively on manufacturers’ claims. In making this decision they must assess whether or not a product’s potential can be achieved on the site in question. This will depend on a range of factors, such as: the availability of trade skills and of materials, the working conditions, facilities for storage and supervision. If the designer cannot control for these, it would be prudent to consider fail-safe solutions or to design-in sufficient redundancy to lessen the likelihood that ordinarily poor workmanship will result in failed performance.

[1] Outwing Construction v Thomas Weatherald, 13 September 1999, Case no. 1998 O 011, AC 1027198