Roof drainage and guttering

Siphonic rainwater disposal systems can be very effective if they are designed and installed properly. However, failures can occur as a result of:

• inappropriate selection of rainfall intensity;
• gutters with insufficient capacity and no overflow;
• priming of pipework being too slow to deal with a peak rainfall event;
• negative pressure in system overcoming the capability of the pipework;
• the arrangement of tail pipes and collector pipes such that priming does not occur properly;
• insufficient drop between gutter sole and collection pipe to allow flow to be generated to fill the downpipe;
• a lack of provision for surcharging of underground drainage system;
• poor maintenance; and
• condensation on uninsulated gutters or downpipes.

Pearson Education Ltd v Charter Partnership Ltd [2005] EWHC 2021

The subject building was a book warehouse constructed in 1989 and situated in Leicestershire. The roof was drained via a syphonic drainage system designed to a capacity of 75mm/hr (0.0208l/s) under the then Building Regulations. Comparison with current standards would suggest that for this location and the sensitivity of the contents, a better choice would have been 0.072l/s.

Two intense rainfall events, each with a return period of the order of 1 in 100 years, resulted in flooding as the system's design capacity was exceeded. The events took place within 8 years of one another, on 24 July 1994 and 30 July 2002 respectively.

The court held that the defect in the roof drainage system was a latent defect enabling the limitation period for claims to be extended for 15 years from the date of the design error. The architects were found liable to the lessees of the warehouse for £2.1m plus interest.

Baxall Securities Ltd & Norbain SDC Ltd v Sheard Walshaw Partnership [2000] EWHC Technology 53

The subject building was one of a series of rectangular 2-bay warehouses completed in 1991 as a speculative development. The roof of the building was arranged in a pair of pitches running longitudinally, with a valley gutter between them. The outside slopes drained to eaves gutters. No overflows had been provided to the central valley.

A siphonic drainage system had been agreed in an attempt to reduce excavations for underground drainage in a somewhat difficult site. At the time of construction, siphonic systems were fairly rare within the UK. The specialist subcontractor selected a design rainfall rate of 75mm/hour, a figure that was not challenged by the architect. On 29 May 1995 and 4 September 1995 severe flooding occurred through the roof as a result of incapacity in the central gutter. The damage to stock within the warehouse amounted to some £740,000.

The court found that the first flood was due to a lack of maintenance in the system coupled with a lack of overflows; the storm intensity did not appear to have exceeded the design capacity of 75mm/hr. However, the second event did exceed 75mm/hr and by that time a regular gutter cleaning regime had been put in place.

To enable the potential tenderers for the roof drainage to carry out their technical design, they should have been given information of the design requirement for the rate of rainfall. That requirement was not given. The tenderers provided their own design requirement and, left to their own devices, would have been tempted to put in the lowest and cheapest requirement. The judge agreed with expert evidence, namely that any shortfall in the capacity of the gutters to dispose of short duration rainfall appropriate to the location of the building would be as a direct result of the failure of the architect to incorporate the necessary information within the specification requirement.

Under the then code, a design rate of 75mm/hr would have been appropriate if some leakage could have been tolerated. Arguably any reasonably minded tenant would have a difficulty with that concept, and so the architect should have insisted on a rate of 150mm/hr, i.e. category 3 under the code. Arguments that the building was speculative did not cut much ice with the court who found the architect negligent in failing to insist upon a higher standard.

Furthermore, the architect had originally specified overflows, which had not been fitted; he had failed to identify the omission when he should reasonably have done so and insisted on their provision.

Although the architect did not have a direct contractual relationship with the claimant, the court found 'that it was reasonably, indeed easily, foreseeable by the defendants that a future occupier of the premises falling within a class of persons likely to occupy the premises would suffer damage to his goods as a result of the defects that the defendants caused or allowed to exist in the premises'.