Roof drainage and guttering

Introduction

Rainwater drainage from pitched and flat roof areas is in some cases an underspecified and misunderstood function, often taken for granted or based on rule of thumb. Even contractors used to installing roofs and gutters on a regular basis sometimes adopt a cavalier approach to design, perhaps ignoring it altogether in the belief that the capacity is being calculated by someone else.

With a conventional eaves gutter, or a flat roof that is able to discharge over its perimeter, the consequences of under-capacity are less significant. However, if there is any risk that the water could be contained in some way, design becomes critical. Failure to design for particular rainfall intensity can bring disastrous consequences, not only to the fabric of a building but to the entire business operation as well.

In the UK, Part H of the Building Regulations deals with the design of rainwater disposal systems in buildings. Guidance is given for the sizing of eaves gutters, but for buildings containing parapet or valley gutters or where siphonic systems are used, reference must be made to BS EN 12056-3, which contains design methods. The key to proper sizing is the selection of appropriate rainfall intensity for a 2 minute peak intensity (2 minutes being the average time for the entire roof area to contribute to flow into the gutter). It is a general rule that the longer the event, the lower the intensity of rainfall. Gutters must be sufficient to deal with periods of high summer rainfall in which large amounts of water fall in a short period of time. The likely frequency of these events can be used to estimate the probability of an event taking place during the lifetime of the building.

You could design on the basis of the worst possible scenario, but this would for most purposes result in considerable overdesign, and there is a need to balance risk and cost. So standard design practice is to consider the return period of a storm event and the chance that this may be exceeded in any given year. For normal eaves gutters and flat roofs, the return period is usually taken as one year - these roofs create a low risk of water penetration in the event of a sudden storm.

In the UK, for valley and parapet gutters the return period is usually based upon 1.5 times the anticipated life of a building (for example 1.5 x 30 years) since the consequences of a sudden storm would be severe if the gutters overflowed. If a higher degree of protection is needed (for example, for a data centre or archive) the return could be based on 4.5 times the anticipated life of the building or in the alternative the maximum possible intensity.

Rainfall intensities vary according to geographical location, with lowland areas most likely to suffer sudden intensive storms. BS EN 12056-3 provides a series of rainfall intensity maps corresponding to different return periods to enable maximum flows to be calculated; other European countries publish their own data to accord with specific circumstances.

Example rainfall intensity for a notional site in Nottingham

From BS EN 12056-3 the intensity of rainfall could be taken as:

  • Return period 1 year = 0.020 l/s per m2
  • Return period 5 years = 0.032 l/s per m2
  • Return period 50 years = 0.048 l/s per m2
  • Return period 500 years = 0.072 l/s per m2
  • Maximum = 0.160 l/s per m2

Assume that the design life of the building is 30 years, but that it has paparpet gutters. Given the increased risk of water penetration arising from parapet gutters, allow an additional factor of safety, i.e. 1.5 x anticipated life = 45 years. There are no charts for 45 years, the nearest data is for 50 years at 0.048l/s per m2, so use this as a basis for design.

So for a drained roof area of 100m2 the maximum flow into the gutter could be 4.8 l/s, which equates to a rainflow intensity of 17.28mm per hour. (To convert l/s to mm/hour multiply by 3,600).

In the figure below, outlet B drains a total area of 138m2 and so will take the greatest amount of rainfall. For the example above, the outlet would be required to discharge at 6.6l/s. This example ignores the pitch of the roof, which strictly speaking should be factored in to give an effective catchment area. To calculate the effective catchment area when measuring on plan adjust the calculation by taking the eaves to ridge measurement (Ah) when measured on plan and adding this to half of the height of the pitch in metres.

Drawing showing 3 outlets in a gutter draining a monopitched roof