Roof drainage and guttering

Having established the suitable rainfall intensity for a given area, the gutters and outlets must be sized. The NFRC publish simplified tables, but otherwise detailed calculations need to be prepared using the methodology set out in BS EN 12056-3. The calculations are not straightforward, although a number of software programmes are now available to simplify this. Note that the size and configuration of the gutter outlets are often the governing factor in gutter design. The speed at which a gravity drainage system can discharge water depends on the head of water at the outlet and the profile of the outlet. A tapered outlet is generally more effective than one with straight sides. Improved flow can be provided by setting the outlet in a sump or box receiver, as this both increases the head and the length of the weir over which water can flow. Outlets into the side of a gutter are less efficient.

For valley and parapet gutters it is usual to provide at least two outlets. However, roof drainage is not a simple matter of 'if one outlet becomes blocked the other will take over' since the depth of water in the gutter will be much greater and there is the risk that the gutter could be overtopped. Gutter design involves the provision of a freeboard to provide security against water splashing and wind induced waves. While this offers some notional emergency capacity, it may not be sufficient to deal with what is in effect an increase in the area of roof drained by one outlet.

To safeguard against accidental overtopping, a weir overflow or overflows should be provided. These are often installed without much thought as to capacity, so for example a 150 x 75mm rectangular outlet is often found at the end of a valley gutter so arranged as to discharge water clear of the inside of the building in the event of outlet blockage or sudden peak rainfall in excess of that designed. The problem with this approach is that the overflow may have a capacity that is much less than that of the normal outlet: so the gutters could still be insufficient. To provide a reliable alternative, the capacity of the overflow should be calculated. This may reveal a need to increase the dimensions of the overflow to 450mm or more.

Poor outlet detail, strainer reduces flow, no box receiver

Box receiver in a gravity drainage system. Z is the overall height of the gutter, W is the effective design depth to allow a freeboard margin. The head of water at the box receiver is shown as h. A tapered outlet is shown - water entering the pipe forms a vortex and runs down the outside walls of the downpipe. Air is drawn into the vortex and reduces the efficiency of the drainage system.

At just over 1m centres the support brackets for this parapet wall gutter are too far apart for comfort. Design conditions will vary, but in this case, 600mm centres were preferable.

In some cases a weir overflow may not be practicable, perhaps due to structural obstructions. As an alternative, vertical pipes can be used, fitted to the sole of the gutter and projecting above the normal water design level in the same way as the overflow to a conventional WC cistern.

Gutters to industrial buildings are usually of metal, either galvanised steel or aluminium. Some roofing systems still employ cement fibre gutter sections, while older buildings may have asbestos cement gutters.

Galvanised steel gutters may be galvanised after manufacture and range from 3-6mm in thickness. Alternatively, galvanising before manufacture is possible, but these gutters are usually 1.6-3mm in thickness. Galvanised steel provides good protection, but given the aggressive conditions that exist in gutters, not to mention foot traffic, it is usual to coat the inside surface with bitumen paint after the initial weathering has taken place. (If applied too early or in wet conditions the paint may lose adhesion.)

Aluminium gutters are usually fabricated from 1.5-3mm thick sheet. Joints in these gutters can also be welded to reduce the potential for leakage at joints.

With all types of gutter, proper and rigid support is necessary. A large box or boundary wall gutter will hold a considerable volume of water and so will weigh an appreciable amount. Support brackets must be robust enough to support this and also the weight of a person or persons cleaning the system.

Generally, gutters in industrial buildings are laid level and are not therefore self-cleaning. Regular inspection and cleaning is essential.

Ineffective valley gutter owing to lack of maintenance