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| Drainage for Pavements |
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Why drainage mattersMost of the paving types discussed on this site are generally impermeable, ie, water does not drain through them, but 'collects' on the surface and therefore all pavements must be designed to drain towards a gully (a grid in popular parlance!), a linear drain, or some other handy disposal point, such as a ditch, a soakaway, or towards the public footpath/highway. |
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Failure to properly drain a pavement can cause all sorts of problems. Water on the surface encourages mosses, algae and other vegetation to colonise the paving; in icy conditions even shallow puddles can become extremely dangerous ice rinks and over the longer term, standing water can actually damage the paving itself. And don't forget the basic premise behind having a pavement in the first place - to provide a sound surface usuable in all weathers!
For domestic paving projects, small 150x150mm opening gullies or linear drains are the most popular methods of draining a pavement. On larger projects, much bigger gullies, combined kerb/drain units or heavy duty linear drains are likely to be used, but the essential principles remain the same - the paving is sloped to create 'fall' and so direct surface water off the paving and into the drainage system as quickly and efficiently as possible. |
Typical domestic gully |
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In the definitions section of the Drainage Introduction page, the 3 classifications of drainage (Foul, Surface and Combined) were introduced. Modern properties tend to have two separate drainage systems installed - the foul water system collects the kitchen and bathroom wastes for processing at the local effluent treatment plant whilst the surface water system collects water from the roof and the paving, often discharging this relatively clean water into a local watercourse, such as a ditch, stream or river to reduce demand on the effluent treatment plant.
You might be able to identify two separate systems on your property by reading the information given on the Foul or Storm page and checking inside access/inspection chambers for the tell-tale signs. |
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GradientThe gradient usually quoted as being the minimum required for draining a pavement is 1 in 40, which is easier to remember and set up as one inch per metre (excuse the mixing of SI and Imperial measurements - it should be 25mm per metre). With some surfaces we typically work to minimum gradients of 1 in 80 (one inch per 2 metres) with no problems as some surfaces drain more efficiently than others. We find that block or brick paving usually drains well at shallower gradients (up to 1:80), assisted by the chamfers on the top edges of the blocks/bricks. Tarmac is notorious for holding water or 'ponding', due to small deviations in surface level resulting from the inaccuracies inherent in hand-laying tarmac. This type of surfacing is best laid to a steeper gradient such as 1 in 40. |
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Where to drain toThe simplest scenario is a pavement/driveway that slopes down towards the public footpath or highway, where it will be directed into road gullies or similar, and away into the storm water sewers. If this is not possible, the surface water should be directed to existing surface water gullies, found beneath downspouts from the roof, or into new gullies installed specifically for the purpose of draining the paving. Alternatively, the surface water can be directed into linear drain units that will carry the run-off into the storm-water sewers, or to filter drains and soakaways. Paving should not be allowed to discharge surface water onto a neighbouring property - it can cause problems on your neighbour's property for which you may be held responsible.If at all possible, use of SUDS should be considered for drainage of all pavements. This is known as "Source Control" - dealing with the precipitation and surface water where it lands, rather than piping it several kilometres for it to be "dealt with" at a Treatment Plant. SUDS comprises a range of simple technologies that can be used for any scale of projects, from a small patio in the back garden to a residential estate or retail outlet car park. Read more about the idea and technolgies used with SUDS on the SUDS Introdution page. |
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Arrows denote falls in paving to drain to gullies, the garden and the public highway
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On flat sites, we will often use a system known as 'summits and valleys' to create the necessary falls to ensure adequate surface drainage. In this system, gulleys are positioned at the lowest points on the surface of the paving, known as the valleys, and the adjacent paving is sloped so that it drains towards the gulley, up to a point known as a summit. On the drawing above, summits are inicated 'S' and the valleys are 'V'.
Worked examples of calculating the required falls for a pavement are given on the Setting-out page. Always bear in mind that the summits, ie the highest points on the surface of the paving, should be at least 150mm below damp proof course level. |
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Draining towards a buildingWhenever possible, a pavement should slope away from a building. This is done to prevent surface water 'hanging' against the masonry of a building, which could lead to problems with damp. However, this is not always possible. |
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| In the example given above, the area of paving around the conservatory at the rear of the property is shown to be draining towards the house, to be collected by the gullies located at X and Y. Because the lawned area is higher than than the paving level near the house, to slope away from the house would require either the lawn/garden level to be dropped or some form of retaining structure to be built, neither of which are desirable on this project. |
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| When draining back towards a property, we must ensure that no surface water will be given the chance to 'hang' against the masonry. This can be achieved in a number of ways:- | ||
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Using a channel unit, such as those featured on the road kerbs page. More aesthetically pleasing channel units are available to complement block paving schemes, as shown opposite. |
Keychannel® by Marshalls |
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Using a linear drain. This solution is more expensive than the use of a channel, but is popular, especially in areas where large volumes of surface water are anticipated. | |
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Creating a channel within the paving. This solution requires the contractor to create a v-shaped channel some distance out from the masonry by laying the paving to strict levels. For example, with block paving, the soldier course units are laid with at least 20mm of fall AWAY from the masonry, so that the actual lowest point on the pavement is 200mm away from the brickwork. With flags/slabs, the v-channel may be 300mm out, with, say, 30mm of back fall. |
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For non-elemental surfacings, such as concrete or tarmac, the laying operative will need to scallop the required profile into the surface before it sets. Whichever method is chosen, it is essential that there is end fall, directing the surface water towards a gully or other disposal point, as well as crossfall away from the masonry. | |
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Each project will use an appropriate solution from those listed above. In all cases, it is preferred that no surface water is allowed to lodge or run against any masonry or other part of a building and careful consideration must be given to those areas where this situation could potentially arise. |
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How many drainage points?We follow a simple rule of thumb for calculating the number of 'point' gullies, such as those beneath a downspout, required to efficiently drain an area of paving. We allow one such gully for each 75 m² of paving, although we often drain a much smaller area than that into each gully.The actual number of gullies or drainage points required for any given area is determined by the theoretical maximum flow rate of the pipework serving the gully. This, in turn, is a function of pipe diameter, pipe smoothness and pipe gradient. It is the size of the connecting pipework, not the size of the gully or other drainage fitting, that determines how much surface water a system can shift per unit time. Assuming that we have a gully served by a 100mm diameter clayware pipe laid at the minimum acceptable gradient of 1:100, and further assuming that we use the 'heavy' rainfall figure for the UK of 75mm per square metre per hour, then a single gully could, theoretically, be sufficient to drain up to 310 m² , although the usual stipulation is that no gully in a footway or other paved area should drain an area greater than 200m²; Road gullies, as used in carriageways, may drain an area up to 250m² However, when relying on existing gullies or other drainage points to drain a new or extended pavement, then allowance has to be made for any area already utilising that gully for drainage, such as a roof, another pavement or any other hard surface. It is better to have plenty drainage points rather than too few, and all drainage should be installed and completed before laying the paving. You will need to decide how your paving is going to be drained, making best use of existing gullies, and then installing any extra drainage that may be required. |
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Other methods of draining a pavementNarrow or short paths can be drained onto the garden as the amount of run-off is minimal, and unless you have waterlogged land, the garden should be able to cope with this small quantity of extra water. Never drain an area greater than 10m² onto the garden, unless you have a sandy, well-drained soil, or a land-drainage system in place as discussed elsewhere on this site. If in doubt, use a gully or other type of drain, such as a linear drain or a narrow fin drain. |