How to get it right: Drainage of paved areas

One area commonly overlooked within building work is paved areas outside of the property. However since 2002 there has been a need to consider the adequate drainage of paved areas around buildings.

The 2015 addition to H3 imposed a requirement for paving, which provided access to buildings where M1, M2 or M4 (1), (2) or (3) imposed a requirement. This is in addition to access to solid waste storage H6 (2) or common passageways.

Domestic buildings should be provided with a drainage system to remove surface water from paved surfaces, such as an access route that is suitable for disabled people, without endangering the building or the health and safety of people in and around the building caused through freezing.

The paved surface should be laid to ensure rainwater run-off is not close to the building.

Paved surface drainage systems should be designed, constructed and installed in accordance with the recommendations of Section 2 of the Approved Document Part H for areas up to 4,000m².

Free draining surface water run-off may be appropriate for small hard surface areas, such as access paths and driveways and can be achieved by laying the surface to a fall, sufficient to avoid ponding, that allows the water to drain to a pervious area, such as grassland, provided the infiltration capacity of the ground is not overloaded.

Also the discharge should not be adjacent to the building where it could damage the foundations.

The AD H Section 2 specifies examples of free draining surfaces, pervious paving and drainage systems appropriate for paved areas.

A designed rainfall intensity of 0.014 litres/ second/m² is assumed for normal situations, however, the recommendations of Diagram 2 in AD H may be followed. Most of the paving types will be free draining to a pervious area or constructed of pervious paving, however, if the paving is impermeable, i.e., water does not drain through them, but gathers on the surface such paving should be drained towards a disposal point, such as a gully or drainage grid and soakaway.

This would be the situation for small domestic paving projects where open gullies and grids are the most popular methods of paving drainage that connects to a soakaway.

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Example

How to calculate the drains capacity necessary for 95m² of paving

The calculation for drainage is based on the equation Q = (A x r) where:

• Q is the runoff capacity measured in litres per second
• A are the areas to be drained m²
• r is the rainfall intensity in litres/second. (ADH quotes 0.014 litres/second/m² for normal situations.) 

So, Q = (A x r)

Q= 95 x 0.014 = 1.33 litres/second

So the drain should have a flow capacity of at least 1.33l/s.

Using ADH diagram 3 – it would appear that a 75mm diameter drain/channel laid at 1:200 gradient would be capable of handling this rainfall intensity. However, a 75mm diameter surface water drain serving more than one building would be unacceptable (3.14) and 75mm and 100mm diameter drains and should be laid to a minimum of 1:100 gradient (3.15).

This runoff capacity needs to be taken into account if the drain connects into the surface water system If a soakaway is proposed then it needs to be able to handle the surface runoff and be able to disperse into the surrounding soil quickly enough for the soakaway to be able to cope with the next storm.

They are usually formed from square or circular pits, filled with rubble or constructed using attenuation crates wrapped in Terram geotextile membrane and externally backfilled with gravel and be a minimum of 5m from any buildings.

For small soakaways serving 25m² or less, a design rainfall of 10mm in five minutes is quoted in the Approved Document H surface water drainage, is assumed as the worst case. However soakaways for larger areas should be designed in accordance with BS EN 752-4 or BRE Digest 365 and uses a percolation rate (how quickly rainwater infiltrates the ground) for design.

The final option is to use pervious paving but again the storage layer beneath the paving needs to be designed in the same way as soakaways to prevent retention and possible movement on sloping sites.

Further information

If you found this post useful, why not read some of our other articles about working with drainage:

• Level thresholds - landing and drainage
• Who is responsible for sustainable urban drainage (SuDS)?
• Building regulations guidance from LABC

Please Note: Every care was taken to ensure the information was correct at the time of publication. Any written guidance provided does not replace the user’s professional judgement. It is the responsibility of the dutyholder or person carrying out the work to ensure compliance with relevant building regulations or applicable technical standards.