Mechanical
Drainage design
Above and below ground drainage design to BS EN 12056 and BS EN 752, including stack sizing, gradients and adoption requirements.
- Discipline
- Mechanical
WHAT IT IS
Drainage design covers foul and surface water from the appliance to the point of connection: stack sizing and ventilation above ground, gradients, falls and pipe sizing below ground, and the requirements of whoever will adopt or receive the discharge.
Below ground drainage is constrained by levels. Once the finished floor level, the external levels and the invert of the receiving sewer are fixed, the drainage either works by gravity or it does not, and a pumped station is a permanent maintenance and resilience liability.
WHEN YOU NEED IT
From Stage 2 on any new build or extension, and earlier where the site has a level constraint, a limited discharge rate, or a combined sewer that the receiving authority wants surface water separated from.
DELIVERABLES
What you receive
- Above ground drainage layouts and stack sizing
- Below ground drainage layouts with levels and gradients
- Surface water and attenuation calculations
- SuDS strategy where required
- Adoption drawings for the receiving authority
STANDARDS
Standards applied
The editions in force at the time of design. Where a standard is amended mid-project we confirm which edition the appointment is based on.
- BS EN 12056Gravity drainage systems inside buildingsBSI
- BS EN 752Drain and sewer systems outside buildingsBSI
- Approved Document HDrainage and waste disposal (England)GOV.UK
- Technical Handbook Section 3Environment (Scotland)Scottish Government
- Sewers for Scotland 4th EditionScottish WaterScottish Water
- CIRIA C753The SuDS ManualCIRIA
QUESTIONS
Common questions
What gradient does a foul drain need?
Approved Document H gives a minimum of 1 in 40 for 100 mm drains serving fewer than the equivalent of one WC, and 1 in 80 for 100 mm drains where at least one WC is connected. Flatter gradients risk solids deposition; steeper gradients can cause the liquid to run away from the solids.
What is SuDS and when is it required?
Sustainable drainage systems manage surface water close to where it falls, using infiltration, attenuation and treatment rather than piping it straight to a sewer. It is required on most new development in Scotland and increasingly in England and Wales, and is designed in accordance with CIRIA C753, the SuDS Manual.