What is a discharge stack?
A discharge stack, often called a soil stack, is the vertical main pipe of a building’s above-ground drainage system. It collects waste water from toilets, showers and basins on each floor and carries it down to the below-ground drains and the sewer. In Dutch it is called a standleiding; the Dutch standard NEN 3215 defines it as a drain that deviates no more than 45 degrees from vertical. In the UK the relevant standard is BS EN 12056, alongside Approved Document H of the Building Regulations.
At the top, the stack normally continues as a vent pipe that terminates above the roof (the soil vent pipe). This keeps the system open to the atmosphere and prevents the suction or pressure that occurs when a toilet is flushed from pulling or blowing out the water seals of the traps. Without proper venting you get smells and gurgling drains.
Application
- Keep it as straight as possible. Each offset slows the flow and can cause pressure fluctuations. If an offset is needed, use two 45-degree bends rather than one 90-degree bend.
- Use a large-radius bend at the base. The connection to the horizontal drain is best made with a long-radius bend or two 45-degree bends.
- Avoid connections just above the base bend in taller buildings. Positive pressure can build up there.
- Vent it. Run the stack through the roof, or use an air admittance valve only where permitted. An air admittance valve lets air in but does not relieve positive pressure.
- Reduce noise. A stack in a wall next to a bedroom is clearly audible. Use acoustic pipe or wrap it in insulation.
- Fire stopping. In flats, pipes passing through floors often need intumescent fire collars.
- Provide access for rodding at the base.
Common mistake: capping or boxing in the vent pipe during alterations without providing ventilation. Traps get sucked dry and sewer smells enter the house.
Related terms
- Soil vent pipe
- Air vent
- Drain trap
- Sewer
- Standpipe
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