14 Oct Sizing Soil And Waste Stacks In High-Rise Buildings
Handover snagging lists from tall residential blocks have a habit of repeating one complaint: a smell in the bathroom on the upper floors that nobody can locate. The pipework passes its tests. The traps are full when anyone checks. Yet the complaint keeps coming back, usually on the floors immediately below a stack offset or near the top of the building.
What is happening is almost always a pressure problem, not a leak. Water falling down a stack drags air with it, and that moving air produces suction ahead of the flow and positive pressure behind it. If the system cannot feed air in and let it out fast enough, those pressures pull or push water out of the nearest trap, and the seal that was keeping drain air out of the room disappears. Stack diameter and the uPVC vent pipe arrangement are the two levers that keep those pressures inside a survivable range.
What Actually Determines Stack Size?
Stack diameter is set by the discharge load it has to carry, expressed as the combined loading of the appliances connected to it and adjusted for how likely those appliances are to discharge at the same moment. A stack serving forty flats is not sized for forty simultaneous discharges, because that never happens; it is sized on a probability-based loading figure.
The second constraint is that a drainage stack is never designed to run full. Water clings to the pipe wall and falls as an annular film with an air core down the centre, and that air core is what allows the system to breathe. Push the flow beyond roughly a quarter to a third of the cross section and the film starts to bridge across the bore, sealing the air path and generating exactly the pressure transients that empty traps. Sizing tables in the applicable code therefore cap the flow well below the pipe’s hydraulic capacity.
Loading figures, probability factors and permitted stack capacities are all set out in the governing code, and in Singapore sanitary and drainage works fall under PUB’s requirements. The current edition of the Code of Practice on Sewerage and Sanitary Works is the document to size from, and it should be read in its published form, not from a summary.
Why Tall Buildings Behave Differently
Height changes the problem in two ways. Flow reaches its terminal velocity within the first few storeys and does not keep accelerating after that, so a thirty-storey stack does not deliver water to the base at thirty storeys’ worth of speed. What does keep accumulating is the volume of air being dragged along, and with it the scale of the pressure swings the system has to absorb. The taller the building, the more the ventilation strategy carries the design.
Where Ventilation Fits In
A vent gives air a route in and out of the drainage system so that pressure changes equalise instead of acting on trap seals. The simplest arrangement extends the stack itself full bore to open air above roof level, which handles the majority of residential layouts on its own.
Taller or more heavily loaded stacks usually need more. A relief or secondary ventilating stack run alongside the main stack and cross-connected at intervals gives air a parallel path around the sections where pressures build, and individual branch ventilation protects traps on long or heavily used branch runs from siphoning themselves. Which arrangement applies depends on stack height, loading and branch geometry, and the choice belongs to the drainage engineer working to the code, not to a rule of thumb.
Vent pipework is sized in its own right rather than being taken as a fixed fraction of the stack. Undersized ventilation is a frequent cause of the intermittent, hard-to-reproduce smell complaint, because it performs adequately at moderate flows and only fails during the morning and evening peaks when several floors discharge together.
Offsets, Bends And The Base Of The Stack
Anywhere the stack changes direction is a pressure event. At an offset, falling water is thrown against the outside of the bend, the annular film breaks down and the air core is briefly interrupted, which produces a pressure spike immediately below and suction above. Offsets in the wet portion of a tall stack are therefore designed with care, ventilated across the offset where the code requires it, and avoided altogether where the layout allows.
The base of the stack is the other pressure hot spot, because the flow has to turn from vertical to horizontal there and decelerate into the drain. Standard practice is a large radius turn, formed with two long radius bends in preference to a single sharp one, and a minimum clear distance before the first branch connects into the horizontal drain. Connect a ground floor appliance too close to that turn and it will be subjected to the full back pressure of the stack every time the building above discharges.
Which Standards Apply To The Pipework Itself
Above-ground soil, waste and vent pipework in Singapore is manufactured to SS 213, published by Enterprise Singapore as the specification for unplasticised PVC pipes and fittings for soil, waste and vent applications. Its scope is above ground and indoor use, which is the distinction that catches people out on submission: the below-ground sanitary drainline running out to the boundary sits under a different standard, SS 272, and specifying one where the other belongs is a straightforward way to attract a query.
Jointing method affects the finished system as much as the pipe specification does, and the choice between solvent cement vs rubber ring joints shapes how a tall stack accommodates movement and how quickly a defective section can be cut out and replaced.
Vicplas manufactures its soil, waste and vent range to recognised Singapore Standards, with over 400 mould designs across the pipe and fitting range.
Conclusion
Sizing a tall stack well is less about finding the largest diameter the riser will take and more about keeping the air core intact from roof to drain. Load the stack within the code’s limits, give the air a proper route in and out, treat every change of direction as a pressure event, and the traps look after themselves.
If you are working through a riser layout and want to talk over fitting options or the range available for a particular configuration, our team would be glad to help. Reach out to us with your drawings and we will take it from there.