Getting SWR pipe sizing wrong on a multi-storey building doesn't show up on day one. It shows up eighteen months later as a gurgling sound from a ground-floor bathroom every time someone flushes on the fifth floor, or as a slow, recurring blockage at exactly the same point in the stack. These are sizing and venting problems, not installation defects, and they're considerably more expensive to fix once the building is occupied than to get right at the design stage.
This guide focuses specifically on the sizing decisions that matter for multi-storey buildings: how soil, waste, and rainwater stacks should actually be sized, why venting is what prevents trap siphonage, and how to phase bulk SWR procurement sensibly against a building's construction schedule. For installation steps and jointing practice, our SWR pipe installation guide covers that ground in detail. This post picks up at the design and specification stage, before installation begins.
Soil, waste, and rainwater: three separate systems, three separate sizing logics
A common mistake on smaller projects is treating "drainage pipe" as one undifferentiated category. In a properly designed multi-storey building, soil, waste, and rainwater are distinct systems, each sized according to its own load logic.
Soil stacks
Soil stacks carry discharge from water closets. Indian plumbing practice, guided by IS 5329 (the code of practice for sanitary pipe work above ground) and NBC 2016 Part 9 on plumbing services, typically sizes soil stacks and branches at 100 to 110mm, since WC discharge volume and solids content demand this diameter regardless of building height in most residential and commercial configurations.
Waste stacks
Waste stacks carry discharge from basins, sinks, and floor traps, generally at a smaller diameter than soil, commonly in the 32 to 75mm range depending on the specific fixture, with floor traps and shower waste typically sized toward the upper end of that range and basin waste toward the lower end.
Rainwater stacks
Rainwater stacks are a genuinely separate system from soil and waste, collecting roof and terrace runoff, and should never be combined into the same stack as sanitary discharge. Rainwater stack sizing is driven by roof or terrace catchment area and expected local rainfall intensity, not by fixture unit count, which means it follows an entirely different calculation from the sanitary stacks in the same building.
Keeping these three systems clearly separated in both design and labelling matters for a multi-storey project specifically because a building with many floors has enough total pipe runs that confusion between systems during construction becomes a real risk if the drawings and material specifications don't distinguish them clearly.
Stack sizing by fixture unit load, not guesswork
The core principle behind proper stack sizing, whether for soil or waste, is that diameter should be determined by the cumulative discharge load the stack actually carries, not assumed from habit or copied from an unrelated project.
Why this matters more in multi-storey buildings
A single-storey home's soil stack carries the discharge of one floor's fixtures. A ten-storey building's main soil stack, particularly in its lower sections, carries the cumulative discharge of every floor above it. Sizing every section of the stack identically, based on a single floor's load, risks under-sizing the lower sections where cumulative flow is highest.
Practical sizing approach
- Calculate cumulative fixture unit load at each section of the stack, recognising that the base of the stack, where it transitions into the building drain, sees the combined discharge of every floor above, while sections higher up carry a progressively smaller cumulative load.
- Confirm whether the lower sections of a tall building's main stack need to step up in diameter beyond the standard 100 to 110mm used higher up, particularly for buildings with a high fixture count per floor or an unusually large number of floors connected to one stack.
- Don't undersize based on cost alone. The cost difference between correctly sized SWR pipe and undersized pipe is genuinely small relative to a multi-storey building's total construction cost, while the cost of remedial work on an occupied building's drainage stack is considerably higher.
Avoiding offsets in vertical stacks
A vertical soil or waste stack works best as a single, unbroken vertical run from top to bottom. Offsets, where the stack shifts horizontally partway up the building to route around structural elements, disrupt the stack's self-cleansing flow characteristics and can create a pressure zone that interferes with venting further up or down the stack.
Where an offset is genuinely unavoidable due to structural constraints, it should be treated as a specific design detail requiring its own relief venting around the offset point, not simply routed and left to function like the rest of the stack. Flagging potential offset locations during structural coordination, before the drainage design is finalised, avoids discovering this problem mid-construction when options for resolving it are more limited.
Venting: what actually prevents trap siphonage
Every trapped fixture, a basin, a floor trap, a WC, relies on a water seal in its trap to block sewer gas from entering the building. When drainage flows past a trap without adequate venting, it can create a pressure difference strong enough to siphon that water seal out, and once a trap loses its seal, sewer gas and odour have a direct path into the room.
Where trap siphonage risk is highest
Long fixture branches, fixtures on lower floors closer to the base of a tall stack where flow volume and pressure fluctuation are greatest, and back-to-back fixture arrangements sharing a single branch are the classic locations where inadequate venting causes gurgling, slow drainage, and odour complaints.
How venting actually addresses this
A stack vent, the extension of the soil or waste stack itself above the highest connected fixture and typically continuing through the roof to atmosphere, is the baseline venting most stacks rely on. This allows air to move freely within the stack, equalising pressure so that a large discharge event doesn't create enough negative pressure downstream to pull water out of a trap seal.
For multi-storey buildings specifically:
- Very tall buildings or those with a high number of floors connected to one stack often need supplementary relief venting beyond the simple stack vent extension, since the pressure fluctuations in a long stack under heavy simultaneous use can exceed what a single vent stack comfortably equalises. This should be confirmed against current NBC 2016 Part 9 and IS 5329 provisions for the specific building height and fixture count involved, since exact thresholds are height and load-dependent rather than a single fixed rule.
- Individual venting for long branch runs protects fixtures at the end of an extended horizontal branch, where the stack vent alone may not adequately equalise pressure at that distance.
- Never assume venting is optional to save on material and labour. A stack that appears to drain adequately during initial testing can still develop siphonage problems under real, simultaneous multi-floor usage patterns that a single-fixture test doesn't replicate.
Slope and gradient for horizontal branches
Horizontal drainage branches need a controlled fall, steep enough to keep flow self-cleansing but not so steep that solids outrun the water and get left behind. A gradient in the range of 1 in 40 to 1 in 60 is typical guidance for 100 to 110mm soil branches, with waste branches following a comparably steady, consistent fall along their length. In a multi-storey building with branches running to a common stack from multiple units per floor, maintaining consistent gradient across every branch, not just the ones easiest to route, is a detail worth checking floor by floor during construction rather than assuming uniform compliance.
Phasing bulk SWR procurement against the construction schedule
Multi-storey projects rarely need their full SWR quantity delivered at once, and phasing procurement sensibly against the construction schedule avoids both storage problems and cash flow strain.
A practical phasing approach
- Align delivery phases with floor-by-floor construction progress, ordering stack and branch pipe for the floors currently under plumbing rough-in rather than stockpiling the entire building's requirement on site from the start.
- Order stack pipe for a given diameter from a single batch wherever possible, since consistent wall thickness and dimensional accuracy across a stack matters more in a continuous vertical run than in isolated branch connections.
- Coordinate fittings procurement with the pipe delivery schedule, ensuring bends, tees, and access fittings for each floor's connections arrive alongside the stack pipe for that phase rather than as a separate, potentially mismatched order.
- Confirm sizing for every floor before committing the full project's bulk order, since discovering a sizing correction is needed after several floors' worth of pipe has already been purchased is considerably more costly than confirming the specification once at the outset.
Specifying SWR correctly from the start
Multi-storey building drainage depends on treating soil, waste, and rainwater as genuinely separate systems, sizing stacks for their actual cumulative fixture unit load rather than a flat assumption, avoiding offsets or properly venting them where unavoidable, and providing adequate venting throughout the system to prevent the trap siphonage that causes the gurgling, odour, and blockage complaints builders most want to avoid after handover.
Trity Pipes manufactures SWR pipes and fittings suited to the sizing range multi-storey buildings need, from soil stacks through waste branches to rainwater systems. You can review our full SWR pipes and fittings range for the sizes that fit your project, and our comparison of SWR and PVC pipes if you're still confirming SWR is the right choice for your drainage system.
Specifying SWR for a multi-unit project? Get sizing support and a bulk quote phased against your construction schedule, and our team will help you confirm stack and branch sizing before you order.
Frequently asked questions
What size should the main soil stack be in a multi-storey building?
100 to 110mm is the typical diameter for soil stacks and branches in most residential and commercial buildings, though the lower sections of a very tall building's main stack, where cumulative discharge from many floors converges, may need to step up beyond this standard size. Confirm sizing against actual fixture unit load for your specific building rather than assuming a fixed diameter regardless of height.
Can rainwater and soil drainage share the same stack?
No. Rainwater stacks should always be kept separate from soil and waste stacks, since rainwater sizing is driven by roof or terrace catchment area and local rainfall intensity, an entirely different calculation from sanitary fixture unit load. Combining them risks both under-sizing during heavy rainfall and cross-contamination concerns.
Why does my building have gurgling sounds and odour on lower floors?
This is a classic sign of trap siphonage caused by inadequate venting, particularly common on lower floors closer to the base of a tall stack where flow volume and pressure fluctuation are greatest. A stack vent extension is the baseline solution, and very tall buildings or those with long branch runs may need supplementary relief or individual venting to fully resolve it.
Should a soil stack ever have a horizontal offset partway up a building?
Avoid it wherever possible. Offsets disrupt a stack's self-cleansing flow and venting characteristics. Where structural constraints make an offset genuinely unavoidable, it needs specific relief venting designed around that point, not just routed the same way as the rest of the stack.
How should I phase SWR pipe orders for a multi-storey construction project?
Align delivery with floor-by-floor construction progress rather than ordering the full building's quantity upfront, ordering stack pipe from a single batch per diameter for consistency, and coordinating fittings delivery with the pipe schedule for each phase. This reduces on-site storage strain and gives you the chance to confirm sizing is correct before committing to the full project quantity.
What's the difference between a stack vent and a vent stack?
A stack vent is the extension of the soil or waste stack itself above the highest connected fixture, typically continuing through the roof, and serves as the baseline venting for most buildings. A vent stack is a separate, dedicated vertical vent pipe installed specifically to provide additional air circulation to the drainage system, generally used in taller or higher-load buildings where a stack vent alone doesn't adequately equalise pressure throughout the system.