Drainage Stack Sizing
Drainage stack sizing selects the diameters of vertical soil and waste stacks — and the venting that protects them — from accumulated fixture-unit loads, so gravity drainage runs full-bore-free, traps keep their seals, and sewer gases stay in the pipe.
Why it matters on an MEPF project
Drainage fails socially before it fails hydraulically: gurgling traps, smells in bathrooms, and blockages are all symptoms of stacks and vents sized or routed casually. The fixture-unit method (the NBC Part 9 basis) accumulates probable simultaneous discharge down the stack; venting — open stack tops, and relief/loop vents where layouts demand — keeps pressures near atmospheric so trap seals survive the slugs of water passing by.
Detailing decides longevity: offsets handled properly (each offset is a pressure event), accessible cleanouts at code intervals, correct slopes on horizontal drains (too flat strands solids; too steep strands them differently), and separation disciplines (soil vs waste vs storm) per local bye-laws. High-rises add their own physics and deserve engineered single-stack variants only where genuinely designed, not copied.
How it's specified in practice
| Parameter | Typical / working position | Governing reference |
|---|---|---|
| Loading method | Fixture units accumulated per stack | NBC 2016 Part 9 basis |
| Venting | Stack vents + relief venting per layout | NBC 2016 Part 9 practice |
| Slopes & cleanouts | Self-cleansing slopes; access at intervals/turns | plumbing practice |
| Separation | Soil/waste/storm segregation per local rules | ULB bye-laws |
Common mistakes
- Vents value-engineered out — the smells arrive with occupancy.
- Offsets mid-stack with no relief, siphoning traps below.
- Storm water tied into soil stacks "temporarily".
- No cleanouts where blockages statistically happen.
Related on this site
Frequently asked
Why do my floor traps smell despite water seals?
Pressure transients siphoning or blowing the seals — a venting problem, occasionally an evaporation one in unused traps. The vent design, not perfume, is the cure.
What size soil stack does my building need?
Whatever the accumulated fixture units and the NBC Part 9 tables say for the height and layout — it is lookup engineering once loads are counted honestly. Our pipe-size calculator handles the pressure-side siblings; drainage follows the code tables in design.
Standards referenced: NBC 2016 Part 9 · ULB bye-laws