Plumbing pipework is one of the few things in a building that becomes almost impossible to fix once walls are plastered and floors are laid, so getting the pipe material, slope and jointing right the first time matters far more than in most other trades. CPVC is the standard choice today for hot and cold water supply lines because it handles both temperature and pressure well, UPVC is used mainly for cold water and drainage lines, and GI pipe, once the default, is now used more selectively because of its tendency to corrode and reduce bore over time.
Drainage and soil lines depend entirely on gravity, so their slope is critical — a typical soil line slope of around 1:60 keeps solids moving without the line running dry of water seal or, at the other extreme, without pipes being laid so flat that the flow slows and blockages build up. Every fixture connects through a trap, either a gully trap outside for waste water or a floor trap or nahani trap inside, which holds a water seal that blocks foul gases from entering the room, and every vertical soil stack needs a vent pipe carried up above roof level so that trap seals are not sucked dry by siphoning when fixtures are used.
Before any pipe run gets buried in a wall chase or under a floor screed, it should be pressure tested — filled with water and held under pressure for a set period to check for leaks at joints — because a leak discovered after walls are closed up means breaking finished work to repair it. CP fittings such as taps and showers should be set out at levels checked with the tile setting-out plan so they sit correctly relative to the finished floor and wall tiles, and waterproofing detailing around the nahani trap connection needs particular attention since this is one of the most common points where bathroom leaks originate.
Typical soil line slope
1:60
Test before concealment
Pressure test
Common leak point
Nahani trap connection
Remember on site
- CPVC for hot/cold supply, UPVC mainly for drainage; slope soil lines around 1:60
- Traps and vent pipes maintain water seals and prevent foul gas entry — never skip venting
- Pressure test every line before it gets concealed in walls or floors
Check yourself
Why must plumbing lines be tested before walls and floors are closed in around them?
What is CPVC pipe mainly used for in residential plumbing?
Why has GI pipe usage declined for water supply in favour of CPVC/UPVC?
What is a typical slope given to soil drainage lines?
What is the function of a trap (like a floor trap or gully trap) in a drainage line?
Why does a soil stack need a vent pipe carried above roof level?
What does a pressure test on a plumbing line check for?
Why should CP fitting levels be checked against the tile setting-out plan?
Which connection point is a particularly common source of bathroom leaks?
UPVC pipe is mainly used for which application in a residential building?
What happens if a soil line is laid too flat, well below the recommended slope?
Why is it risky to skip venting on a drainage stack even if it seems to work fine initially?