IS 14735 : 1999Unplasticized Polyvinyl Chloride (UPVC) Injection Moulded Fittings for Soil and Waste Discharge Systems for Inside and Outside Buildings Including Ventilation and Rain Water Systems

IS 14735:1999 specifies unplasticized PVC (uPVC) injection-moulded fittings for soil and waste discharge systems — the bends, tees, junctions, traps, reducers and access fittings used with uPVC soil/waste/vent pipework inside and outside buildings. It is the fittings companion to the uPVC soil-pipe specification.

It is read with the drainage/plumbing stack:

• IS 13592 — uPVC pipes for non-pressure drainage & sewerage (the pipe these fittings join)
• NBC 2016 Part 9 — plumbing services · IS 1742 Part 1 — drainage outside buildings
• IS 5329 — sanitary pipework / building drainage code of practice
• IS 12818 / IS 15328 — uPVC system references

For a soil/waste fitting to perform over the building's life, IS 14735 fixes:

• Material — uPVC compound (impact-modified), UV-stabilised for external use
• Dimensions & compatibility — socket/spigot sizes matched to IS 13592 pipe so joints actually seal; angle of bends/junctions (e.g. 87.5°/45°, single/double junctions, swept entries to reduce blockage)
• Wall thickness / class — adequate ring stiffness for the application
• Jointing — solvent-cement socket or elastomeric ring-seal type
• Performance tests — dimensions, opacity, effect on water, impact, thermal stability (Vicat softening), and watertightness of joints

The key plumbing-engineering point: fittings must give swept (radiused) changes of direction and correct vent provision so the system stays self-cleansing and the trap seals are not siphoned.

Scenario: a uPVC soil stack with branch connections, fittings to IS 14735.

Step 1 — compatibility: confirm fitting socket sizes match the IS 13592 pipe class on the job (mixed makes/classes is the No.1 leak cause).

Step 2 — geometry: use swept/45° branch junctions into the stack, not square 90° tees — square entries cause cross-flow, blockage and trap-seal loss.

Step 3 — venting: size and connect the vent so branch discharges don't siphon trap seals (per NBC Part 9 / IS 5329) — fittings include the vent entries for this.

Step 4 — jointing: solvent-cement joints made on cleaned, deburred, chamfered pipe with correct cement and witness/cure time; ring-seal joints with the seal seated.

Step 5 — acceptance & test: check IS 14735 dimensional + impact + Vicat compliance on the delivered lot; hydrostatic/air test the installed stack before concealment.

1. Mixing makes/classes of pipe and fittings. Sockets that don't match the IS 13592 pipe OD/class never seal reliably — specify pipe + fittings as one compatible system.

2. Square 90° junctions on soil stacks. Cause turbulence, blockage and induced siphonage of trap seals — use swept/45° fittings.

3. Ignoring venting. Without correct vent fittings/sizing, branch discharge siphons the trap water seal and the bathroom smells — a fitting-layout failure, not a pipe failure.

4. Bad solvent-cement technique. Un-deburred/un-chamfered pipe, wrong cement, or insufficient cure → joint failure; the commonest defect on uPVC drainage.

5. External fittings without UV grade / no test before concealment. UV-degraded fittings crack outdoors; concealed un-tested joints are hugely expensive to chase later.

• IS 13592 — uPVC pipes for non-pressure drainage & sewerage
• NBC 2016 Part 9 — plumbing services · IS 5329 — sanitary pipework / drainage code of practice
• IS 1742 Part 1 — drainage outside buildings
• IS 12818 — uPVC fittings (other) · IS 15328 — uPVC underground sewerage
• IS 1172 — water supply, drainage & sanitation basic requirements

IS 14735 is reaffirmed and uPVC soil/waste systems have largely displaced cast-iron in Indian buildings on cost, weight and corrosion grounds — so this fittings spec is in play on virtually every modern project. The material is rarely the problem; the failures are systemic and workmanship-based: incompatible pipe/fitting makes, square junctions causing blockage, missing/incorrect venting that siphons trap seals, and poor solvent-cement joints.

The practitioner discipline is to specify pipe + fittings as one compatible IS 13592 / IS 14735 system, insist on swept junctions and correct venting per NBC Part 9 / IS 5329, enforce solvent-cement jointing procedure, and pressure/air-test every stack before it is boxed in. Get those right and uPVC drainage outlasts the building's other services; get the venting or junction geometry wrong and you get a permanently smelly, blocking system that no amount of pipe quality fixes.