House Drains & House Sewers
Chapter 12 governs the pipe that carries a building's excreta drainage from the building drain out to the point of disposal — the house (building) sewer. It sets the rule for when you must connect to the public sewer versus a private septic system, the approved materials, the burial clearances that protect both the structure and the potable water service, and the minimum size and slope of the sewer.
Sewer Required & Public-Sewer Connection
When you must connect to the public sewer — and when a private system is allowed
Every building containing plumbing fixtures, and all premises with drainage piping, shall connect to a public or private excreta sewerage system. The route of disposal depends on whether a public sewer is available:
- Public sewer available (within 61 m of the building/drainage facility) — connection to the public sewer is mandatory.
- Public sewer not available (more than 61 m away) — connect to an approved private sewage disposal system, e.g. a septic tank (see Chapter 10).
Materials & Under-Slab Installation
Approved house-sewer pipe and the rules for buried/under-slab runs
| Material | Use | Restriction |
|---|---|---|
| Cast iron / ductile iron | ✓ Approved | Most durable for under-slab and building sewer |
| Series 1000 PVC-DWV | ✓ Approved | Most common in Philippine practice |
| Galvanized steel / wrought iron | ⚠ Limited | Under a slab on grade: requires a machine-applied outside protective coating — galvanizing alone is NOT adequate |
| Lead, copper, brass | ✓ Approved | Per material standards |
| Extra-strength vitrified clay | ⚠ Outside only | Prohibited inside/under a building or where pump/ejector-pressurized; bury with ≥0.3 m earth cover |
| Concrete sewer pipe | ⚠ Outside only | Same restriction as vitrified clay |
Layout, Clearances & Excavation
Separation from structures and the potable water service — Table 12-1
No house sewer may be buried within 1.5 m of any building foundation, wall, or structure drawing support from the same soil.
House sewers shall be laid at least 0.6 m below finished ground surface.
Water-Pipe Trench Isolation
A house sewer trench must stay completely clear of any water service line unless both of these are met:
- The bottom of the water pipe is ≥0.3 m above the top of the sewer line at all points; and
- The water pipe sits on a solid, unexcavated shelf on one side of the trench, with a minimum 0.3 m clear horizontal distance from the sewer.
Table 12-1 — Minimum Horizontal Distance from Building Sewer
| Element | Minimum Horizontal Distance |
|---|---|
| Buildings or structures (incl. porches, steps, breezeways, roofed patios, carports, covered walks/driveways) | 0.6 m |
| Property line adjoining private property | Clear (per §1208) |
| Water supply wells | 15.2 m (7.6 m if sewer is of within-building–approved material) |
| Streams | 15.2 m |
| On-site domestic water service line | 0.3 m (parallel) |
| Public water main | 3.0 m (parallel) |
Crossings of a public water main require approval of the Health Department / authority having jurisdiction.
Sanitary (house) sewer — requirements & installation. The building sewer runs from 0.6 m outside the foundation to the public sewer or private disposal system, on a uniform 2% grade, minimum 102 mm diameter, with cleanouts at the upper terminal, at each change of direction >45°, and at intervals along its length. Keep ≥1.5 m from foundations and isolate from the potable water service per Table 12-1. (NPC §1202)
Sizing & Slope Matrix
Minimum diameter, standard grade, and permitted slope reductions — Table 12-2
| Pipe Size | Standard Grade | Reduced Grade (with approval) |
|---|---|---|
| 102–152 mm (4″–6″) | 2% (20 mm/m) | down to 1% (10 mm/m) |
| 203 mm (8″) and larger | 2% (20 mm/m) | down to 0.5% (5 mm/m) |
Reduced grades are permitted only where a 2% slope is structurally impossible, subject to formal engineering review and Administrative Authority approval.
Table 12-2 — Maximum / Minimum Fixture-Unit Loading on Building Sewer Piping
| Size of Pipe | s = ½% (5.3 mm/m) | s = 1% (10.5 mm/m) | s = 2% (20.9 mm/m) |
|---|---|---|---|
| 152 mm (6″) | per Table | per Table | no minimum loading |
| 203 mm (8″) | 1,950 | 2,800 | 3,500 |
| 254 mm (10″) | 3,400 | 4,900 | 6,800 |
| 305 mm (12″) | 5,600 | 8,600 | 11,200 |
Values are maximum drainage fixture units (DFU) permitted on the building sewer at the given slope (the steeper the grade, the higher the capacity).
Building Sewer Size & Slope Checker
Enter the proposed pipe diameter and grade to check §1204 compliance.
Abandoned Sewers
Capping and plugging disused house sewers