The 7-step process
The DRB lays out a fixed sequence. Skipping a step often means the wrong repair gets specified.
| Step | Output | DRB ref |
|---|---|---|
| 1. Assess responsibility | Liability for repair work | Part 2 |
| 2. Site investigation | General site information | 3.2 |
| 3. Drain condition grading | Structural grade per drain | 3.3 |
| 4. Drain serviceability | Functioning correctly Y/N | 3.4 |
| 5. Identify need for repair | Repair / replace / no work | 3.5 |
| 6. Causes of failure | Root cause | 3.6 |
| 7. Select repair technique | Recommended method | 3.7 |
Three investigation types
| Type | Trigger | Pressure test required? |
|---|---|---|
| Reactive | Blockage, flooding, smells, rats | No — flow restoration first |
| Subsidence | Building structural problems | Yes — leakage test required |
| General | House-purchase survey, follow-up | Only if CCTV indicates |
⚠️ Jetting pressures — get this wrong and you damage the pipe
Most jetting rigs operate at pressures that damage plastics and pitch fibre pipes. The damage is often invisible on CCTV — it shows up later as small holes in the invert. Material identification first, pressure setting second.
| Pipe material | Max pumping pressure | (bar) | (psi) |
|---|---|---|---|
| Clay / concrete / asbestos cement | 340 bar / 5000 psi | 340 | 5000 |
| Plastics | 180 bar / 2600 psi | 180 | 2600 |
| Pitch fibre | 100 bar / 1500 psi | 100 | 1500 |
| Unknown material | 100 bar / 1500 psi | 100 | 1500 |
It's common — especially with pitch fibre runs — to find that the first 1-2 pipes leading from a manhole are clay (laid as part of the manhole construction). Check the run material visually further down before deciding pressure.
Best-practice jetting
- Work from the access downstream of the blockage. Nozzle goes upstream and is then pulled back through the blockage.
- Break the blockage into small pieces. Don't let the nozzle smash it loose en masse — it'll re-block downstream and damage bends/traps/gullies.
- Steady rewind rate 100-200 mm/sec for cleaning.
- Special nozzles for hard-scale and FOG (fats/oils/grease).
- Operative must hold a valid high-pressure water jetting training certificate.
Leakage testing
Water 'drop' test
- Plug downstream end, fill with water, monitor at gully or chamber.
- Maximum head 1.5 m above lowest invert.
- Fill to top, top up after 5-min conditioning.
- Stand 10 min; measure water needed to refill = leakage.
- Pass criterion (drains >5 m): <2 L per m² wetted internal area in 30 min.
- Short drain alternative: <25 mm drop in 10 min = pass.
Acceptable leakage — Table 3.3
| Diameter (mm) | 5-10 m | 10-20 m | 20-30 m |
|---|---|---|---|
| 100 | 0.2 L | 0.4 L | 0.6 L |
| 150 | 0.3 L | 0.6 L | 0.9 L |
| 200 | 0.4 L | 0.9 L | 1.3 L |
| 225 | 0.5 L | 0.9 L | 1.4 L |
Air test (per BS EN 1610:2015)
- Plugs isolate pipe lengths.
- Pressure raised to 110 mm head water (manometer); condition 5 min.
- Adjust to 100 mm head; measure change after 5 min.
- Maximum allowable head loss = 25 mm in 5 min.
Condition grades — A, B, C
The most important triage skill in the DRB. Get this right and the repair selection follows.
- Rigid pipes (clay, concrete, AC): insufficient arching support when deformation >10% of diameter → Grade C
- Non-rigid pipes (plastics, pitch fibre): insufficient arching support when deformation >20% of diameter → Grade C
- Fully displaced joints (displacement ≥ wall thickness, code "JX") → Grade C
Serviceability checklist
A drain is serviceable if functioning correctly today AND likely to continue. ANY of these = unserviceable:
- Failing to discharge normal household flows (recurring blockage)
- Evidence of leakage (infiltration or exfiltration on CCTV)
- Intermittent surface water flooding under normal conditions
- Existing/future root growth likely to cause continuing blockages
- Failed leakage test (subsidence investigations only)
- Defects make the drain unserviceable
Repair selection — Table 3.6
| Grade A | Grade B | Grade C | |
|---|---|---|---|
| Serviceable | No repair | No repair | Excavate & replace |
| Unserviceable | No repair* | No-dig (CIPP/patch) preferred if arching intact, else excavate | Excavate & replace |
*Recurring blockage with no defect → unserviceable but treat the cause (gradient, FOG management) rather than re-line.
Repair techniques — quick reference
Causes of failure — what to look for
| Cause | CCTV signature |
|---|---|
| Hoop overload (vehicle on shallow drain) | Longitudinal cracks at 12, 3, 6, 9 o'clock |
| Beam failure (soft ground below) | Circumferential crack |
| Bearing failure (point load) | Cracks radiating from bearing point; or bulge in pitch fibre |
| Shear (chamber/wall settlement) | Crack at structure-pipe boundary |
| Joint leakage washout | Open displaced joints; soil visible in pipe |
| Tree roots | Roots through joint seals and cracks; ~4 bar forward growth, 8-13 bar lateral |
| Pitch fibre deterioration | Blistering, delamination, bulges, flow narrowing |
| Aggressive effluent | Sulphate-attacked concrete; solvent-degraded plastics |
Pitch fibre + CIPP — special rules
- Treat as asbestos-bearing until proven otherwise. Control of Asbestos Regulations 2012 applies.
- If >20% deformation → CIPP is unsuitable; consider pipe bursting or excavate.
- If ≤20% deformation and uniform → CIPP can stabilise. Reaming first to remove blisters.
- After re-rounding, install CIPP within 4 hours.
- Pre-liner required on inversion CIPP into pitch fibre.
- Polyester resin reduction -10%; epoxy with pre-liner -10%; epoxy bonded to host -25%.
- Polyester CIPP min flexural modulus on pitch fibre = 1567 MPa (vs 1500 MPa standard).
Documentation — what must be retained
Every CIPP job needs a QC record. Retain for audit; supply to owner on request:
- CIPP system description
- Date
- Repair number / claim ref
- Homeowner name + address
- Lining thickness
- Drain diameter
- Lining length
- Resin name + batch number(s)
- Hardener name + batch number(s)
- Accelerator name + batch number(s) if used
- Mix ratio + proportions
- Ambient temperature (ambient cure)
- Cure temperature (hot cure)
- Cure time
CCTV recordings retained ≥3 months, available for audit.
Knowledge check — Module 3
Q1. A site report shows pitch fibre pipework. The maximum jetting pressure is:
Q2. A CCTV survey shows a clay pipe with longitudinal cracks at 12, 3, 6 and 9 o'clock. Most likely cause?
Q3. A clay drain shows 12% deformation. WRc condition grade?
Q4. Air test maximum allowable head loss in 5 minutes:
Q5. A patch repair within an undamaged section of a pipe is:
Q6. CCTV recordings must be retained for at least:
Q7. Best-practice jetting works:
Q8. Rocker pipe maximum length:
Q9. A reactive investigation. Drain blocks once, blockage cleared by rodding, no risk of property damage. WRc says:
Q10. A fully-displaced joint (displacement ≥ wall thickness):
Q11. Pitch fibre re-rounding before CIPP — install window:
Q12. The water 'drop' test. A 150 mm pipe, 15 m long. Acceptable leakage in 10 minutes is approximately:
Q13. A clay pipe is in Grade B and serviceable, in a general (non-subsidence) investigation. WRc recommendation?
Q14. The minimum flexural modulus for a polyester resin CIPP lining installed on pitch fibre host is:
Q15. A subsidence investigation finds a Grade B drain that fails the leakage test. WRc recommendation?