Discovering that a drain pipe under your concrete slab has failed is one of the most stressful plumbing scenarios a property owner can face. The conventional solution, breaking the slab to access and replace the pipe, is disruptive, expensive and takes days. Drain relining offers a far less invasive alternative: a new structural pipe is installed inside the old one from access points at either end, with no concrete breaking required in most cases.
The quick answer
Yes, pipes under concrete slab foundations can be relined without breaking the slab. A CCTV inspection maps the pipe condition and confirms whether the bore is intact enough to accept a liner. If it is, a resin liner is installed from access points outside the slab (toilet pans, inspection shafts or pits). The cure takes 30-90 minutes and the result is a new pipe-within-a-pipe with a 50+ year service life.
What pipes typically run under a slab
In Central Coast homes built on concrete slabs (common from the 1970s onward), the following pipe types are typically found:
| Pipe type | Era | Location under slab |
|---|---|---|
| PVC (uPVC) | Post-1990 | Toilet pans, basin wastes, floor drains |
| AC cement | 1960-1985 | Main sewer drain running to boundary |
| Terracotta | Pre-1970 | Less common in slab homes but present in older builds |
| Cast iron | Pre-1960 commercial | Some heritage slab buildings |
The pipes most likely to fail under a slab are AC cement and older PVC. AC cement becomes brittle and delaminating over time. PVC from the 1980s, 1990s can suffer from low-grade formulations, UV-affected sections (where exposed before the slab was poured), or joint failures at rubber ring connections.
Why under-slab pipe failures are common
Slab movement
Concrete slabs move over time, they settle, expand and contract with temperature and moisture. This places physical stress on pipes passing through or under the slab. AC cement pipes are brittle and crack under this stress. Even rubber ring PVC joints can be pushed out of alignment.
Root intrusion
Tree roots follow moisture through the soil under a slab. If a pipe under the slab has a joint gap, roots enter. Under the slab, you can’t see root damage developing, the first sign is often a slow drain or back-surge inside the house.
No inspection access
The problem with under-slab pipes is that there’s no way to inspect them visually without either breaking the slab or using a CCTV camera. Problems develop silently for years before symptoms appear.
Soil movement under the slab
On the Central Coast, clay-bearing soils under slabs can shrink in dry weather and swell in wet weather, causing the slab to flex slightly with the seasons. This repeated movement fatigues pipe joints over decades.
Symptoms of under-slab pipe failure
| Symptom | What it suggests |
|---|---|
| Slow drains across multiple fixtures | Partial blockage or collapse in main drain under slab |
| Effluent smell inside the house with no visible source | Cracked pipe under slab allowing sewer gas to escape |
| Warm or wet patch on slab floor (in older homes) | Sewer exfiltrating under the slab |
| Cracks in slab near toilet/bathroom | Possible soil washout caused by pipe leaking under slab |
| Sewer gas smell in sub-floor space | Cracked pipe in sub-floor or under slab |
| Back-surge through floor drain | Blockage or collapse in drain line |
Any warm or damp patch on a concrete floor near a wet area should be investigated promptly, it can indicate a slow leak that’s undermining the soil under the slab.
How under-slab drain relining works
Access without breaking the slab
The key insight in under-slab relining is that you don’t need to access the pipe along its entire length, just at entry and exit points. These are typically:
- The toilet pan connection (inside the bathroom)
- The inspection shaft or pit outside the building
- An existing cleanout point
The toilet pan is removed temporarily, giving access to the drain pipe below. The camera is fed in, and the liner follows the same path.
The CCTV inspection phase
A camera is fed through from the toilet pan or cleanout point to map the pipe condition. In under-slab situations this is especially important because:
- You’re committing to a liner that you can’t easily retrieve or redo
- You need to confirm the pipe bore is intact, if a section has completely collapsed, excavation at that point is unavoidable
- You need to identify all junctions that will need to be reinstated
Liner installation
The resin liner is pushed or inverted through the pipe from the access point. It travels the full length of the pipe run, including the under-slab section, and is inflated against the pipe wall. The liner conforms to bends, gradients and existing pipe shape.
Curing
UV cure is preferred under-slab because it’s fast, precise and unaffected by moisture or temperature conditions inside the pipe. A UV lamp is drawn through the liner on a slow, controlled pass.
Reinstatement
The toilet pan is reconnected. Junctions (branch pipes connecting in) are reinstated by robotic cutter from inside the relined pipe. The final CCTV confirms a full, sealed installation.
When excavation is still needed
Drain relining under a slab is not possible when:
- A section of the pipe has completely collapsed, the bore is gone and the liner can’t pass through
- The pipe offset or displacement is so severe that the liner can’t navigate the bend
- A pipe junction needs to be relocated (not just reinstated)
In these cases, a hybrid approach is common: break the slab at the specific failure point, repair or replace that section, then reline the rest. This typically means breaking a much smaller area of slab than a full replacement would require.
Cost guide for under-slab drain relining on the Central Coast
| Job type | Typical price range |
|---|---|
| CCTV inspection (with toilet pan access) | $300, $550 |
| Single point repair under slab | $1,200, $2,500 |
| Full under-slab reline (8-15 m) | $4,500, $8,500 |
| Full under-slab reline (15-25 m) | $7,000, $13,000 |
| Hybrid: slab break at collapse + reline balance | $5,000, $15,000+ |
Under-slab work is typically priced at the higher end compared to the same pipe length in an open-access yard. The complexity of access, the need for toilet pan removal and reinstatement, and the higher stakes (no ability to redo from outside) justify the premium.
See the 2026 Central Coast drain relining cost guide for broader pricing context.
Comparing relining vs breaking the slab
| Factor | Drain relining | Break and replace |
|---|---|---|
| Slab damage | None | Significant, concrete coring/breaking |
| Floor tile damage | None | Significant in wet areas |
| Duration | 1 day | 3-7 days |
| Cost | 40-60% less typically | Full excavation + concrete reinstatement |
| Disruption | Low, toilets off for 2-4 hours | High, bathroom out of service for days |
| Noise | Low | Very high (concrete saw and jack hammer) |
| Structural risk | None | Some risk depending on slab design |
The case for relining is particularly strong in bathrooms and kitchens with tiled floors. The cost of retiling after slab break is substantial, and tile matching for older homes can be impossible.
Frequently asked questions
Does drain relining work in homes with post-tensioned slabs? Post-tensioned (PT) slabs are common in newer Central Coast homes. Breaking a PT slab requires specialist structural assessment because the steel cables are under tension. Drain relining is strongly preferred in PT slab homes, it avoids the need to touch the slab at all.
Can a pipe under a slab be relined if there’s a 90-degree bend? Yes. Modern liners and pull-in-place systems can navigate 90-degree bends with appropriate access. Sharp bends can sometimes require a short cut access pit immediately before the bend, but often a pull-in-place system can manage the turn.
How do I know if my Central Coast slab home has AC cement under-slab pipes? Homes built on slabs between approximately 1965 and 1985 on the Central Coast almost certainly used AC cement for the main drain. A CCTV inspection will confirm the material and condition.
Will the liner restrict my drain’s flow capacity? Minimally. The liner wall thickness is typically 4-8 mm, which slightly reduces internal diameter but actually increases flow efficiency because the liner surface is much smoother than old clay or AC cement. Net flow performance is equal or better.
What happens if the liner fails years down the track? It can be re-relined, a second liner can be installed inside the first. This is rare (liners are rated for 50+ years) but it’s the standard maintenance approach if needed.
Concerned about pipes under your slab? Book a CCTV inspection to find out what’s there.