Under Slab Plumbing: What to Check Before Pouring Concrete

A concrete slab gives your new home a solid foundation, but what happens underneath matters just as much as what sits on top. One misplaced pipe or poorly planned drain can turn your dream build into an expensive headache. And once the concrete goes down, fixing those mistakes becomes far more complicated.

If you’re building a home in Ulladulla, you probably spend plenty of time thinking about floor plans, kitchen finishes and bathroom designs. Fair enough. However, the plumbing beneath your slab deserves just as much attention. After all, nobody wants to break through a brand-new floor because a toilet outlet sits in the wrong spot.

That’s why careful Under Slab Plumbing planning matters before your builder schedules the concrete pour. Getting the pipe layout, drainage connections and inspections right from the start helps you avoid costly rework and unnecessary construction delays. So, what exactly should you check before the concrete truck arrives? Let’s walk through it.

1. Check Your Under-Slab Plumbing Layout Against the Floor Plan

Before your plumber starts laying pipes, make sure the plumbing layout matches your final building plans.

This sounds obvious, but small design changes can create big problems underground.

For example, imagine moving your ensuite toilet 300 mm to make room for a larger vanity. That minor adjustment might affect the waste pipe position, drainage route and slab penetrations.

I recommend checking these fixtures carefully:

• Toilets: Confirm the waste outlet position and connection requirements.
• Showers: Check the floor waste location, drainage arrangement and planned floor levels.
• Kitchen sinks: Allow for the final cabinet layout, including any island bench.
• Laundry fixtures: Confirm the washing machine and laundry tub drainage points.
• Floor wastes: Check where the design requires them and how water will reach them.

Compare the latest architectural plans with the plumbing drawings. Don’t rely on an earlier design if your builder has made changes.

Quick tip: Ask your plumber and builder to walk through every wet area before they finalise the underground pipework.

2. Check Pipe Gradients and Drainage Routes

Water needs the right pathway to leave your home. Gravity does most of the work, but your plumber must give it a little help.

Every gravity drainage pipe needs an appropriate slope, or fall, so wastewater can flow towards the sewer connection.

If a pipe has too little fall, wastewater may move slowly and allow solids to accumulate. Poorly planned changes in direction can also create maintenance problems.

However, making a drain steeper doesn’t automatically improve its performance. Your licensed plumber must choose the correct gradient for the pipe size, drainage design and applicable standards.

Before the concrete pour, ask your plumber to check:

• Pipe gradients and flow direction.
• Drainage junctions and changes in direction.
• Pipe sizes and suitable materials.
• Connections between fixtures and the main drain.
• The separation of sanitary drainage and stormwater systems.

Under Slab Plumbing also needs proper pipe bedding and support. Suitable bedding helps protect drainage pipes from sharp materials and uneven pressure.

3. Confirm NSW Plumbing Approvals and Inspections

Here’s something you shouldn’t leave until the last minute: inspection requirements.

In Ulladulla, plumbing and drainage work falls under NSW regulations, with Shoalhaven City Council carrying out relevant local regulatory functions.

According to the NSW Government’s regional plumbing and drainage inspection guidance, plumbers generally need to arrange drainage inspections after completing the work but before covering or backfilling it.

Your licensed plumber should also confirm which approvals, notices and inspection stages apply to your particular project.

For construction projects in the Shoalhaven area, this may involve Section 68 approval, a Notice of Work and council inspections, depending on the nature of the work.

I suggest asking three questions:

1. Have we obtained the necessary plumbing and drainage approvals?
2. Has the plumber arranged the required pre-cover inspection?
3. Has the relevant authority cleared the work to proceed?

Don’t assume your building inspection automatically covers every plumbing requirement.

Remember: Your plumber and builder should confirm the required inspection outcome before anyone covers the drains or pours concrete over the relevant installation.

4. Test the Plumbing System Before Covering It

Imagine discovering a leaking pipe underneath your new lounge room floor. Not exactly the housewarming surprise you had in mind.

That’s why testing plays such an important role in Under Slab Plumbing.

Before covering underground drainage, your licensed plumber should carry out the appropriate tests and checks under the applicable plumbing standards and inspection requirements.

These checks help identify leaking joints, damaged pipes and connection problems while workers can still reach the installation.

Depending on the system, your plumber may use an approved water, air or other test method.

Here’s what these checks help identify:

Plumbing check	What it helps identify
Drainage integrity test	Leaks or faulty connections
Pipe alignment check	Incorrect routes or displaced pipes
Gradient check	Drainage sections with unsuitable fall
Pipe support inspection	Inadequate bedding or unstable support
Fixture location check	Outlets that don’t match the floor plan

Your plumber should select the test method and acceptance criteria. Leave these technical checks to a suitably licensed professional rather than attempting your own pressure test.

Finding a problem now usually gives the team a much easier opportunity to correct it.

5. Check Pipe Locations Around Slab Footings

A house slab does more than hold your flooring. Its footings help support the building, and plumbing pipes must work around that structural design.

Poor pipe placement can interfere with footings, reinforcement or other important structural elements.

For that reason, your plumber and builder should review the engineering drawings before installing the underground drainage system.

Pay particular attention to:

• Pipes that pass near or through footings.
• Areas where structural beams cross drainage routes.
• Approved sleeves and pipe penetrations.
• Clearances around building foundations.
• Any engineer-specified requirements for pipe movement.

Never let someone cut through a footing or move reinforcement simply because a drainage pipe needs more space.

The structural engineer must approve any necessary design changes.

Good coordination between trades helps protect both the building structure and the plumbing system.

6. Consider Soil Conditions Around Ulladulla

Not every building site behaves the same way.

Some properties around Ulladulla and the NSW South Coast have sandy ground, while others contain clay, rock, fill or areas that retain water.

These conditions can affect how your builder prepares the slab and how your plumber installs the underground drainage.

For example, poorly compacted fill may settle after construction. Ground movement can then place stress on buried pipes and their joints.

On sites with reactive or unstable ground, the approved design may require special bedding, pipe supports or flexible connections that accommodate movement.

Before finalising Under Slab Plumbing, I recommend checking the soil classification and geotechnical recommendations with your builder.

Your plumber should coordinate with the structural engineer where the ground conditions call for additional protection.

The goal is simple: keep the drainage system working even when the ground experiences normal movement.

7. Make Sure Pipes Have Proper Protection

Once you pour concrete, you lose easy access to the pipework underneath.

That’s why pipe protection deserves attention before the pour.

Your plumber needs to check that the installation uses suitable materials, follows the approved design and allows for movement where necessary.

Some water supply arrangements may require protective conduits or other compliant solutions. In many cases, designers can also avoid running water supply pipes beneath slabs altogether.

Sanitary drainage pipes have different installation requirements, so don’t treat every underground pipe the same way.

Check whether the approved Under Slab Plumbing design includes appropriate:

• Pipe bedding and surrounding material.
• Sleeves where pipes pass through structural elements.
• Protection against ground movement.
• Clearances from other underground services.
• Arrangements for water supply lines, where applicable.

A protective sleeve or conduit doesn’t fix an incorrect pipe layout, though. Your plumber must get both the design and installation right.

8. Plan for Future Drainage Access

A good plumbing installation doesn’t just work on day one. It also gives plumbers practical ways to investigate problems years later.

That’s where inspection openings, accessible cleaning points and sensible drainage routes matter.

Your licensed plumber should confirm where the design requires access points and keep them available after construction.

Also check the location and finished levels of any required overflow relief gully. This part of the drainage system helps manage certain sewer overflow situations, so its position matters.

I also recommend keeping accurate records of the underground pipe layout.

Ask your plumber or builder to photograph the pipework before backfilling and pouring concrete. Include clear reference points, such as external walls or fixed building corners.

These photos won’t replace the formal sewer service diagram, but they can help future plumbers understand the installation.

9. Watch for Last-Minute Changes Before the Pour

Construction projects rarely follow every drawing perfectly. Sometimes, the kitchen layout changes. Other times, someone decides to move a bathroom wall.

These changes can affect Under Slab Plumbing, even when they look minor on paper.

Before concrete work begins, make sure the builder, plumber and relevant trades agree on the final layout.

For example, moving a toilet or adding a second bathroom fixture may require a different drainage arrangement.

Don’t ask a concrete worker to shift a pipe during the pour to solve a last-minute problem. Your licensed plumber should review the change and make any necessary corrections beforehand.

Also confirm that other trades haven’t damaged or displaced the installed pipes while preparing reinforcement or working around the slab.

A short coordination meeting can prevent a surprisingly long list of problems.

10. Complete a Final Under-Slab Plumbing Checklist

By this point, you’ve covered the important technical details. Now you need one final check before the concrete truck arrives.

I recommend using this quick pre-pour checklist with your builder and licensed plumber.

Final check	What you need to confirm
Plumbing layout	All outlets match the latest approved plans
Drainage gradients	The plumber has checked the required falls
Pipe bedding	The installation has suitable support and protection
Plumbing testing	The plumber has completed the required tests
Structural clearances	Pipes comply with the approved engineering details
Inspection requirements	The plumber has completed the necessary booking and inspection process
Drainage access	Required inspection openings and access points remain accessible
Documentation	The relevant records and photographs are available

Don’t treat this checklist as a replacement for professional inspections. Instead, use it to guide a final conversation with your construction team.

Your builder should also confirm that the plumbing installation won’t interfere with the slab preparation, reinforcement or concrete placement.

If something doesn’t look right, raise it before the pour.

What Happens If You Get Under-Slab Plumbing Wrong?

A small underground plumbing mistake can create an expensive repair job once your home reaches the finishing stage.

The actual consequences depend on the problem, but common risks include:

1. Blocked or slow-moving drains

Incorrect pipe gradients, poorly planned junctions or damaged drainage pipes may contribute to recurring blockages.

2. Wastewater leaks beneath the slab

Faulty joints or pipe damage can allow wastewater to escape into the surrounding ground.

3. Expensive floor repairs

If plumbers cannot reach a faulty pipe through another practical route, repairs may require opening part of the concrete floor.

4. Construction delays

Missing inspections or non-compliant installation work can interrupt the building schedule while your team resolves the issue.

5. Problems during future renovations

Incomplete pipe records make it harder to locate underground services when you want to change a bathroom or kitchen layout.

Getting Under Slab Plumbing right during construction helps reduce these risks and supports a more reliable home drainage system.

Conclusion: Get the Plumbing Right Before the Concrete Goes Down

A good home starts with a solid foundation, but it also needs plumbing that works quietly and reliably underneath it.

Before pouring concrete, focus on the essentials: accurate pipe placement, proper drainage gradients, suitable materials, testing, structural coordination and required NSW inspections.

I always think of Under Slab Plumbing as one of those jobs where attention to detail matters more than speed. A little extra checking before the pour can save plenty of frustration later.

If you’re planning a new home in Ulladulla, Mollymook, Milton or the surrounding NSW South Coast, contact a licensed local construction plumber to review your underground drainage requirements. Ask for a clear scope of work, confirm the inspection process and organise a quote before construction reaches the slab stage.

Getting expert advice early gives your project a much better starting point.

Frequently Asked Questions (FAQs)

1. How deep should under-slab plumbing pipes be?

The required depth depends on the drainage design, pipe gradient, structural footings and site conditions. Your licensed plumber should follow the approved plans and applicable Australian plumbing standards rather than use one standard depth for every home.

2. Can you pour concrete before a plumbing inspection?

You must complete the inspections that apply to your project before covering the relevant plumbing work. In the Shoalhaven area, your licensed plumber should confirm council requirements and obtain the necessary inspection clearance before the pour.

3. How do plumbers check under-slab pipes for leaks?

Licensed plumbers use appropriate testing methods to check the integrity of underground plumbing and drainage. They select the method according to the system, applicable standards and inspection requirements, then correct any faults before covering the pipes.

4. Can you repair under-slab plumbing after pouring concrete?

Yes, but repairs can become more complicated and expensive. Depending on the fault and pipe location, a plumber may need to access the pipe through the slab or consider another compliant repair method. Proper planning and testing help reduce the risk of future repairs.