Drilling Into a Post Tension Slab: What to Check First

Drilling into a post tension slab without scanning it first is one of the highest-risk tasks on a construction or fit-out site. The steel tendons inside are under high tension, and cutting one can injure workers and damage the structure. At Dowd's Pipe and Cable Locating, we scan post-tensioned slabs across Wollongong, Sydney and the Southern Highlands so trades can drill and core in the right places.

Key Takeaways

• Drilling into a post tension slab without scanning is a serious safety and structural risk. The tendons are under high tension and can release that energy suddenly if cut.
• The first step before any drilling, coring or cutting is to locate the tendons, rebar, conduits and embedded services.
• We use ground penetrating radar (GPR) to detect post-tension cables, rebar, conduits and voids without damaging the slab.
• Striking a tendon or a live service means injury risk, structural repairs and project delays, all of which are avoidable with planning and scanning.
• Call 0434 635 134 to arrange a scan before any drilling starts.

What Is a Post Tension Slab and Why It Matters Before You Drill

A post tension slab is a concrete slab reinforced with high-strength steel tendons that are tensioned after the concrete has cured. That's the key difference from conventional reinforced concrete, where the steel sits passively in the slab.

The tendons run through ducts or sheathing cast into the concrete. Once the concrete reaches the required strength, they're stressed with hydraulic jacks and anchored at the slab edges. This puts the concrete into compression, which resists cracking and lets engineers design thinner slabs with longer spans.

The tendons usually follow a draped profile through the slab rather than sitting at one depth. Their depth changes across the span, so there's no way to guess a safe drilling depth. Cutting one can release stored energy suddenly and compromise how the slab performs. That's why we scan before any penetration on PT slabs in Wollongong and Sydney.

Where You'll Commonly Encounter Post Tensioned Concrete in NSW

Post tensioned concrete is common in modern multi-storey and commercial buildings across NSW, so trades run into it more often than they expect.

• Apartment and unit buildings across Sydney metro and Wollongong
• Suspended floors and podium slabs , often spanning over basement car parks
• Commercial buildings , including car parks, schools, hospitals and office fit-outs
• Residential ground slabs on sites where the engineer has specified post tensioning
• Bridges and infrastructure , though trades are less likely to be drilling these

If a building has thin floor slabs, long spans between columns and few visible beams, there's a good chance it's post tensioned. We regularly scan slabs in apartment buildings and commercial sites from Wollongong to the Sydney CBD before plumbers, electricians and HVAC installers make their penetrations.

Why Drilling Into a Post Tension Slab Is High-Risk Work

Drilling into a post tension slab is high-risk because the tendons hold a large amount of stored energy, and cutting one releases it without warning.

A tendon strike can cause:

• sudden release of the tendon, with risk of injury to anyone nearby
• concrete spalling around the strike point
• loss of prestress, leading to deflection, sagging and cracking
• water getting in through damaged ducts
• engineered structural repairs that cost far more than the original job

PT slabs also often carry electrical conduits, data cabling, water pipes and fire services within or just below the slab, and those can be hit too. The risk applies to every penetration method: hammer drilling, diamond core drilling, chasing and saw cutting. Have an emergency plan in place on any site where PT slabs are being penetrated.

How to Tell If Your Concrete Slab Is Post Tensioned

You can't confirm a post tensioned slab by looking at it, but several clues suggest one before you arrange a scan.

• Warning signs or plaques such as "Post-Tensioned Slab: Do Not Drill or Cut", often in plant rooms, basements or near wall openings
• Anchor pockets at slab edges, visible in basement car parks or on exposed soffits
• Structural drawings or as-builts that mention post tensioning, PT slabs, strand spacing or anchorages
• Building form: thin slabs, long spans and few beams

Drawings help, but actual tendon positions can differ from the design because of construction tolerances and site changes. If you're unsure, treat the slab as post tensioned, talk to a structural engineer, and arrange a scan before anyone drills. We help builders and strata managers across the Illawarra and Sydney work out what they're dealing with.

Why Concrete Scanning Is Essential Before Any Drilling

Concrete scanning is essential because it shows what's inside a slab, including tendons, rebar, conduits and voids, without damaging it, so drilling locations can be moved or depth-limited before anyone starts.

At Dowd's Pipe and Cable Locating, we use ground penetrating radar to detect post-tension cables, rebar, conduits and voids in concrete slabs. The scan maps the position and alignment of each feature, and we mark safe zones directly on the slab so the drill or core operator can see where to go and where not to.

GPR suits suspended PT slabs, ground slabs and other structural concrete on live sites. Dense reinforcement or wet concrete can affect the signal, and we'll tell you on site if conditions limit the results. You can learn more about how we scan on our concrete scanning page, and our article on how deep ground penetrating radar can see explains what affects GPR performance.

How Dowd's Approaches a Safe Drilling Job on a PT Slab

Dowd's Pipe and Cable Locating follows a clear process on PT slab jobs so every trade knows where it's safe to drill.

1. Site assessment: confirm the slab type, access and proposed penetration points, and review any drawings you have.
2. GPR scanning: scan each area, often in more than one direction, to map tendons, rebar, conduits and voids.
3. Surface marking: mark detected features directly on the slab so the operator can see where tendons and services run.
4. On-site discussion: agree safe drilling zones with the builder, plumber, electrician or HVAC installer.
5. Documentation: provide photos of the mark-ups so future penetrations can be planned around the layout.

This gives everyone on site in Wollongong, Shellharbour or Sydney a clear plan before the first hole is drilled.

Practical Do's and Don'ts When Drilling a Post Tension Slab

These are the rules for drilling a post tension slab safely, whether you're on the tools or managing the job.

Do:

• scan before any drilling, coring or cutting
• confirm the slab type from drawings, signage and a site inspection
• use depth-controlled drilling where the engineer allows it
• follow the structural engineer's directions, especially near columns and anchorages
• wear appropriate PPE
• record the location, depth and diameter of every penetration

Don't:

• guess tendon positions from assumptions or old drawings
• rely on a stud finder or basic metal detector
• drill near anchorages without engineering sign-off
• ignore warning signs on the slab or in the building
• keep drilling if you hit metal or feel unexpected resistance. Stop and reassess.

Larger penetrations, such as new risers or big duct openings, should always be engineered rather than decided on site. We scan for single penetrations and full grids alike across Wollongong and Sydney.

Legal, Safety and Insurance Considerations in NSW

Under the NSW Work Health and Safety Act 2011, duty holders must take reasonably practicable steps to prevent foreseeable incidents, and drilling into an unscanned post tension slab is a foreseeable risk.

• A damaged tendon or embedded service can be seen as preventable, which may affect insurance claims and defect liability.
• Project specifications and engineering documents often require scanning on PT slabs before penetrations.
• Many projects use formal penetration request forms and engineer sign-off for larger holes or critical zones.
• Keeping scan records, mark-up photos and penetration logs helps show due diligence during audits or defect periods.

At Dowd's Pipe and Cable Locating, we're fully insured with $20M public liability and professional indemnity cover. We provide documentation you can attach to your project records.

Why Use Dowd's Pipe and Cable Locating for PT Slab Work

Dowd's Pipe and Cable Locating is a local, Wollongong-based specialist in concrete scanning and underground locating, servicing Wollongong, the Illawarra, the South Coast, Sydney metro and the Southern Highlands.

We bring more than 25 years' experience in civil and underground work and are accredited with DBYD, Telstra, NULCA and Certloc. Our GPR scanning identifies rebar, post-tension cables, conduits and voids so trades can drill or core with confidence. Whether it's one hole for a new pipe or a full grid of penetrations across a basement slab, scanning first is the most cost-effective protection you can buy. Same-day and next-day callouts are available, and quotes are free and no-obligation.

Call 0434 635 134 to talk through your PT slab drilling plans before work starts.

What to Tell Your Trades Before They Drill or Core

If you're a building owner, strata manager or site supervisor, tell your trades clearly that no one drills, cores or cuts a post tension slab until it has been scanned.

• Confirm whether the slab is post tensioned and share any drawings or as-builts.
• Make scanning a requirement before any penetration, with no exceptions.
• Pass on previous scan results, mark-up photos and engineering reports for the building.
• Walk the slab with the scanner and the drill operator to agree on safe locations.
• If a penetration point moves during the job, scan the new location before drilling resumes.

Clear communication at this stage is much easier than dealing with a cut tendon. We're happy to brief trades on site in Wollongong and Sydney when we scan.

Frequently Asked Questions About Drilling Into Post Tension Slabs

Can I drill into a post tension slab to run new services in an existing apartment?

Often yes, but only after the slab has been scanned and, for larger holes, a structural engineer has approved the penetration. Apartment owners should also check with their owners corporation or building manager before any work starts.

How deep can I safely drill into a PT slab?

There is no generic safe depth. It depends on the slab thickness, tendon profile and reinforcement cover at each location, and because tendons are draped, the depth changes across the slab. Scanning shows where the tendons are so safe locations can be chosen with your engineer.

Is a standard stud finder or metal detector enough to locate tendons?

No. Stud finders and basic metal detectors aren't reliable for mapping tendons or multiple layers of reinforcement, and they can give a false sense of safety. Professional GPR scanning is the appropriate method for PT slabs.

Will scanning damage the concrete slab or finished floor?

No. GPR scanning is non-destructive and is done from the surface of the slab. Markings are made with removable paint or tape, and we can discuss options in advance for sensitive finishes like polished concrete or tiles.

How do I book scanning before drilling in Wollongong or Sydney?

Call Dowd's Pipe and Cable Locating on 0434 635 134. Have any plans, photos and slab details ready so we can scope the job quickly. You can also visit our concrete scanning page for more about the service.