Do I need GPR before drilling?

The short answer is: almost always, on any structural concrete element where you do not have complete and reliable as-built records.

Why pre-drill scanning matters

Drilling into reinforced concrete without scanning carries real risks:

• Severed electrical cables — safety incident, outage, expensive remedial work.
• Severed water and gas services — flood, gas escape, statutory consequences.
• Damaged conduit and comms — chasing-out and re-pouring slab segments.
• Damaged post-tension tendons — the most expensive avoidable mistake on any modern slab.
• Damaged sprinkler pipework — flood damage to occupied space below.
• Project delays — even minor strikes typically cost more in programme than the survey would have cost in time.

The cost of a half-day pre-drill scan is much smaller than the cost of any of these.

When it is non-negotiable

• Post-tension floors. A struck PT cable can cause structural redistribution, costly repair, or worse.
• Live electrical environments (data centres, healthcare, manufacturing).
• Heritage and listed buildings where invasive damage would be irreversible.
• Engineered slabs (transfer slabs, post-tension floors, beam-and-block) with specific reinforcement layouts.
• Drilling close to slab edges or known services where geometry is uncertain.

If the slab might be post-tension and you do not know for sure, treat it as PT and scan. The downside of caution is half a day; the downside of a strike is enormous.

When it might not be needed

For small fixings on lightly reinforced ground-bearing slabs in non-critical environments, pre-drill scanning is sometimes skipped — but the cost-benefit tilts against this for almost any structural element. The honest position: when in doubt, scan.

What good practice looks like

Pre-drill GPR is part of the contractor’s RAMS for any drilling into structural concrete. The standard pattern:

1. Mark the proposed drilling positions on the slab.
2. Brief the surveyor on the structure type, history, and known plant.
3. Survey the marked positions and a reasonable buffer.
4. Mark detectable reflectors found.
5. Reposition any drilling target that conflicts.
6. Sign off the drilling plan against the survey.

Insurers and main contractors increasingly require this loop on any non-trivial drilling on RC structures.

Cost vs benefit

A pre-drill scan on a typical area is a few hundred pounds. A struck cable is thousands. A struck PT tendon can cost orders of magnitude more. The arithmetic favours scanning on virtually any non-trivial drilling operation.

When it is skipped

The temptation to skip the scan usually comes from one of three places: tight programme, tight budget, or “it’s just a few small holes”. The first two are addressable in planning. The third is a category error: a small hole through a PT tendon causes the same damage as a large one.

If your project has reinforced concrete and you have any doubt at all about what is inside it, the right answer is almost always to scan first. For a fuller treatment, see our article GPR scanning before drilling — why it matters.