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: one of the most serious avoidable mistakes on a slab.
• Damaged sprinkler pipework: flood damage to occupied space below.
• Project delays: even minor strikes can cost more in programme than the survey would have taken.

The cost of a pre-drill scan is usually 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 a short delay; 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 should be part of the contractor’s RAMS for drilling into structural concrete. A typical 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. Have the engineer or contractor approve the final drilling positions.

Cost vs benefit

A pre-drill scan usually costs far less than repairing a struck cable, and a struck PT tendon can cost far more again. The cost case 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.