Ferro scanning is a non-destructive way of locating ferrous reinforcement inside concrete and measuring how deep it sits and how big the bars are. It uses electromagnetic detection, sometimes called a cover meter or ferroscan technique, to sense the steel below the surface and report its position, cover and estimated diameter. Engineers and contractors use it to confirm rebar layout, check cover for durability or structural assessment, and find steel before drilling or fixing.
The name comes from ferrous, meaning iron or steel. Ferro scanning is tuned specifically to metal reinforcement, which makes it very good at what it does and clear about what it does not do. This guide explains the method, what it measures, how it differs from ground penetrating radar, and where it fits in a concrete investigation.
How ferro scanning works
A ferro scanner generates a magnetic field and measures how that field is disturbed by ferrous metal beneath the surface. Steel reinforcement changes the field in a way the instrument can read, and from that reading it works out how close the bar is (the cover) and, within limits, how thick it is (the diameter). The operator moves the device across the surface, and the readings combine into a map of the bars, their spacing and their depth.
Because the method responds to magnetic properties rather than sending a radar pulse, it is selective. It sees steel and largely ignores plastic conduit, timber and voids. That selectivity is exactly why it is useful when the question is specifically about reinforcement, and why it is often used alongside other methods when the picture is more complicated.
What ferro scanning measures
A ferro scan is typically used to establish:
- The position and spacing of reinforcement bars
- Concrete cover to the steel, which matters for durability and fire protection
- An estimate of bar diameter, within the instrument’s stated accuracy
- The layout of a reinforcement mat before drilling or fixing
Cover is often the headline result. Too little cover leaves reinforcement vulnerable to corrosion and can breach the design or the relevant code; too much can indicate the bars are not where the drawings suggest. Measuring it directly, non-destructively, gives an engineer real data rather than an assumption.
Ferro scanning versus GPR
Ferro scanning and ground penetrating radar are complementary, not competing, and understanding the difference helps you scope the right survey.
Ferro scanning is specialised for ferrous reinforcement. It gives reliable cover and good bar-size estimation at shallow to moderate depths, and it distinguishes steel clearly. Its limitation is that it only sees ferrous metal, so it will not find plastic ducts, non-ferrous services or voids, and its useful depth is limited compared with radar.
GPR sends a radar pulse and reflects off a much wider range of objects and interfaces, including post-tension tendons, conduits, services and voids as well as reinforcement. It reaches greater depth but does not automatically label what it finds, so interpretation is central. On many jobs the two are used together: ferro scanning to pin down cover and bar size, GPR to build the wider picture of everything embedded in the element. You can read more about the radar method in our guide on what a GPR survey is.
When you need a ferro scan
Ferro scanning is commonly called for:
- Structural assessment, where an engineer needs measured cover and reinforcement layout
- Durability or condition surveys on ageing concrete
- Checking as-built reinforcement against design drawings
- Confirming there is enough cover before drilling shallow fixings
- Locating bars to avoid before coring, cutting or chasing
On refurbishment and remedial projects, where drawings are missing or unreliable, a ferro scan turns guesswork about the steel into recorded measurements the design team can act on.
Strengths and limits
The strengths are precision and clarity on the one thing it is built for. Ferro scanning gives good, repeatable cover readings and reliable bar location without touching the concrete, and it is quick to deploy over an element or a floor plate.
The limits follow from the same specialisation. It only detects ferrous metal, so anything non-metallic is invisible to it. Depth is restricted, closely spaced or layered bars can be hard to separate, and diameter is an estimate within a tolerance rather than an exact figure. Where a full understanding of the slab is needed, ferro scanning is combined with GPR and, where appropriate, physical verification.
Common questions
What does ferro scanning detect?
Ferro scanning detects ferrous reinforcement, meaning steel bars, inside concrete. It measures their position and the depth of cover over them, and it estimates bar diameter. It does not detect non-metallic services, plastic ducts or voids, which is where ground penetrating radar is used instead.
Is ferro scanning the same as using a cover meter?
They are closely related. A cover meter is an electromagnetic instrument for measuring concrete cover to reinforcement, and ferro scanning uses the same electromagnetic principle, often with the ability to map bar layout and estimate diameter as well as cover. The terms are frequently used interchangeably on site.
How accurate is ferro scanning for cover and bar size?
Cover measurement is generally accurate within the instrument’s stated tolerance at typical cover depths, which is why it is trusted for structural and durability assessment. Bar-diameter estimation is less precise and is treated as an estimate. Accuracy falls off as depth increases and where bars are closely spaced or layered.
Can ferro scanning find post-tension cables?
It can detect the presence of ferrous metal, but reliably identifying a draped post-tension tendon and distinguishing it from ordinary reinforcement is a job for GPR and skilled interpretation. If you are working in a post-tensioned slab, radar-based detection is the appropriate method.
If you need reinforcement located and cover measured, or a full picture of what is inside a slab before you drill, our concrete scanning service combines ferro scanning and GPR and marks the findings on site. For a wider view of how the radar method compares with older techniques, see our guide on concrete scanning versus X-ray.