Ever wondered how an inspector can tell whether a wall is wet without tearing it open? The short answer is a mix of instruments, observation and an understanding of where water likes to travel. The longer answer follows.
What tools are used?
Most inspectors carry at least two kinds of meter. A non-invasive meter, sometimes called a capacitance or radio-frequency meter, is pressed against a wall or ceiling and gives a relative reading based on how the material beneath responds to an electrical field. It doesn’t leave a mark and can cover a lot of ground quickly, which makes it handy for scanning around windows, showers and skirting boards.
A resistance meter, by contrast, uses two pins that are pushed into timber. The current flowing between them indicates the moisture content of the wood itself. Where there is access, such as in a roof space or subfloor, it can be used directly on framing. To reach framing hidden behind cladding, small holes are drilled through the exterior, which is why this approach is known as invasive testing.
Thermal imaging cameras are common too. They show differences in surface temperature, and because damp areas often read cooler as moisture evaporates, a camera can hint at a problem. A thermal image on its own doesn’t measure moisture, though, so it is normally paired with a meter to confirm what the camera suggests.
Where in the house are readings taken?
You’ll find most attention goes to places where water gets in or where plumbing runs. Typical spots include the bottom corners of windows and doors, walls below or beside decks and balconies, around showers and baths, behind toilets and vanities, under kitchen sinks, beneath roof valleys and at ceiling level below internal gutters or flat roofs. Walls on the side of the house that takes the prevailing weather often get a closer look as well.
Should I ask for invasive testing?
It depends on the house. For a weatherboard home with decent eaves, a cavity and no warning signs, non-invasive readings taken during a standard building inspection may be enough. For a plaster-clad house built without a drained cavity, or any property where the non-invasive scan turns up worrying readings, invasive testing gives much clearer answers.
Because it involves drilling into someone else’s property, you’ll need the vendor’s written permission. Some vendors agree readily; others refuse, which in itself can tell you something. The holes are sealed afterwards, but agreeing the scope and number of test points in advance avoids disputes.
What do the numbers actually mean?
Readings from non-invasive meters are relative rather than absolute. An inspector compares readings across the same wall and between dry and suspect areas, looking for patterns. A spike near the corner of a window that tapers away is more telling than a single odd number in the middle of a wall.
Timber moisture content from a resistance meter is expressed as a percentage, and the figure is affected by timber species, treatment and temperature, so inspectors apply corrections and their own judgement. Higher figures suggest a growing risk of decay, but a single reading is not a diagnosis. Persistent dampness over time is what causes rot, and one test only shows the conditions on that particular day.
Weather plays a part as well. Testing after a dry spell may produce lower readings than testing after heavy rain, which is one reason many inspectors record the conditions on the day in their report.
If your report flags something, ask the inspector how confident they are and what the next step would be. It might be further invasive testing, a specialist weathertightness assessment, or a quote from a builder or cladding contractor. Knowing the difference between “keep an eye on this” and “investigate this before you buy” is the real value of the exercise.