How We Investigate: Non-Destructive First, Intrusive Only Where It Earns Its Place

Every DML investigation follows the same logic: understand the symptom, list the possible causes, test each one until only the real cause is left, and write it down with the evidence attached. We use non-destructive testing (NDT) first. Intrusive investigation, such as lifting a floorboard, cutting a small inspection hole or a trial hole outside, is used only when the non-invasive results point to a specific place and the answer cannot be had any other way. You are told before anything is opened, and the reinstatement is included in the repair scope.

Stage 1: the conversation and the history

When did it start, does it change with rain, season, heating or bathing, has anything been repaired or decorated over, and what has anyone else already told you? Previous damp proofing guarantees, insurance claims, a water bill that has gone up or a boiler that keeps losing pressure are all evidence. We ask for photographs and, for a pre-purchase survey, the valuer or surveyor’s comment that triggered the instruction.

Stage 2: non-destructive testing

• Pin (resistance) moisture meter. Two pins measure electrical resistance between them. Good for timber and for comparing one point on a wall with another. Readings on plaster and masonry are relative, not a percentage of water, and can give false readings on foil-backed plasterboard, metal lath, conductive salts and some wallpapers and paints. We treat them as a map, not a verdict.
• Pinless (capacitance) meter. A flat sensor that reads a few centimetres into the material without marking the surface. Fast for moisture profiling across a wall or floor and for finding the edge of a wet area behind tiling. The same caveats about metal and foil apply.
• Hygrometer and surface thermometer. Air temperature, relative humidity, surface temperature and the calculated dew point tell us whether a surface is cold enough to condense. This is the core of a condensation and mould assessment, and we record it room by room.
• Thermal imaging. Shows temperature patterns: evaporative cooling over wet areas, warm plumes from heating pipes, cold bridging at lintels and slab edges. Always confirmed with a meter before it goes in the report. See thermal imaging surveys.
• Salt tests. Simple on-site indicator tests for nitrates and chlorides in plaster. Ground salts suggest true rising damp or chimney salt contamination; their absence points to a leak, condensation or penetrating damp. Laboratory analysis of a plaster sample can be arranged where the finding will decide an expensive repair.
• Pressure testing and isolation. For leaks, we test the incoming main, the hot and cold distribution, the heating circuit and, where fitted, underfloor heating loops separately. Isolating circuits one at a time is how a drop on a gauge becomes a named pipe. A meter drop test is used where there is a water meter.
• Dye testing. Coloured dye in a bath, shower tray, toilet cistern or gully shows whether a specific fitting or drain is the source and, for drains, whether there is a cross connection between foul and surface water.
• Acoustic listening and tracer gas. For pressurised pipes under floors and gardens. Used where appropriate, after the simpler tests have narrowed the area. See acoustic and tracer gas.
• CCTV drain camera. When drains are suspected of contributing to damp or of being defective, a camera survey with on-screen chainage records where each defect sits. See CCTV drainage surveys.

Stage 3: deep-wall and timber checks

Where surface readings are ambiguous, deeper measurement settles it. A calcium carbide (Speedy) test on a drilled sample gives the actual moisture content of masonry rather than a relative meter value. For disputes and lender reports, gravimetric analysis of a drilled sample, the laboratory method described in BRE Digest 245, separates moisture held by hygroscopic salts from free moisture, which is often the difference between “needs a damp-proof course” and “needs re-plastering”. Timber is probed, and joist ends, bearing ends, wall plates and purlins are checked wherever there is access because that is where wet rot, dry rot and beetle attack start.

Stage 4: intrusive investigation, only when it earns its place

Sometimes the answer is under a floor, inside a boxed-in duct or below ground. A lifted floorboard at the threshold, a small inspection hole in plasterboard, or a trial hole against an external wall to see the ground level, the damp-proof course and the drain run are all legitimate when the non-invasive work has already said where to look. We explain the exposure works before we start, keep them as small as the question allows, and write the making-good into the repair scope.

Why some investigations take two visits

Damp and leaks are often time-dependent. A heating leak shows best with the system hot; penetrating damp shows after rain; condensation shows in a cold snap; a drain blockage may need clearing before the run beyond it can be surveyed. Where a second visit is the honest way to a firm answer, we say so at the quotation stage rather than guessing on the day.

What the report contains

• The symptom as reported and as found
• Each possible cause and the test that confirmed or excluded it
• Readings, thermal and visible photographs and, where taken, laboratory results
• The cause, its location and the mechanism
• A repair scope written so that any contractor can price it
• What to check afterwards, and when it is safe to redecorate
• Negative results recorded as well as positive ones, because “the drains are sound” is a finding

Independence

DML is paid for the investigation. Repairs are quoted separately, by our team if you want a price, or by any contractor you choose. The report says the same thing whoever instructs it. Read more about us.

We investigate across Greater London and Surrey. See areas we cover.

Frequently Asked Questions

Related services

• Damp surveys
• Leak detection
• Mould inspections
• CCTV drainage surveys
• About DML