A moisture meter is not the test that matters. The test that matters is drilling the wall, collecting the dust and measuring the water in it chemically, and that is the one that says this wall contains no water at all.
I am a chartered building surveyor and about half my work is damp and timber.
The two instruments, because the whole trade argues about the first one and nobody argues about the second.
A resistance moisture meter has two pins. It passes a current between them and reads the resistance, and in timber that translates to a moisture content because the relationship is known. In masonry it does not translate to anything, which is why the needle on those meters is labelled relative, and a competent surveyor uses it to find where to look and not to decide.
Salts send it to the top of the scale. A wall full of nitrates from old plaster or chlorides from a leaking flue will read soaking wet when it is dry as a bone, and that is the single commonest misdiagnosis in British housing.
The instrument that settles it is a carbide meter, or a laboratory gravimetric test. You drill the wall, catch the dust at depth, and either react it with calcium carbide in a sealed vessel and read the pressure, or weigh it, oven dry it and weigh it again. Either way you get the percentage of water by weight, and that is a fact.
Below about two percent is a dry wall. Above five is wet. A wall with rising damp is wet at the bottom, drier as you go up, and salty at the top of the rise.
The north wall of the dining room in this house reads full scale on every resistance meter I own, to a height of about a metre and a quarter.
The carbide test says nought point two percent.
Nought point two is drier than the air in most houses. It is drier than the wall next to it, which came back at one point one and is a normal internal wall.
What I did, in order, over two visits in 2021 paid for by the owner and a third that I did not charge for.
Four resistance meters, three makes. All full scale over the same area, and the area has a top edge you can trace with the pins and it is about as level as a spirit level.
A capacitance meter, which is a different principle entirely and reads through the surface without pins. Also high.
Six carbide tests at two depths. Twenty millimetres and a hundred millimetres, along the wall at three positions. The highest reading of the twelve is nought point four percent.
A salt test, because salts are the obvious reconciliation. You take the drillings and test for nitrates and chlorides with a field kit, and a wall that reads wet and is dry is almost always a salty wall.
Negligible. Both. On all six samples.
I sent two samples to a laboratory in Sheffield for gravimetric and salt analysis, out of my own pocket, because at that point it was personal.
The report says the samples are dry, with no significant hygroscopic salt content, and that the masonry is a hard fired brick in a lime mortar in good condition.
What the house is and what is behind that wall.
Eighteenth century, solid brick, lime plaster, suspended timber floor with decent ventilation.
The north wall of the dining room is an external wall and outside it is a yard and then a stone boundary wall.
I have been outside with a thermal camera and a hose and a ladder. The pointing is sound, the ground level outside is two courses below the internal floor, which is unusually good for the age, and there is no defective gutter, no leaking downpipe and no bridged cavity because there is no cavity.
Why I cannot leave it.
A resistance meter reads a current passing between two pins. Something has to carry that current.
In a dry, salt free, hard fired brick wall, nothing does. That is not a matter of interpretation: that is the physics the instrument is built on. Those meters read high on water, on salts, on metal, on carbon, and on conductive contamination, and the laboratory has excluded all of the chemical ones.
So there is something in that metre and a quarter of brickwork conducting electricity, and I have taken dust out of it at two depths in six places and had it analysed, and the dust is brick.
What I wrote in the report, which is the part I am proudest of and which lost me the job.
I wrote that there is no rising damp in that wall, that the carbide and laboratory results are conclusive, that no damp proof course injection is required, and that the owner should not pay anybody who tells her otherwise.
Two damp proofing companies had already quoted her for a chemical DPC and replastering at four and six thousand pounds respectively on the basis of a resistance meter reading, and my report cost her three hundred and eighty and saved her all of it.
She did not like the report. She wanted to know what the meters were reading and I could not tell her, and she had a third company in who told her it was rising damp, and as far as I know the wall has been hacked off and injected and rendered in a sand cement mix, which will make it genuinely wet in about ten years.
The one thing I did not put in the report.
The top edge of the area.
A rising damp pattern has a ragged top edge, because it follows the mortar courses and the variations in the brick, and it is higher in some places than others.
This edge is straight and it is level and it is one metre and twenty six centimetres above the floor, and I measured it at nine points and the worst deviation was four millimetres.
And on my third visit, the one I did for nothing, I measured it again because I wanted a photograph with a tape in it.
One metre and thirty one.
I have the two photographs. Five centimetres in seven weeks, and the wall is dry, and the instrument that says it is wet is the only thing in that room that has noticed anything at all.