A tell-tale across a crack measures movement to a tenth of a millimetre. Cracks in old walls open and close with the seasons. This one only closes, by about a tenth of a millimetre a month, for eleven years.
I am a structural engineer and a good part of my work is the boring, useful end: is this crack a problem.
Most are not, and the way you find out is to measure.
A tell-tale is two overlapping plastic plates, one screwed each side of the crack, with a grid on one and a hairline on the other. If the crack opens, the plates slide and the hairline moves across the grid. You read it to a tenth of a millimetre.
I also use studs and a demountable gauge for anything I care about: two stainless discs epoxied into the masonry a fixed distance apart, and an instrument that measures between them to a hundredth. That is the proper way and it is what I used here.
What you expect in an old building is seasonal movement. Clay shrinks in summer and swells in winter and a wall on a shallow footing goes with it, so a crack opens in August and closes in February and the trace looks like a sine wave that goes nowhere.
The farmhouse is late seventeenth century, solid stone, on clay, with a crack running diagonally through the ground floor gable, about four metres of it.
It was reported by the owners in 2014 after they bought it and I put studs on in the January of that year.
It closes.
Every reading, in eleven years, is smaller than the one before, by between six hundredths and fifteen hundredths of a millimetre a month.
There is no seasonal component. That is the first thing that is wrong.
Every other crack I have on a clay site does the sine wave. This one does a straight line down, through summers and winters, through the drought of 2018 and the wet winter of 2020, when every other building I monitor in that parish did something visible.
Over eleven years it has closed by about eleven millimetres.
Eleven millimetres is a lot. A crack that has closed by eleven millimetres is a crack that has effectively gone.
Which it has. In 2014 it was a crack you could get a finger in at the widest point. Today it is a hairline you can see only because I know where it is and because there are two stainless discs either side of it with my initials scratched on the epoxy.
The owners are delighted. They have been delighted for eleven years and they mention it every time and I have never told them what I am about to write down.
A crack closing is not, on its own, impossible.
If a building has settled and then the ground recovers, a crack can close. If somebody underpins one side, a crack can close. If a wall is being pushed together by something, a crack can close.
What is not possible is closing eleven millimetres without anything else in the building moving.
Because a crack is a gap between two pieces of wall, and if the gap shuts by eleven millimetres, those two pieces have moved eleven millimetres relative to each other, and that has to show up somewhere else.
So I measured somewhere else.
Since 2016 I have had a level datum on that gable and four studs on the return walls, and I have surveyed them annually with a precise level, which is a two hour job I do for nothing because I want to know.
The building has not moved. The datum is where it was in 2016 within the accuracy of the survey, which is better than a millimetre. The return walls have not rotated. The floor levels are the same. The door openings are square and were not.
The gable has not leaned, spread, settled or risen.
And the crack has closed by seven millimetres in that time.
The masonry.
If a crack closes and nothing moves, then something has gone into the gap.
That is the only remaining possibility and I want to be plain that it is the one I have worked on hardest, because it is the only one that is not absurd.
The crack is not pointed. Nobody has repaired it: I would see the mortar and I have photographed it every year at the same three points with a scale in the frame.
What is in it is stone.
At the two points where the crack was widest, the faces have grown together. Not filled. The two faces of the same stone, either side of the crack, have closed and the join between them is a hairline with no mortar in it and no fill.
I have put a knife blade in it and it goes in about two millimetres and stops.
What I have written in eleven years of reports.
The correct thing, which is that the crack is closing, that the movement is not seasonal, that the building shows no associated movement, and that the crack requires no repair.
Every one of those sentences is true. Put together they describe something that cannot happen, and no client has ever read them put together, because a client reads the last line, which says no repair required.
I have not shown the file to a colleague. I have thought about it a great deal and what stops me is not embarrassment.
It is that the next question anybody competent would ask is whether the same thing is happening in the rest of the building, and the honest answer is that I have only ever put studs on the one crack.
I read it every February.
Last February it was one point one millimetres and closing at about a tenth a month, which means that some time in the year after next there will be nothing left to measure.
I have already decided what I will do when the gauge reads zero, which is take the studs out, fill the holes, and write a final report saying the movement has ceased and the monitoring is discontinued.
And I will not go back, and I have found that I am relieved about that in a way I would not be able to explain to the owners, who are kind people with a lovely house that is putting itself back together.