Tidemark

I did write the Braydon Court report in the end, on the Tuesday, and in the box marked 'cause' it says 'condensation'. Darren at the agency paid me double time for the weekend call-out and hasn't rung since, though he used to put three or four jobs a month my way. The flat's already back on the market. It's up on Rightmove as 'recently redecorated', and in the third photo you can see the bedroom door. The frame's been painted over in white, one thick coat, with brush hairs dried into it along the edge.

The rest I'll tell as I remember it, and I don't remember all of it. Some of it Siobhan told me afterwards, and some I've probably filled in myself.

I survey damp, nearly always for letting agents. I stick the pins of a moisture meter into the plaster, photograph the mould behind wardrobes and write down where it came from. Nine times out of ten I write condensation, and usually it's true enough. People dry their washing on the radiators and keep the windows shut, and Victorian brick doesn't care for that. That Saturday Darren rang at half eight in the morning. The tenant at Braydon Court had been bombarding them with emails for three weeks and was threatening to go to Camden Council, and the landlord needed something on paper by Monday. Darren also said he'd seen a clothes airer in the middle of her front room with his own eyes. I said yes. All I had on the Sunday was lunch with Holly, who'd promised to come up from Croydon, and the Sunday would also make it twelve years since my last drink, but Holly didn't know about that.

Braydon Court is a short dead end off New Oxford Street: three sooty houses between the back wall of a theatre and a glass office block where not one window is lit at the weekend. There's nowhere to stop. I drove round Bloomsbury for twenty minutes, left the van on Bedford Square, where I later found a ticket under the wiper that I never did pay, and lugged the kit case the rest of the way in a fine drizzle. In October nobody in London puts an umbrella up for that sort of rain.

Siobhan Gallagher opened the door with the boy on her hip. She was a nurse at a hospital on the Euston Road, she had a County Mayo accent, and her face was grey from nights. The boy, Finn, was two, in dinosaur pyjamas, and he looked at me over her shoulder with deep suspicion. The airer was where Darren had said, with tiny socks and a set of blue scrubs hanging on it. I photographed that first, because they'd be sure to ask.

It was on the window that I first spotted the line. The window looked out into the area, onto the steps, and up above, beyond the iron railings, legs went past, and once a bulldog in a yellow raincoat. Across the glass, at about the height of my chest, ran a level brown line, and it was on the inside. From the window it carried on to the frame, from the frame to the plaster, and right round the room at the same height, across the door frame, into the bedroom and the kitchen. I've been doing this twenty-three years, and I've never once seen damp climb a pane of glass.

Below the line the meter went off the scale, and the pins came out of the wall dark and sticky. Two centimetres higher it read eleven, which is a dry, healthy wall. Rising damp comes up in tongues, ninety centimetres or so, and the edge of it is always blurred. This was at a metre thirty, dead level, and I even got the laser level out of the case, I couldn't tell you why. The red beam lay along it the full length of the wall.

On the bedroom door frame there were pencil marks with dates in small, careful handwriting: the third of October at knee height, then the seventh, the tenth, the fourteenth and yesterday's, the sixteenth, a hand's width below what I was looking at now. Behind me Siobhan said she marked it every evening and had sent all of it to the agency, and they'd written back telling her to ventilate the property. Lower down on the same frame Finn had been measured in blue biro: at one, at eighteen months, at two. The last blue mark was under the brown.

I ran a finger along the glass. The line was soft and it stuck to my finger, and I'd put the finger to my nose before I'd had time to think. Burnt sugar, liquorice, bread crust, and under all that the sour smell of a pub at eleven in the morning before anyone's mopped the floor. I wiped my finger on my trousers, then went into the kitchen and washed my hands until Siobhan asked if I was all right.

Anyone who's done any drinking round here knows about the brewery on the corner of Tottenham Court Road. On the seventeenth of October 1814 a vat of porter twenty-two feet high burst at the Horse Shoe Brewery, the ones next to it went too, and the beer took out the back wall and poured into the rookery of St Giles, into the cellars where the Irish lived. Eight people died. Five of them were in the same cellar at the wake of a two-year-old boy who had died the day before. Where the brewery stood there's a theatre now. They do musicals.

In 2014, for the two hundredth anniversary, a pub on Sicilian Avenue had a special porter on all day, and I sat in there from lunchtime till it got dark. Somebody at the bar said the vat went at half five, so at half five we raised our glasses to the eight. Then I went home, because Sue was on shift and Holly needed her bath. Holly was two. I remember testing the water with my elbow and her bashing it with a rubber duck. After that all I remember is the tiles against my cheek. I don't remember getting her out. Sue says that when she got in from work Holly was asleep on my lap, wrapped in a bath towel, and I was sitting on the wet floor and couldn't tell her how long it had been. Holly doesn't remember any of it.

Meanwhile Finn was sitting up in his cot by the far wall of the bedroom, holding the bars and rocking and singing. Siobhan said he sang all the time now. He woke at three in the morning, sat up and sang, and if you asked him, he said he was helping the ladies.

I asked what ladies.

'The ones that sit.' At first she'd thought he could hear the neighbours, you can hear everything in this place. Only it's offices upstairs, and there's nobody there at night. Then she said she sometimes heard it herself, a sort of keening behind the wall, old women, and it put her in mind of the keening at her nan's wake back in Mayo. Mind you, she'd been very small then, and maybe somebody had just told her about it later.

I get the thermal camera out in every flat. It's for finding cold bridges. On the screen warm things come up yellow and orange and cold things blue, and a wet wall is always colder than a dry one, so damp usually shows as a blue patch with a ragged edge.

Here the blue went from the floor up to that line in one solid layer with a flat top. I swung the camera round towards the bedroom. Darker patches came up in the blue. First I thought they were pipes, then marks left by furniture that had stood against the wall for years. They were in a row, evenly spaced, narrow at the bottom and rounded at the top, and the tops all leaned the same way, towards the cot.

By then Finn had dropped off, slumped against the bars, and on the screen he was an orange lump, the warmest thing in the flat. Round him the dark blue ones sat with their heads bowed low, the way you bend over a sleeping child to hear if it's breathing.

I lowered the camera and saw the cot, the moon night light, the airer and an empty floor, and when I lifted it again they were sitting where they'd been. A thermal camera lies on gloss and picks up reflections, I explain it to clients myself, but a reflection slides about as soon as you move your hand. I moved my hand. The room on the screen slid, and they stayed sitting where they were. Finn rolled over in his sleep to face the wall, and all of them leaned slowly after him.

After that my memory's got holes in it. Later on, by the cab, Siobhan told me things I don't remember at all, like me asking her the time three times in a row. I remember waiting for half five. I stood in the bedroom doorway with the camera while Siobhan put the kettle on and told me about the gas, which is on a prepayment meter you top up with a card at the corner shop, only the corner shop's shut down and now she has to go up to High Holborn. Half five came and went. Up above, people were walking to the theatre for the evening show, nothing had changed on the screen, and the kettle clicked off.

She gave me my tea in a mug that said WORLD'S BEST NURSE (HONESTLY). I got my phone out to text Darren something, anything, and at the top of the screen was a reminder I'd set myself the autumn before: 25 October, put the clocks back in the van and on the boiler.

British Summer Time started in 1916, during the war, I knew that from some pub quiz. In 1814 nobody put the clocks forward or back, and their half five was our half six. My phone said 18:11.

I told Siobhan she and Finn had to get out, right now, for an hour, anywhere. She didn't answer. She looked at my hands, checking whether they shook. So I gave her the thermal camera. She looked from the screen to the cot and back for a long time, then handed it to me without a word, went into the bedroom and picked Finn up, blanket and all. He grizzled without waking.

The front door wouldn't open. The lock turned, the handle went down, and the door stood there solid, and I couldn't shift it with my shoulder. That's how a basement door holds when there's water up to the ceiling on the other side. I saw one like that in Lewisham after a main burst, and they only got it open once the water had been pumped out. Through the fanlight I could see wet steps and a bit of pavement under a street lamp.

I don't remember how we ended up in the kitchen. The blue on the screen was coming up in surges, with a whitish rim along the top, and dark bits of wreckage were turning over in it that weren't in the room. The room itself was dry and smelled of tea. Somewhere under the floor there was a low hum. Siobhan could hear it too and decided it was the Tube.

The blue came up to her waist, then to the blanket, and when it reached Finn's shoulders he started to cough.

It was a wet cough, with a gurgle on every breath in, and brown froth came out of his mouth onto the blanket. The smell filled the kitchen straight away, warm and heavy, the smell of a beer cellar. Siobhan screamed, turned him face down along her forearm and started slapping him between the shoulder blades, properly, the way they're taught, and it made no difference at all.

I pushed the camera at her and took Finn off her. I got up on the stool, from the stool onto the table, and held him up to the ceiling, right up by the light, where there was still a bit of yellow left on Siobhan's screen. The ceiling's low in there, so I was standing bent over with Finn at arm's length above my head, and he was quiet, and then he started bawling, dry and furious, loud enough to fill the flat, and caught me under the chin with his heel.

Then the blue got to me. Nothing changed in the kitchen. I was standing on a table in dry clothes, but the air went heavy and sweet and stopped going into my chest. I breathed in and something thick and warm and bitter came into my mouth. I couldn't cough it up, I'd have had to lower my arms for that. So I swallowed. I swallowed for a long time, in big gulps, the way you get the first pint down after work, and with every gulp it got easier.

Siobhan stood below me with the camera, shouting something. She told me afterwards that on the screen the blue had gone over her head long before, and she'd been breathing all the same. I don't know why it left her alone.

There was singing as well. Women, low down, no words, holding one note, breaking it off and starting again, very close, all round the table, under my feet.

Siobhan said the whole thing took about four minutes. It felt longer to me, but she could see the clock on the cooker and I had a child in my arms. The blue went down and away through the gap under the skirting board. I climbed down, gave her Finn and stood over the sink for a long time, bringing up something dark, though all day the only thing I'd had to drink was that tea.

That same evening I put Siobhan and Finn in a cab to Kilburn, to a friend of hers. The front door opened at the first push. The line was gone from the walls, and the meter read between ten and twelve all over the flat. The pencil marks on the door frame were still there, the blue ones too. It didn't occur to me to photograph them, and now there's white paint over them.

A week later she texted me, just the once: they were at her mum's outside Castlebar, Finn was fine, sleeping well, and sometimes he sang, but they were songs from playschool, she'd checked. I texted back. She didn't write again.

There were eleven images left on the camera's memory card. I showed them to Kevin, who's got his own firm out in Enfield and has been in the trade longer than me. Kevin scrolled through them for a long while and said it was reflection off a radiator, and then asked how I was doing generally, and whether I was drinking. There's no radiator in that bedroom. I haven't opened the pictures since, but I haven't deleted them either.

On the Sunday morning I went to a meeting where everyone knew what day it was for me, and they clapped. I said twelve years and sat down.

Holly came over for lunch. She's fourteen now. She ate her pasta and told me about some girl in her class, and I took in about one word in two. In the van, driving her back to Croydon, she remembered that her mum wanted me to have a look at the bath: there was a tidemark on the enamel and nothing would shift it.

It was an ordinary tidemark, greyish brown, the sort you get on old enamel when nobody's cleaned the bath for a week, at the height people usually run it to. I didn't bring the meter in from the van. I asked Sue for a sponge and some cleaner and scrubbed for about forty minutes on my knees, until the enamel came up white. Sue stood in the doorway for a bit, said, 'Ray, it's just a bath,' and went to put the kettle on. I threw the sponge in their wheelie bin out the front, and I didn't put it anywhere near my face.