Rapid bursts of what sounds like gunfire emanate from the edge of an airy, single-storey, red-brick building in St Louis, the Missouri city known as the gateway to the American west.

A lanky man, headphones jammed on to his ears, his cap visor pointing down the back of his neck, is firing stitches around the sole of a single leather sandal, a process he completes every five seconds or so. A tangle of rose-gold sandals amasses in an open crate on the floor.

There are shoes everywhere: in pieces, in piles and in boxes, in preparation for the annual spring rush. Hundreds of cutting dies – one for each part of each size of sandal – hang on the wall like cookie cutters in a bakery.

“If the stitches aren’t right, the shoe isn’t right,” says Michael James, 48, who has worked for the Hoy Shoe Company since 2011. He asks me to lift up my foot so he can inspect the sole of my sandal. “Is that one of mine? Yeah, I know my stitches!”

The shoes in question are better known as Salt-Water sandals, a humble woven leather design originally intended for small children but which became a sleeper fashion hit: global sales this year are expected to reach 900,000 pairs.

If you aren’t already a convert, you probably know someone who is: Salt-Waters are popular with everyone from Sarah Jessica Parker and Alexa Chung to the Instamum crowd, who love the sandals for their combination of glamour and practicality; they were christened for their ability to withstand wading in the waves. The original woven style runs from a baby size 3 to a women’s US 11 (UK 9), which means that twinning mum-and-kid snaps are rife on social media.

Salt-Waters’ rise to summer fixture has been a slow journey: the first pair was made here in St Louis during the second world war by Walter Hoy, a British immigrant from Norwich, using leather scraps left over from cutting out military boots. Hoy made the first pair for his little girl, Margery; an early bronzed pair worn by her brother, Bob, sits on shelves in a corridor.

‘If the stitches aren’t right, the shoe isn’t right’: Michael James, Salt-Water stitcher. Photograph: Jennifer Silverberg/The Guardian

The stitching is the company’s mainstay, says Ric Gebel, Walter Hoy’s grandson, an affable midwesterner in his mid-50s who shows me around the St Louis workshop. (Hoy’s descendants still own and run the company he started in 1944.) “We still do what my grandfather did. We’re lucky we can keep selling them.”

Family relationships dominate the workshop floor, as well as the front office. Michael James’s brother, Steven, is scanning silver sandals which are due to be shipped in a fortnight. The foreman, Rick Sumpter, is eating a sandwich at a desk opposite his sister, Phyllis Davis, 62, who assembles every pair of Hoy shoes that gets boxed up in St Louis. Rick’s wife, Cheryl, works the die-cutting machines that stamp out leather straps; their daughter April helps sort stock.

British fans have Rachael Laine, a former marketing executive, to thank for launching the brand in the UK, after she stumbled across the sandals on a Brooklyn-based blog in August 2009. “I was on my second maternity leave, bored, as you are with a tiny baby, scrolling through blogs thinking, ‘Oh my God, I need to see what’s going on in the big wide world’,” she remembers.

Two days later, Rachael saw her stylish neighbour wearing a pair a friend had brought back from New York and felt that she had to persuade the US sandal-maker to let her help them launch in Europe. “I thought that if I didn’t do it, someone else would. This was at the peak of Croc-mania, when you couldn’t get that stripped-back aesthetic here. I loved their Swallows And Amazons vibe.”

Laine’s persistence paid off, and by 2011 she was dragging a suitcase of samples around British boutiques. What she had envisaged as a marketing role became an entire distribution business: she expects to sell 150,000 pairs in the UK this year, up from 700 that first summer. She even has the rights to sell Salt-Waters in Asia (Rachael had to add the hyphen in the UK because a clothing brand, Saltwater, had got there first).

‘We stick to doing the same thing’: Ric Gebel, grandson of the company founder. Photograph: Jennifer Silverberg/The Guardian

“We took out a £10,000 loan from HSBC for our first website, and paid it back seven years early. Now they lend us millions for stock,” Rachael tells me. “What started with me on a laptop in my bedroom is now a business with eight people working in the UK.”

Salt-Waters, which come in 16 colours and a handful of styles, are now sold in Office, John Lewis, Selfridges and Harrods – as well as a string of independent shoe stores. A retro plaited version will be launched next year; Rachael found the original 1970s design in an old box while visiting the St Louis factory and convinced the family to reissue it.

RicGebel laughs at the memory of Rachael hunting through old sandals. “We’ll have to hide that box next time she comes.” He sits on a big green leather sofa, with Kaitlin, his 28-year-old daughter (who represents the fourth generation to work for the family firm), and Cindy Barylski, executive administrator and the only non-relative to work in the front office. Kaitlin, who handles PR, remembers “growing up playing hide-and-seek among all the shoe boxes”; her first job was packing sandals. Ric succeeded his mother, Margery, 81, as chief executive when she retired last December; she still owns 90% of the company.

Later, after the lunch break, I watch Phyllis’s fingers deftly weave three pieces of tan leather into a child’s Sun-San sandal, a tweaked design with a cushioned sole. She manages 15 cases a day, or 360 pairs, pushing her running total into the millions (she has worked for the company since 1975).

“At one point there were 10 people doing the job I do,” she says. She hung on to her position during the lean years of the early 2000s, when the company struggled both to sell sandals and recruit enough staff to keep the St Louis factory viable.

Sandal-makers Cheryl Mueller (on left) and Joanne Copeland. Photograph: Jennifer Silverberg/The Guardian

“We’d hire 40 people, only to keep a couple,” president Scott Downs, 71, remembers. “Our quality suffered and our costs rocketed.” Nobody wanted to work in a factory, especially one that wasn’t air-conditioned, in a city where summer temperatures hover in the mid-30s. (Staff can’t even keep their feet cool in sandals: federal regulations require closed-toe footwear in factories.)

The solution was a compromise: shift some production to China and keep some in St Louis. “Thank God we did it, because all of a sudden our sales picked up; if we had the shoes, we sold the shoes,” Ric says.

Now the St Louis team handles “special makeups”: mixed colours, vintage styles and emergency runs in colours that have sold out – namely tan, a bestseller, and rose-gold, the fastest-growing colour since its debut three years ago.

“I thought we’d eventually not manufacture here, but it’s just too easy to make our product,” adds Ric. There is also too much history tied up in the factory for the family to walk away.

Kaitlin Gebel, PR director and family member. Photograph: Jennifer Silverberg/The Guardian

Back on the factory floor, we pause to watch a line of what look like amputated feet being strapped into sandal uppers, ready to stamp on the polyurethane, spurting out of a giant hydraulic arm, into a row of moulds. Ric makes me press the warm liquid waiting to become a sole on a child’s Sun-San Surfer. He designed this “direct attach” process and still fixes the machine on occasion – “so my mom can’t fire me,” he jokes.

Despite the male-heavy top team, neither Ric nor Jeff Downs, Scott’s son and global sales director, is keen to expand into sandals for men, insisting that sports versions like Keen or Teva are all that most American men will tolerate when it comes to baring foot flesh. That could change, however, if Rachael Laine cracks Asia. The sandals are also sold in Australia and New Zealand.

Meanwhile, expansion must be slow. “We don’t want to be a trend,” says Kaitlin, conscious that overexposure can break a brand. She disappears for a few minutes, returning with some Salt-Waters that are the antithesis of mass market: a formerly white pair dazzling with 3,800 rhinestones that she stuck on, one by one, for her wedding. “I got the idea from Instagram,” she explains.

Glittering one-offs aside, Ric nails the quality that makes his grandfather’s company unique. “Most shoe companies are like car companies: they make something different every year. We don’t. We have what we call the Hoy Shoe Company Code. We keep doing the same thing, and we stick to it.”

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