The gracious triplexes indigenous to Montreal’s Plateau and Mile-End neighborhoods, long admired for their stone and brick facades and wrought-iron outdoor staircases, captured Erik Rydingsvärd’s fancy soon after he arrived in the city from Copenhagen in 2010. “My first thought when I moved here was, I would love to buy one of those triplexes and just live in one [of the units] and rent out the other two,” he says. “I love the row houses; for me, coming from Scandinavia, it’s very different.”

But first, Rydingsvärd, a fashion executive who relocated for work, had to make sure he was staying put. His first two months in Montreal were “an interim period,” he says, which meant hastily securing a Plateau apartment on Craigslist. That led to a second short-term lease, with rented furniture, and then—once he knew he’d be in Montreal for the long haul—a downtown condominium that he regretted purchasing as soon as winter arrived. The Plateau “lives in the winter,” he explains, “whereas, in downtown Montreal, everything dies. After six? Whoosh.”

So Rydingsvärd quickly sold the condo for a small profit and, in 2012, returned to the Plateau, this time spending a year in a rented house while he searched, block by block, for that quintessential triplex he’d first imagined. “I used to take my bike at night,” he says, “and check, like, Where is there no traffic at night? Because I’m a bit obsessed with being able to sleep with the windows open.” He narrowed his hunt to a pair of streets that run north-south between the Plateau and the adjacent and similarly artsy Mile-End. On one of the streets, his agent found a building perfectly suited to his vision: a circa 1910 triplex with three floor-through units. The bottom two had been renovated recently enough that they could be rented with minimum upgrades. The top apartment, however, was “disgusting,” Rydingsvärd says, which made it a perfect candidate for the top-to-bottom renovation he envisioned for his own home.

Chief among his must-haves: an open-air space to entertain and a sauna—both on the roof. “I was pretty fixed on both ideas,” he says, “because I had an apartment in Copenhagen where I had a sauna on the roof as well.” He also wanted to convert the apartment—which crammed four closed rooms and a kitchen and bathroom into 1,000 square feet—into a one-bedroom with a large, open kitchen. Outfitting a small alcove with a Murphy bed would give Rydingsvärd just enough room to accommodate houseguests.

If methodical exploration was what led Rydingsvärd to the right property, serendipity supplied the right designer for the job. He first came across Maria Rosa Di Ioia’s work when he attended a store opening in the tony enclave of Westmount for a competing Scandinavian fashion brand. “It felt a bit homey,” Rydingsvärd recalls of the retail space. “Nice materials, a lot of wood. I asked my friend, ‘Who did this?’” Not long afterward, he read an account in Dwell about Di Ioia’s renovation of a flat in Habitat 67, the iconic modular residential complex that Moshe Safdie conceived for the Expo 67 world’s fair. “I called Maria,” he recalls, “and I said, ‘I bought a triplex, and would you be interested in talking?’”

“The first thing we wanted to do was open it up completely,” Di Ioia says of Rydingsvärd’s top-story unit. “And so we toyed with, How do we open it up without having to intervene too much on the lower floors?” Emilie Bédard, the architect Rydingsvärd hired to design the space along with Di Ioia (the two have since joined forces under the name EM Architecture), decided that installing cellar-to-rooftop columns and new beams to support a terrace would be too onerous. Instead, Bédard says she decided to remove about three-quarters of the roof structure toward the back of the building and install new joists. This move, along with their decision to strip away the old ceilings at the front of the building for a loftlike effect, allowed her to augment the ceiling height throughout the apartment to just over ten feet from around nine feet, helping to create room for the staircase to the terrace and sauna. Only a small portion of the original roof—its underside beautiful and intact after the apartment’s ceilings were peeled back—was preserved, to act as a new ceiling for Rydingsvärd’s street-side bedroom.

With the problem of properly supporting the terrace solved—and with Rydingsvärd deciding to “screw the budget” to extract maximum enjoyment from the 550-square-foot outdoor space Bédard had designed—all that was left to do was execute. “It was fairly straightforward,” Di Ioia says. The wrought-iron spiral staircase, an aesthetic nod to the neighborhood’s signature outdoor stairways, allowed more sunlight to filter down into Rydingsvärd’s living space, which was renovated with fir flooring and teak cabinetry, materials chosen for their Scandinavian feel. “I’ve always been surprised how, in Montreal, they clutter things more than I find necessary,” Rydingsvärd says. “They paint it red and black—crazy colors—and you’re like, Why? Just paint it white.”

With construction completed in late August 2013, Rydingsvärd was able to host his first alfresco get-together right before Montreal’s early-autumn chill crept in. Since then, the roof has become a warm-weather respite for him and his fiancée, Valerie Gohier. “We use it every night for barbecuing, just sitting out there eating,” he says. “We have a beautiful sunset. You see the airplanes landing at the airport; it feels like you have the whole city.”

And for the long winters, there’s the sauna. Constructed inside the cupola-like stairwell, which added 100 square feet to the apartment’s footprint, the sauna is outfitted with glass paneling and oriented to capture views of Mount Royal, the hill at the center of town that gave Montreal its name.

“You sit in the sauna and you look out and you have the whole mountain,” Rydingsvärd says. “For a Scandinavian, it’s like therapy.”