Visit San Francisco during the week, and you can take advantage of a bounty found in no other American city — downtown rooftops open to the public, many with vistas that would make a cinematographer swoon.

Our warmer October days are an ideal time of year to explore the secluded summits — if only to make the building owners scowl.

Even though they’re required by city planning codes when commercial buildings are added downtown, too many are managed as though we outsiders aren’t welcome. Some buildings require visitors to sign in or show a photo ID, and the guard then walks you to the elevator. One space doubles as a venue for nighttime banquets, complete with a plastic tent that isn’t removed during “public” hours.

One took out its tables last year, though supposedly they’re coming back. Another — 1 Montgomery St., wildly popular at lunch with Financial District workers — has wooden benches that look as though they haven’t been replaced since the space opened in the mid-1980s.

That’s when the city began requiring developers of new buildings in and near the Financial District to include what planners call “privately owned public open spaces.” Their size is determined by the size of the complex, and many are conventional plazas alongside office towers.

The ground-level spaces are easily accessible, on full view to anyone walking by. The rooftops are another matter, hidden destinations that tend to be created because there’s no room on the ground. Two are only at the equivalent of the third floor, but several are in the 15-story range and offer revelations from unexpected vantage points.

There’s a terrace atop a 13-story building from 1929 in the heart of the Financial District, a space entered from the low tower’s new wing. A public rooftop on Howard Street sits amid the new towers of Transbay and Rincon Hill. Some come with generous landscaping, and several seem tailored for a leisurely lunch or a discreet rendezvous.

These days, thanks to legislation passed by the Board of Supervisors in 2012, almost all the spaces are announced by large signs along the sidewalk. But no rules dictate what hurdles can be placed in your path once you walk through the front door.

“The code is not explicit in terms of what restrictions an owner can or cannot place” on getting up to the space, said Scott Sanchez, the Planning Department’s zoning administrator. “If someone felt they weren’t welcome ... that’s probably something we should look at.”

In today’s security-conscious world — and in a city where screaming, troubled people are an all-too-common sight on downtown streets — some sense of vigilance is to be expected.

“Roof decks are extensions of the lobby, and that’s problematic,” said Jerold Kayden, a professor at the Harvard University Graduate School of Design and a longtime champion of making required spaces as accessible as they can be. “If you’re going to do a rooftop public space, or the city is going to call for one, the design has to take getting there into account.”

That will be the case at the 5.4-acre park that will top the new Transbay Transit Center when it opens late in 2017. Not only will a tram suspended from cables glide up from the adjacent ground-level plaza, but also an express elevator from the plaza to the rooftop will be part of the Salesforce Tower now under construction next door.

But for the free public spaces that now beckon in the clouds, here’s the best solution to improve the connection to the public at large: Put them to use.

If more and more people stop by for lunch, or for a few moments of calm amid the urban turbulence, then there will be more pressure on the city to come up with a clear set of guidelines that spell out what sort of check-in procedures are reasonable and which ones are unfair.

There might also be deserved shaming of building owners or managers who let the spaces go to seed, or act as if putting out a few tables is a burden.

Other cities don’t have such rooftop spaces. San Francisco does. It’s up to us to make them part of our daily lives.

John King is The San Francisco Chronicle’s urban design critic. Email: jking@sfchronicle.com Twitter: @johnkingsfchron

Online extras

For a full map of downtown San Francisco’s privately owned public open spaces, go to the Planning Department website: http://bit.ly/1QUqv0m

To see a video on the rooftops: http://bit.ly/sf-rooftops