Peter York: Sky’s the limit

"When this old world starts getting me down/ And people are just too much for me to face/ I climb way up to the top of the stairs/ And all my cares just drift right into space."

I find that the words of the Drifters' lovely "Up on the Roof" segue naturally into Petula Clark's "Downtown" – "Things are much brighter there". Clark sang of New York, of course, but we have roof gardens here, too, including the biggest in Europe – the Kensington Roof Gardens, one and a half acres on top of what was Derry & Toms, later Biba's. And there's a clever roof garden along from me, where the sixth storey of a house is an open garden behind a windowed façade.

But the real thing – the wonderfully artificial real thing – belongs on the top of a tall building in a city of tall buildings, where the contrast, the ridiculous luxury of turf and trees on top of steel and glass, seems so acute. There's a roof garden on top of the Rockefeller Centre, surrounded by later, huger buildings, and one on Chicago City Hall. For me, real roof gardens are absolutely rooted in the American Century – the 20th.

Roof gardens are for showing off, for eating out in, for extraordinary light and views. Not really for growing your own organic food or for planting a miniature Sissinghurst. The roof garden impulse is a bit like the underground pool one, the "because I can" gesture.

Roof gardens are often laid out just like outdoor rooms, with sharply delineated spaces. Modern garden design works better here, too. Lots of tubs and planters with clean lines, precise slabs rather than York stone.

A small roof garden will have decking, planters and a sitting-room set-up of chairs with magically impervious upholstery. A more formal, corporate one will almost always involve some sort of dining-room affair. What's the point of anything if you can't do lunch? The decoration could be that bit Resort, with bright cushions and china stored in the corporate serving station, and there's a great temptation to go sculptural and exhibit your patron-of-the-arts credentials.

The giant roof garden pictured here (there's twice as much again on the other side) is, amazingly, in London. You can tell by the skyline – one lonely modest tower. But it's got the New York ambition; stone balls for a start. And the pond affair, surrounded by planters. Plus the exotic veg – in plain English pots, but distinct ambitions for Marrakech, with a touch of Torquay.

On the other side, on the turf carpet, there's a table and chairs, statuary and what looks like a privet hedge. A privet hedge in the sky! You could be in East Sussex! But look closely under the tallest palm; there's one of those little, bronzy, oriental chaps that the law requires of all roof-garden owners.