Londoner with a flat roof and a question.

Hi Everyone

I am looking for some advice. I'm wholly ignorant about birds, so I apologise in advance if these questions seem ridiculous.

Okay

We live in central London, and our house has a 30 square meter flat roof at the back.

We want to attract birds such as tits etc, who might find it harder to get a purchase in the area.  Using the roof, how can we best provide feeding and nesting space to birds with declining populations?

The roof is unused for other purposes but has no walls to fit nests.

Thanks in advance for any suggestions.

LondonNewbie