Roof greening in cities provides a range of environmental, economic, and social benefits. However, the lack of motivation among property owners in high-rise buildings poses an intractable obstacle to its wide implementation in compact cities. Cooperation amongst stakeholders, from individual-building to city-block scales, could facilitate adoption of green roofing, with implications for urban sustainability. This study is an attempt to evaluate the net gain from roof greening in terms of apportioned collective costs and benefits to a group of property owners. With government tax exemption to encourage green-roof installation, a fair-allocation scheme, based on the Shapley value, is adopted to distribute the net gain in a partnership structure. A case study in Hong Kong serves to illustrate the application of the method to allocate theoretically the sharable gain of roof greening translated into monetary terms. The results verify the importance of individual owners in different coalition configurations in moulding the benefit profile. The crux of the financial incentive scheme is the enhanced rewards to optimized cooperation, and the pump-priming triggering of cooperation and action. The benefits of green roofs could be maximized by their widespread and contiguous, rather than piecemeal, installation. Green roofs could reduce both capital and recurrent public expenditures in stormwater management, healthcare, and green-space provision, the savings from which would be more than enough to fund the tax-exemption scheme. The findings yield convincing justifications for government financial incentives to promote public – private partnerships and cooperative coalitions of stakeholders in roof greening in compact urban areas.