A roof deck is popular and can provide a great opportunity for condo/coop owners to interact socially, and provide outdoor space where it is otherwise non-existent or limited. It is also a great selling point for future buyers -- especially if the building does not have many amenities and if the condo fees are reasonable. It is very difficult to compute a ROI, but I do think it could make units more competitive for resale purposes. Determining the cost per unit owner for the construction, maintenance and insurance for the roof deck, would give you some idea. Add that to the current value of a unit and compare to most recent sales in the building.