As urban land values drive new development upward to skyscraper heights, the flat roofs of existing buildings become fertile ground for construction. Add a floor or two to an older building and the result may be increased property value and greater income potential.

Owners of multi-unit residential, commercial or industrial buildings may be overlooking opportunity when flat roofs remain unused space. The green movement has encouraged the growth of rooftop gardens, but this is only one aspect of the development potential. The level, open space on top of older buildings may represent ideal sites for construction of new living or work space in "pop-up" structures, or through the addition of a new floor or two. Top-floor apartments could be transformed into two storey penthouses with a resulting increase in property value and income; offices in the same location could be expanded upwards into prestigious, customized bi-level commercial space. These renovations could be stand-alones or undertaken concurrent with building system modernization projects like updating of elevator or electrical services.

While increased land values make building up worth a closer look, manufactured housing offers a practical, cost-effective alternative to on-site construction that makes vertical intensification projects feasible. One exemplary renovation project recently received a 2006 CMHC Housing Award from the federal housing agency for its innovative application of manufactured space to intensification of a social housing building:

Prefabricated Rooftop Addition to St. Clare's Multifaith Housing at 25 Leonard Street, Toronto, by Toronto-based Levitt Goodman Architects Ltd. . In 2005/2006, St. Clare's intensified its supportive housing project at 25 Leonard Street by adding a fifth and sixth floor which included 26 self-contained 212-square-foot bachelor apartments. The new units were prefabricated housing modules, which cleverly maximize living space. The manufactured housing was organized around an exterior landscaped courtyard located on the roof of the existing building. St. Clare's internal subsidy program allows 80 per cent of the tenants, many of them coming direct from shelters, to pay a rent equal to the shelter component of welfare.

"We are used to thinking of roofs as an end rather than a beginning," said Architect Dean Goodman. "We are used to looking across at buildings, but now we are building up, so we look down. When we look down on roofs, they are ugly. We need to look at [roofs] differently."

Concrete buildings are preferable, but engineers can determine rooftop capacity for any building using existing drawings and on-site surveys. Many factors dictate whether it is feasible to build and what to construct, including the condition of the foundation, essential structural upgrades and the extent of the renovation. Cost-benefit analysis must also consider time frames, financing and levels of inconvenience for those who live or work in a building.

Goodman suggests most owners should at least consider the possibilities. Owners who aren't familiar with development might not understand their building's potential nor realize that there is a need for a particular type of space in the area. Investigating the potential may only involve a small expenditure for the services of an engineer and consultation with an architect.

Prefab rooftop renovations can overcome intensification cost barriers and add flexibility that may convert innovative design and green thinking into increased property value and financial gain:

• There is no land cost involved in development. Buildings in great locations or with terrific views can be expanded vertically when land costs or zoning hinder tear-down and re-development.

• The addition of one or two floors, connected to existing top-floor units does not require an elevator or modification of the existing elevator service. Stair access to the second floor of these apartments or office is sufficient. If existing elevator banks must be refurbished, this may be an excellent time to consider prefab rooftop development to add new space.

• Although plumbing and electrical services represent significant cost in complete renovations, the extension of these services from existing top-floor units to the new units is cost effective.

• Manufacturing with steel creates non-combustible units which should meet building codes for non-combustible construction above the third floor.

• Panelized systems, which create smaller units or rooms that are assembled on site, is ideal for small lots or narrow access routes where the use of large equipment is problematic.

• Renovations on inhabited buildings cause minimal, short term construction disruption for existing tenants and businesses since most of the work is carried on off site.

• Manufactured off-site, prefabricated room units or entire floors may allow removal of the existing roof and installation of the prefab unites within a day, which reduces weather vulnerability.

• Green development approaches and environmentally-friendly materials can be incorporated into designs along with roof-top garden areas. Alternative energy sources like solar or wind power may be employed for long-term cost savings and to demonstrate corporate social responsibility.

• Where zoning allows work/live usage, roof-top residential or office units could increase income and property value.

• Prefabricated units may be completed down to carpeting, furniture and appliances prior to installation, so planning for the move-in phase is more predictable.

Has your building reached its full potential?