Many green roofing manufacturers offer modular products that can be laid down in the manner you describe. These products are similar to plant flats, allowing for easy installation; they're lightweight, come pre-planted, and are more flexible than installing a permanent green roof system. A green roof constructed of modular units will still have all the benefits of a permanent, built-in green roof, including stormwater management, insulation, and reduced heat island effect.
Although a modular green roof will weigh less than an intensive green roof system, I still recommend you have an engineer complete a review of the roof structure to ensure it can accommodate the extra load. The cost for an engineer's site visit shouldn't be too much (I would expect $500-$1,000) and will be worth it for the peace of mind. If an engineering consultation or necessary retrofits to your roof structure prove too expensive for a green roof to be viable, try a white roof to achieve similar insulating benefits.
Benefits of green roofs and white roofs
Green roofs help reduce runoff into a city's stormwater system by retaining water in the soil until it naturally evaporates or is used by plants. Water quality is also improved by the natural filtering provided by green roofs.
A typical dark-colored roof can reach temperatures well over 160 degree F. It absorbs the sun's heat and transfers that heat into your home. A green roof adds insulation value, reducing your heating/cooling loads. White roofs, on the other hand, while not as beneficial in the winter, can save energy in the summertime by reflecting the sun's rays. The installation of cooler roofs in urban environments combats the urban heat island effect.
Modular green roof products
Most modules come in a standard 2'x2' size, and you can arrange them in whatever design you desire. For example, you could put plants around the edge of your roof, or you could integrate plant modules into the design of a roof deck or seating area.
Modular systems include Green Roof Blocks, TectaGreen, and GreenGrid. Individual manufacturers can tell you how much their product will weigh at its heaviest (after a rainstorm) and can offer technical advice on the feasibility of the project. They can also recommend approved installers in your area.
Be careful not to block any drainage, and make sure the current drainage system is adequate to prevent standing water on your roof. Laying down a waterproofing membrane below the modules will help ensure protection against water leaking into your house.
In Harrisburg, you won't have a problem with wind loads, but readers who live in hurricane zones or areas with strong winds should check local building codes and ask manufacturers for specific product recommendations.
For more information:
Read Susan Wisniewski's "What's the difference between a green roof and a white roof?" for more advice on the topic of energy-saving roofs.