A living roof, also known as a green roof, is a complete vegetative system installed over a waterproofing membrane that covers a building’s roof structure. These systems transform an unused rooftop into a living landscape, delivering numerous benefits to the structure and the environment. Homeowners are often drawn to living roofs for their ability to provide natural thermal insulation, reducing heating and cooling costs. The system also helps manage stormwater runoff by absorbing rainfall, while adding aesthetic value to the property.
Structural Requirements and Preliminary Planning
The primary concern before installation is ensuring the existing roof structure can safely bear the system’s additional weight. This weight is calculated as the dead load (the permanent weight of the roof structure and all green roof layers) and the live load (temporary weights like maintenance workers or snow). A saturated green roof system adds significant weight, with lightweight extensive systems adding up to 30 pounds per square foot (psf) when fully wet, which often exceeds the capacity of a standard residential roof designed for 20 psf.
Consulting a licensed structural engineer is a required step to assess the current load-bearing capacity and determine if structural reinforcement, such as adding or strengthening joists, is necessary. The engineer calculates the total wet weight of the proposed system against the roof’s capacity to ensure safety and compliance with local building codes. The roof slope should also be assessed; a gentle pitch of 1 to 3 degrees is often ideal to allow for adequate drainage without the substrate washing away.
Choosing the Right Green Roof System
The choice of system is governed by the roof’s structural capacity and maintenance goals. The two primary types, extensive and intensive green roofs, are distinguished by the depth of the growing medium they utilize.
Extensive green roofs are the lighter, low-maintenance option, featuring a shallow substrate layer typically 2 to 6 inches deep. This system is suitable for retrofitting existing buildings because its lower weight places less stress on the structure. Plant selection is limited to hardy, drought-tolerant species, and the primary benefits are ecological, such as stormwater management and insulation.
Intensive green roofs are much heavier and behave more like a traditional garden, utilizing a substrate depth of 6 inches up to several feet. This depth allows for a diverse planting palette, including shrubs, trees, and lawns, but requires significant structural support and a higher level of ongoing maintenance. Semi-intensive systems exist as a middle ground, often using a substrate layer between 6 and 12 inches to offer greater plant diversity than extensive roofs without the full weight of an intensive design.
Step-by-Step Installation of Roof Layers
Installation involves laying several specialized layers onto the prepared roof surface, starting with the waterproof and root-resistant membranes. A high-quality waterproofing membrane is placed directly on the roof deck to prevent water intrusion; this is the most important layer for the building envelope’s longevity. A separate root barrier must be installed on top of the membrane, especially if the waterproofing material is not root-resistant, to prevent plant roots from penetrating and causing damage.
Next, a protection layer (often heavy-duty fabric or foam insulation) is laid down to shield the delicate membrane from puncture damage during installation. The drainage layer is then installed, typically consisting of a plastic drainage mat or a layer of lightweight aggregate. This layer ensures that excess water can flow efficiently toward the roof drains, preventing waterlogging of the system while also potentially retaining a small amount of moisture for the plants.
A geotextile filter layer is placed over the drainage component to prevent fine particles from the growing medium from clogging the drainage mat. The final structural component is the growing medium (substrate), engineered to be significantly lighter than standard garden soil. This substrate is composed of lightweight aggregate (like crushed brick or shale) and a small amount of organic matter to provide nutrients, ensuring excellent drainage and minimizing saturated weight.
Selecting and Establishing Plant Life
The final layer is the vegetation, chosen based on the system type and local climate conditions. For extensive green roofs, plant selection focuses on extremely hardy, low-growing, drought-tolerant species with shallow root systems. Succulents from the Sedum genus are the most common choice because they store water in their leaves and survive long periods without irrigation.
Other successful choices for extensive systems include mosses, herbaceous perennials, and native grasses that can withstand the harsh, exposed conditions of a rooftop. Intensive systems offer a wider range of options, allowing for deep-rooted grasses, ornamental perennials, and even small shrubs or trees. Regardless of the system, initial establishment requires careful attention, including regular watering to help roots anchor into the substrate and occasional weeding to prevent aggressive, non-desired species.