The Details of a Vegetative Roof

Managers need to ensure proper design and installation by engaging a professional. Each roof is different, so managers should not rely on “boxed” systems.

Before installation of the overburden, managers should schedule a flood test of the system. Water should be at least 2 inches deep at all details. If a detail fails, the installer must retest it after repairs.

Vegetative roofs offer significant benefits related to sustainability that building owners and managers seek. But to ensure system longevity, managers must specify proper materials and detailing. Otherwise, the cost to repair or replace a leaking vegetative roof system dramatically reduces these sustainability benefits.

When specifying a vegetative roofing system, managers must consider system details carefully.

Base flashing. For conventional roofs, the top edge of the base flashing must be at least 8 inches above the membrane surface. For vegetative roofs, the top edge of the flashing system should be at least 8 inches above the top of the planting system.

Penetrations. For all roof systems, managers must minimize penetrations to reduce the potential for leaks. Due to access constraints and challenges, this issue is more important in vegetative roofs.

Gravel. Managers should specify that installers use gravel at perimeters and around penetrations and details so water can flow promptly away from these areas.

Drains. Managers need to specify dual-level drains to ensure water can drain from the top surface of the overburden and the waterproofing-membrane layer.

Planters. For plants with deep root structures, such as trees and shrubs, managers should specify independent planter boxes to separate the roots from the waterproofing system.

Roofing Trends: Roofs and Energy

As we head into the next decade, the biggest trends in the roofing industry—which are inherently connected to the direction of the entire construction industry—are related to energy efficiency and sustainability. This push towards energy efficiency begins with a well-insulated facility.

Over the past decade, higher and higher levels of insulation have been mandated with each subsequent edition of building and energy codes. The result is an increased thickness of overall roof insulation, which certainly is beneficial to a building’s energy efficiency.

Testing One, Two, Three
Roof systems are required by building codes to have minimum fire and wind resistance appropriate for the building type and location. Testing agencies, like Underwriters Laboratories and FM Approvals, perform fire and wind resistance tests and classify roof systems.

So how does this impact the energy efficiency trend? In order to achieve the maximum insulating benefit from roof systems, it is best to use at least two layers of insulation and stagger the board joints. This approach provides a more stable substrate for the roof membrane and minimizes the likelihood of unwanted air movement within a roof system.

A single layer of roof insulation means there is a high possibility of air movement from the deck to the underside of the membrane. And because air movement accounts for a significant portion of a building’s heat loss; staggered board joints are quite beneficial to the overall R-value of a roof system.

Proper design and installation of a roof system’s insulation layer are critical to the accuracy of the energy modeling of a building. If you assume a roof system has a design R-value of 30 but the insulation board joints align vertically and mechanical fasteners are used to secure the entire thickness of insulation, this could present a problem.

Studies have shown that this configuration can create a loss of R-value of over 15%. In fact, the in place R-value would be closer to R-25.

Reflected Energy
White and light colored, highly reflective roof surfaces also are trending up and have been for some time, mostly in the form of TPO and PVC single-ply systems. However, there are also reflective BUR cap sheets that provide similar reflectance. The benefit of roof surface reflectivity should be looked at from two perspectives: the specific building’s energy savings and the overall environment by reducing the ‘urban heat island’ effect.

Low rise buildings with large roof areas (relative to the entire building envelope) are likely to be more energy efficient during periods of air conditioning operation when a highly reflective roof surface is used to reduce solar gain into the building. On the other hand, a highly reflective roof will reflect the sun’s heat energy, subsequently reducing rooftop temperatures. This is overall beneficial for the environment regardless of building size, proportions, and location. Understanding the different perspectives about the benefits of using a reflective roof surface is important when determining if a building will or will not glean reduced energy consumption.