Vegetative Roofs: Pay Attention to Base Flashing and Penetrations

By Eric J. Seaverson, P.E.  
OTHER PARTS OF THIS ARTICLEPt. 1: What are the Benefits of Vegetative Roofs?Pt. 2: Vegetative Roofs: A Look at the DisadvantagesPt. 3: Specification Tips for Extensive and Intensive Vegetative RoofsPt. 4: Vegetative Roofs: Secondary Components Include Root Barriers and Vapor RetardersPt. 5: Vegetative Roofs: Ensuring Proper Design and InstallationPt. 6: This Page

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.


The GreenSave Calculator from Green Roofs for Healthy Cities (GRHC) allows managers to compare roofing systems to determine which roof has the lowest life-cycle cost. It can determine whether reduced energy use and lower maintenance and operating costs justify the higher initial cost of certain roofing systems. The calculator also determines whether some roofs have lower initial costs but cost more throughout their life cycles. To learn more about the calculator, visit GRHC.

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  posted on 12/30/2009   Article Use Policy

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