MEC&F Expert Engineers : CAUSES AND CONTRIBUTING FACTORS OF FAILURES OF VEGETATIVE (GREEN) ROOFS

Thursday, October 16, 2014

CAUSES AND CONTRIBUTING FACTORS OF FAILURES OF VEGETATIVE (GREEN) ROOFS

https://sites.google.com/site/metropolitanforensics/cause-of-failures-of-vegetative-roofs

CAUSES OF FAILURES OF VEGETATIVE ROOFS

Failed Aquascape roof in 2011.

There has been a steady increase of claims for damages associated with green technology issues. This blog will address one of these types of claims we have been dealing with: the failure of the vegetative roofs.

The failure of the largest vegetative roof in North America

Perhaps most of you have heard of the “famous” green roof failure at the headquarters of the pond construction company Aquascape in St. Charles, Illinois. This failure (shown above) occurred about 3 years ago. The project was designed to be the largest vegetative sloped roof in North America. The 256,000 sq. ft. building had received a LEED Silver rating. In February 2011, during a heavy snow storm that dumped almost two feet of snow, followed by a freeze/thaw cycle, the roof failed. The melted ice and snow overwhelmed a clogged the drainage system, leading to the failure of a portion of the roof (roughly a 700 x 40 foot section collapsed). The roof was rebuilt one year later, using only 75 percent of the original vegetative cover, especially over the area of the roof covering the parking garage – that was the weakest area of the roof and supporting foundation.

Closer view of the Aquascape roof failure

The faulty structural design of the roof over the parking area, the lack of maintenance of the vegetative roof, incorrect vegetative roof design, and the undersized drainage system also played a big role in its demise. Aquascape and its insurer, Charter Oak Fire Insurance Company, filed suit against a large number of defendants. Six subcontractors -- Metallic Building Co., Tricon, Senektekts, Applied Ecological Services, Dewberry Architects and Olsson Roofing Co., were dismissed from the lawsuit in August 2014, provided they can reach settlements and a judge finds the settlements were reached in good faith. The primary defendants in the case are the Madison, Wisconsin-based, Artisan Design Build, Area Erectors Inc. and Tecza Design Build. In these roof failure cases, the building owner has to prove that the roofing contractors and other defendants were negligent or breached the construction contract or warranty.

The water-protective membranes (2 layers of orange material) were both cut through to install the leak monitoring boxes. Only one of the membranes should have been cut.

DESCRIPTION OF VEGETATIVE ROOF COVERS

A vegetated roof cover is a veneer of vegetation that is grown on and covers an otherwise conventional flat or pitched roof, endowing the roof (< 30 degree slope) with hydrologic characteristics that more closely match surface vegetation than the roof. The overall thickness of the veneer typically ranges from 2 to 6 inches and may contain multiple layers, consisting of waterproofing, synthetic insulation, non-soil engineered growth media, fabrics, and synthetic components. Vegetated roofs, also called “green rooftops” can be optimized to achieve water quantity and water quality benefits. Through the appropriate selection of materials, even thin vegetated covers can provide significant rainfall retention and detention functions.

Completed vegetative roof in mid-south U.S.

Depending on the plant material and planned usage for the roof area, modern vegetated roofs can be categorized as systems that are intensive, semi-intensive, or extensive. Intensive vegetated roofs utilize a wide variety of plant species that may include trees and shrubs, require deeper substrate layers (usually > 4 inches), are generally limited to flat roofs, require „intense‟ maintenance, and are often park-like areas accessible to the general public.

Extensive vegetated roofs are limited to herbs, grasses, mosses, and drought tolerant succulents such as sedum, can be sustained in a shallow substrate layer (<4 inches), require minimal maintenance once established, and are generally not designed for access by the public. These vegetated roofs are typically intended to achieve a specific environmental benefit, such as rainfall runoff mitigation. Extensive roofs are well suited to rooftops with little load bearing capacity and sites which are not meant to be used as roof gardens. Semi-intensive vegetated roofs fall between intensive and extensive vegetated roof systems. More maintenance, higher costs and

Important Technical and Liability Questions

There are multiple engineering, construction, maintenance, contractual and legal issues associated with this relatively new technology. Certainly the insurers are concerned due to the projected growth of this industry. Health issues have been brought by tenants and this has also alarmed some insurers. We will address some of these issues at a later blog.

Based on the sheer number of roof failures during severe winters, especially the winter of 2013/2014 in the northeast, and the number of green projects growing steadily, there is certainly an increase in litigation addressing the following issues:

· Was the green roof (like the one at Aquascape) designed appropriately to accommodate ice/water loads in a winter climate?

· With so many roof failures this winter (including, of course, the Aquascape and Minneapolis Metrodome in 2011), is there, or should there be, a duty on the building’s owner/maintenance crew to proactively remove snow accumulations?

· For new vegetative, solar or other green roofs, who assumes the risk if the expected cost savings are not realized?

Main Causes of Failure based on Metropolitan’s Investigations

Based on our investigations of these types of failures, we can point out the following causes of failure:

· Time and field-tested codes and standards, including proper construction sequence do not exist;

· Lack of maintenance or improper maintenance;

· Missing or improperly installed flashings. The integrity of the perimeter flashing is a critical first line of defense against roof failure. Flashing interface details with other trades and products are one of the major causes of roof failures; the weather resistant surfaces, and flashings must be continuous, sloped and must shed water;

· Improper selection and installation of the waterproofing membrane;

· Improper installation of the leak detection system;

· Roof did not have the minimum required 2 percent slope;

· Roof leaks developed due to condensation;

· Incorrect or inappropriate design (orientation, growing media, drainage and plant selection);

· Incorrect construction of the liner, insulation and vegetative cover;

· Pierced waterproofing liner during construction;

· Failure to protect the roofing components during the construction process;

· Clogged drains;

· Debonded membrane base flashings below the overburden area;

· Failure to extend the root barrier up walls, curbs, etc. beyond the height of the top of the growth media depth. At drain areas, the root barrier must extend a minimum of three inches past an aluminum or asphalt-based flashing;

· Inadequate-sized drainage system;

· Wind and water erosion of the growing media;

· Failure to oversee the installation;

· Mixing of trades;

· Failure of roofing contractor’s wind uplift design or installation (incorrect fastener type, layout and number of fasteners);

· Lack of experienced personnel to design, build and maintain the roofs

Roof leak caused by improperly designed and constructed vegetative roof.

COMPONENTS OF VEGETATIVE ROOFS

All flat roof assemblies consist of the same basic elements assembled in a seemingly logical order: a roof deck (composed of wood, metal or concrete), covered with insulation and topped with a waterproofing membrane.

The alleged benefits of vegetation on the roof are that it can limit storm water runoff and, by filtering the runoff through the plants, also improve the quality of the runoff. The plantings not only ballast the insulation, they can, depending on the configuration, also add additional R-value (RSI) to the roof assembly. Vegetative roofs provide habitat for insects and other wildlife and often are considered in buildings applying for USGBC or CAGBC Leadership in Energy and Environmental Design (LEED) certification.

The roof structure must be designed to accommodate the dead load from the additional weight of the plantings (including when they are fully saturated by rainfall and covered in several feet of snow), plus any live load from traffic, if applicable. It is also important to design the roof slope and drainage system to accommodate rain runoff.

Building upon years of in-field experience, the majority of roof leaks in these vegetative roof systems occur at flashings as opposed to the interior field area. The integrity of the perimeter flashing is a critical first line of defense against roof failure.

The field area is protected from physical abuse, UV attack and thermal cycling –all factors that are the primary causes of roof failures – by both the insulation and growing media over the membrane. However, sometimes interior field leaks do occur.

At times, grass, weeds or small trees may grow on both vegetative and conventional roofs. Good roofing practice should include a maintenance program that includes periodic inspection for this type of growth. Any plant growth should be pulled out and, if required, the area treated with a weed killer. Roots from plant growth can sometimes damage the membrane if left unchecked. With a properly designed and constructed vegetative roof system, there should be less chance of this happening since the membrane is protected by the insulation, fabric and vegetative media.

Waterproofing Options

Water tightness First!

To a building owner, the beauty of a lavish vegetative green roof is quickly obscured when the roofing system below it starts to leak. To ensure a watertight installation under a green roof, the following steps should be taken by the building owner, architect, roofer, and green roof installer:

Over Design the Roofing System

In a vegetative green roof application, the roofing membrane is buried beneath the vegetation and the more the waterproofing system can be enhanced to extend longevity, the better the overall application will be. Go with a thicker membrane for better puncture resistance. With single-ply membranes, 60 mil should be considered the minimum thickness to use in a green roof application.

On more elaborate vegetative green roof designs or if extended roofing warranties are desired, consider using the thickest membranes available in that product line.

Strip in the seams. To ensure a strong seam, specify a minimum 6-inch stripping over all seams to provide redundancy. To prevent possible abrasions or punctures, try to eliminate any mechanical fasteners directly below the membrane. If attaching insulation, use adhesives for the top layer.

Flashings in a vegetative green roofing system are vulnerable to damage because they are subjected to the differential movement that exists between the roof deck and vertical surfaces. Flashings should extend a minimum of 8 inches above the surface of the membrane and at least 4 inches above the surface of the growing medium.

Installing metal counter flashings over the membrane flashings will provide additional protection from physical damage throughout the service life of the vegetative roofing system. Sealing the top edge of the membrane flashings with sealant and termination bar is critical to maintaining a watertight seal at the flashings.

Fully Inspect/Water Test the Roofing System

It is imperative that the waterproofing system be tested for leaks prior to the installation of the vegetative roof. It makes the most sense to test the integrity of the membrane for water tightness while it is still fully exposed for easy access. There are three basic methods to test a roofing system for leaks prior to installing the vegetative components:

Flood Test

A standing water test. The roof drains are plugged and dams are built to retain water on top of the waterproofing membrane. Water depth should be a minimum of 2 inches and left on the roof surface for at least 24 hours.

Advantages: Exerts hydrostatic pressure over the entire surface of the membrane, which is the ultimate test of a waterproofing membrane.

Disadvantages: Time consuming because only small areas should be tested to prevent overloading the structure. In addition, slope in the roof may mean much deeper depths downslope to create the required 2 inches on the upslope end.

Flowing Water Test

Continual flow of water over the roof surface for a minimum of 24 hours, without blocking the drains or construction dams.

Advantages: Excellent alternative to flood testing on sloped roofs or on roofs that do not have the structural capacity to support 2 inches of ponding water.

Disadvantages: Only tests the roof surface and does not test the flashings. Does not put the roofing membrane under hydrostatic pressure, which may miss some of the smaller, difficult-to-find voids or punctures.

Electronic Field Vector Mapping

Uses DC current and water (with a surfactant) to test for breaches in the roofing membrane. An electric potential is created by placing a conductor below the roof membrane (generally the roof deck and framing) and energizing the roof surface by the use of a grounded loop placed on the surface. Water spread over the surface of the roof will pass through a puncture and complete a circuit. Since DC is single directional, the puncture can be located by following the flow of current.

Advantages: Very precise. Conductors can be left in place and reused throughout the life of the system, which makes pinpointing leaks in the installed system much easier and eliminates the need to remove large areas of vegetation and overburden.

Disadvantages: Expensive compared to flood and running water testing. Will not work on EPDM and may not work with aluminum-coated membranes, due to conductivity of membranes. Nonconductive roof decks require additional conductor installation before new waterproofing membrane is installed.

Protect the Membrane

All vegetative green roofing systems must have some type of membrane protection, such as protection boards or fabric mats. Also, membrane protection must be increased in areas of heavy traffic and under designated walkways. It is very important that the roofing membrane be protected during the installation process. Plywood and other hard-surface protection boards must be used at access points and travel paths during installation.

Don’t forget the root barrier in the system. Omitting the root barrier will subject the roofing membrane to the penetrating characteristics of plant roots. Only PVC and TPO roof membranes meet Germany’s FLL root barrier requirements. All other membrane types require root barriers.

Maintain the Roofing System After Installation

As with any roofing system, regular maintenance is important. Don’t be misled by thinking that because the membrane is buried under a vegetative green roof, there is nothing you can do to maintain the roof. Here are regular maintenance items that should be done on a green roof:

· Clean drains twice a year

· Inspect flashings for deterioration

· Check sealant at terminations for deterioration

· Inspect after major storms

Understanding Roofing and Waterproofing Principles

Low-slope roofing differs from common steep slope (residential) roofing because low-slope roofing systems must be WATERTIGHT where steep-slope roofing systems need only be WATER SHEDDING. Because low-slope roofing systems must be watertight, the slightest hole or puncture will admit water into the systems, creating a leak.

Capillary Action - What does this mean in the roofer’s world? It means the smaller the hole the farther water will travel. It is imperative that all seams be 100% sealed. The smallest gap, or void, in a seam will draw water through the opening and into the roofing system.

Insulation

Consideration should be given to the types of insulation used in a vegetative green roof application.

Avoid the use of porous insulations and focus on closed-cell-type insulations to avoid trapping moisture within the roofing system. Porous insulations to avoid:

· Perlite

· Wood fiber

· Expanded polystyrene

· Fiberglass

Recommended Closed-Cell Insulations:

· Extruded Polystyrene

· Polyisocyanurate

· Gypsum based Dens Deck

In addition to limiting the porosity of the roof insulation, ensure that higher compressive strength insulation is specified to prevent compression by the overburden.

Workers examining an insulation board that should be installed under the moisture retention/drainage panel

Drainage

Like any roofing system, positive drainage is very important. Minimum slope should be 1/4 inch per foot (2% slope). Structural slope is preferred over tapered insulation. The drainage layer not only provides a means for storm water to exit the roof, it also can provide air movement under the growing media to help dry the insulation. Vegetative green roofing systems should always be internally drained. Edge draining should be avoided to prevent loss of components and growing medium at the building perimeter during high wind or precipitation events.

Retrofit Project Considerations

Built-in-place vegetative green roof systems do not lend themselves to retrofit applications. Modular applications are more “user friendly” when designing a retrofit.

Integrity of existing roof system:

· What has been the service life of the roof? - If the service life of the roof is reaching its end and starting to have small issues such as minor leaks and deterioration, the roof system is probably not conducive to a vegetative roof.

· Is the roof system capable of accepting a vegetative green roof system? - It is important to confirm with the manufacturer the acceptability of a vegetative roof system over an existing roof system. Installing a vegetative roof system on an existing roof system could void the balance of the manufacturer’s warranty.

· Is the roof watertight? - Just as with a new application, the existing roof system needs to be completely watertight and in good repair. You do not want to have to go in and make roof repairs after the vegetation is installed.

· Is the structure capable of handling the additional loads required?

Completed vegetative roof in central U.S.

CONSTRUCTION DEFECT SERVICES

Metropolitan specializes in addressing and resolving disputes involving public work projects, business/commercial disputes, complex construction defect, environmental health and safety issues, construction delay and contractual matters, partnerships, homeowners association, retail centers, and real estate issues. Our three decades of forensic experience includes advanced research and analysis of code, materials and building practices, as well as the development of comprehensive repair methodologies for maximization of repair and restoration efficiency. We have financial and legal education, expertise and experience that we used to better focus our engineering arguments and presentations to the various parties. We have successfully participated in dispute resolution mediation, arbitration, and settlement procedures on numerous occasions.

Metropolitan’s design experience includes civil site improvements for single family housing, multi-family housing, commercial and industrial land development, roadways, public works construction, pipelines, reservoirs, pump stations, hydraulics and hydrology, and environmental impacts of construction defects. We are also proficient in all aspects of claims handling and management, including damage, liability and coverage issues, reserving, risk transfer, auditing, litigation management and expense control measures.

Metropolitan has extensive experience in the fields of moisture/water infiltration/intrusion, roof system failures, lath, plaster, mortar, stucco, drywall, dimensional stone, handset applications, prefab installations, light-gauge metal stud, fireproofing, architectural pre-cast concrete, and EIFS for all types of facilities. Our skills and experience include design, construction estimating, material take-offs, site inspection, corrective construction detail, scheduling, subcontractor and field crew coordination, contract drafting and execution, worker and environmental health and safety.

We have handled thousands of factual issues dealing with:

· Delay, disruption, extra work and impact claims;

· contractual change orders;

· standard of care;

· cost of repair;

· scope of work disputes;

· mechanic’s liens;

· stop notices;

· bid protests;

· errors and omissions;

· environmental matters, including mold;

· construction defects, as well as contract drafting and review;

· green technology defects, including vegetative roof systems and wind turbines;

We are including our web address below, should you wish to find out more about us and our services. We are very competitive. We will beat every competitor’s prices and quality of work. We provide discounts for multiple assignments.