Wind-damaged Roof Systems

Wind damage to roof systems is often catastrophic, placing the building users at a life-safety risk, resulting in interior and furnishing damage and suspension of interior operations, loss of revenues, legal ramifications and great costs to repair. Because of my 30 years of experience in the design of roof systems and forensic investigation, I’m often called upon as an expert witness after wind events. In this article, I’ll review a couple wind-event roof failures, the causes of the failures and how they could have been prevented. I’ll also provide recommendations for failure prevention in the design process for new roof systems, as well as for existing roof systems.

1. The concrete roof deck panels deflected more than 3/4 inch, which the design architect should have accounted for if a thorough field investigation was undertaken.

1. The concrete roof deck panels deflected more than 3/4 inch, which the design architect should have accounted for if a thorough field investigation
was undertaken.

The Perfect Storm

How can it be that when roof systems are to be designed for code-required wind-uplift resistance that so many fail in winds well below the design parameters and/or warranty coverage? The answer could be design-related, material or installation; typically, it involves all three.

Architects and some roof system designers are often not as knowledgeable about roof systems as they should be, have little empirical evidence in how all the components work together as a system, and move beyond their abilities (a violation of their standard of care) when designing roofs where specific detailing is required. In addition, manufacturers are all too often
bringing new products to the marketplace that have not been properly vetted in the field and their long-term performance is truly unknown. Unfortunately, the roofing contractor cannot escape any of this. The lack of proper specification and contract document review; failure to review product data, including installation guidelines for new products; poor project oversight and management; and pressure from general contractors often result in installations that are subpar. The result is a “perfect storm” of design, materials and installation that fail under stress.

Consider the following case studies that I have been involved in as a forensic or “expert” witness when litigation was involved.

Coastal Facility

A large aged warehouse along the eastern seaboard was in need of a new roof system. Because the interior was not conditioned, thermal insulation was not required. The existing roof was an asphalt built-up with aggregate surfacing on high-density fiberboard on precast concrete panels 24-inches wide on a steel structure. The northern portion of the building had overhead doors that were seldom closed. On the interior, an aedicule structure (a building within a building) was constructed approximately 65-feet south of the overhead door, which had a ceiling level 5-feet below the roof deck.

2. The thin, flexible 1/2-inchthick high-density board was found to have little, if any, contact with the full-coverage spray-foam adhesive, making uplift extremely easy.

2. The thin, flexible 1/2-inch-thick high-density board was found to have little, if any, contact with the full-coverage spray-foam adhesive, making uplift extremely easy.

The architect who designed the replacement roof system called for the existing BUR roof to be removed down to the precast concrete roof panels. Then a new 1/2-inch 4- by 8-foot high-density wood fiberboard was set in full-coverage spray polyurethane foam adhesive with a 60-mil EPDM membrane fully adhered to the high-density wood fiberboard.

Additionally, the architectural drawings called for rooftop relief vents to be removed and capped over.

Around June 2008, a Nor’easter (an intense rainstorm), coming in from the east off the ocean, swept into the city. This resulted in the new roof system being lifted off the roof deck. Mode of failure was the fiberboard detaching from the precast concrete roof deck.

Investigation revealed several acts and conditions that contributed to the wind damage.

PHOTOS: Hutchinson Design Group Ltd.

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Insulation Types, Application Methods and Physical Characteristics Must Be Reviewed, Understood and Selected to Ensure Roof System Performance

Designing and constructing roof systems (see my previous articles about roof decks, substrate boards and vapor barriers) continues with the thermal insulation layer. The governing building codes will dictate the minimum R-value required and, based on the R-value of the selected insulation, the thickness of required insulation can be determined. This plays into the design of the roof edge, which will be the subject of future articles. For now, let’s focus on insulation.

Photo 1: Polyisocyanurate (ISO) with organic facers

Photo 1: Polyisocyanurate
(ISO) with organic facers

Thermal insulation has multiple purposes, including to:

    ▪▪ Provide an appropriate surface on which the roof cover can be placed.
    ▪▪ Assist in providing interior user comfort.
    ▪▪ Assist in uplift performance of the roof system.
    ▪▪ Provide support for rooftop activities.
    ▪▪ Keep the cool air in during the summer and out during the winter, resulting in energy savings.

INSULATION OPTIONS

For the designer, there are numerous insulation material choices, each with its own positive and negative characteristics. Today’s insulation options are:

    ▪▪ Polyisocyanurate (ISO)

  • »» Varying densities
  • »» Organic facers (see photos 1 and 2)
  • »» Double-coated fiberglass facers (see photo 3)
  • ▪▪ Expanded polystyrene (XPS) (see photo 4)

  • »» Varying densities
  • ▪▪ Extruded polystyrene (EPS) (see photo 5)

  • »» Varying densities
  • ▪▪ Mineral wool (see photo 6)

  • »» Varying densities
  • ▪▪ Perlite
    ▪▪ High-density wood fiber

With today’s codes, the use of perlite and high-density wood fiber as primary roof insulation is very limited. The R-value per inch and overall cost is prohibitive.

Some attributes of the more commonly used insulation types are:
POLYISOCYANURATE

Photo 2: Polyisocyanurate (ISO) with organic facers

Photo 2: Polyisocyanurate
(ISO) with organic facers

    ▪▪ Predominate roof insulation in the market
    ▪▪ Organic and double-coated fiberglass facers (mold-resistant)
    ▪▪ Varying densities available: 18 to 25 psi, nominal and minimum, as well as 80 to 125 psi high-density cover boards
    ▪▪ Has an allowable dimensional change, per the ASTM standard, that needs to be understood and designed for
    ▪▪ Can be secured via mechanical fasteners or installed in hot asphalt and/or polyurethane foam adhesive: bead and full-coverage spray foam
    ▪▪ Has an R-value just under 6.0 per inch but has some downward drifting over time

EXPANDED POLYSTYRENE (EPS)

    ▪▪ Has good moisture resistance but can accumulate moisture
    ▪▪ Direct application to steel decks is often a concern with fire resistance
    ▪▪ Has varying densities: 1.0 to 3.0 pound per cubic foot
    ▪▪ Very difficult to install in hot asphalt; basically not appropriate
    ▪▪ Certain products can be secured with mechanical fasteners or lowrise foam adhesive
    ▪▪ Has stable R-values: 3.1 to 4.3 per inch based upon classification type

EXTRUDED POLYSTYRENE (XPS)

    ▪▪ Has good moisture resistance and is often used in protected roof membrane systems and plaza deck applications
    ▪▪ Direct application to steel decks is often a concern with fire resistance
    ▪▪ Has varying compressive strengths: 20 to 100 psi
    ▪▪ Not appropriate to be installed in hot asphalt
    ▪▪ Has stable R-values: 3.9 to 5 per inch based on classification type

MINERAL WOOL

    ▪▪ Outstanding fire resistance
    ▪▪ Stable thermal R-value: 4.0 per inch
    ▪▪ No dimensional change in thickness or width over time
    ▪▪ Available in differing densities
    ▪▪ May absorb and release moisture
    ▪▪ Can be installed in hot asphalt or mechanically attached

PHOTOS: HUTCHINSON DESIGN GROUP LTD.

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Vapor Retarders

The need for, use and design of a vapor retarder in the design of a roof system used to be a hotly debated topic. It appears now—when vapor retarders are needed more than ever—the design community seems to have lost interest, which is not good, considering how codes and standards (altered through concerns for energy savings) have changed how buildings are designed, constructed and operated. Most notably, positive building pressures are changing the game.

If not controlled, constructiongenerated moisture can have deleterious effects on new roof systems.

PHOTO 1: If not controlled, construction-generated
moisture can have
deleterious effects on new
roof systems.

A vapor retarder is a material or system that is designed as part of the roof system to substantially reduce the movement of water vapor into the roof system, where it can condense. Everyone knows that water in roof systems is never a positive. Typically, a vapor retarder has to have a perm rating of 1.0 or less to be successful. Through my recent observations, the lack of or poorly constructed vapor retarders contribute to ice under the membrane, soaked insulation facers, destabilized insulation, rusting roof decks, dripping water down screw-fastener threads, compromised fiber board and perlite integrity, mold on organic facers and loss of adhesion on adhered systems, just to name a few. Oh, and did I fail to mention the litigation that follows?

The codes’ “air-barrier requirements” have confused roof system designers. Codes and standards are being driven by the need for energy savings and, as a consequence, buildings are becoming tighter and tighter, as well as more sophisticated. This article will discuss preventing air and vapor transport of interior conditioned air into the roof system and the need for a vapor retarder. The responsibility of incorporating a vapor retarder or air retarder into a roof system is that of the licensed design professional and not that of the contractor or roof system material supplier.

It should be noted that all vapor retarders are air barriers but not all air barriers are vapor retarders. In so much that the roof membrane can often serve as an air barrier, it does nothing to prevent this interior air transport.

WHEN TO USE A VAPOR RETARDER

So the question arises: “When is it prudent to use a vapor retarder?” This is not a simple question and has been complicated by codes, standards, costs and building construction, changing roof membranes and confusion about air barriers. Then, there is the difference in new-construction design and roof removal and replacement design. Historically, it was said that a vapor retarder should be used if the interior use of the building was “wet”, such as a pool room, kitchen, locker shower rooms, etc.; outside temperature in the winter was 40 F or below; or when in doubt, leave it out. In my experience, changes in the building and construction industry have now made the determination criteria more complex.

I find there are typically three primary scenarios that suggest a vapor barrier is prudent. The first is the interior use of the building. The second is consideration for the control of construction-generated moisture, so that the roof can make it to the building’s intended use (see photo 1). The third consideration is the sequence of construction. In all three situations I like to specify a robust vapor retarder that “dries in” the building so that interior work and construction work above the vapor retarder can take place without compromising the finished roof. Consider the following:

BUILDING USE

This characteristic is often the most determinant. If the interior use of the building requires conditioned air and has relative-humidity percentages great enough to condense if the exterior temperatures get cold enough, a vapor retarder is needed to prevent the movement of this conditioned air into the roof system where it can condense and become problematic.

Most designers consider building use only in their design thinking, and it is often in error as the roof system can be compromised during construction and commissioning (through interior building flushing, which can drive moist air into the roof system) before occupancy.

To seal two-ply asphaltic felts set in hot asphalt on a concrete roof deck, an asphaltic glaze coat was applied at the end of the day. Because of the inherent tackiness of the asphalt until it oxidizes, Hutch has been specifying a smooth-surfaced modified bitumen cap sheet, eliminating the glaze coat.

PHOTO 2: To seal two-ply asphaltic felts set in hot asphalt on a concrete roof deck, an asphaltic glaze coat was applied at the end of the day. Because of the
inherent tackiness of the asphalt until it oxidizes, Hutch has been specifying a smooth-surfaced modified bitumen cap
sheet, eliminating the glaze coat.

CONTROL OF CONSTRUCTION-GENERATED MOISTURE

I have seen roof systems on office buildings severely compromised by construction- generated moisture caused by concrete pours, combustion heaters, block laying, fireproofing, drywall taping and painting. Thus, a simple vapor retarder should be considered in these situations to control rising moisture vapor during construction, which includes the flushing of the building if required for commissioning.

CONSTRUCTION SEQUENCING AND MATERIALS

Building construction takes place year round. It is unfortunate decision makers in the roofing industry who are pushing low-VOC and/or water-based adhesives do not understand this; problems with their decisions are for another article. If the roof is to be installed in late fall (in the Midwest) and interior concrete work and/or large amounts of moisture-producing construction, such as concrete-block laying, plastering, drywall taping or painting, are to take place, a vapor retarder should be considered.

How will the building, especially the façades, be constructed? Will they be installed after the finished roof? This creates a scenario for a damaged “completed” roof system.

PHOTOS: Hutchinson Design Group Ltd.

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Substrate Boards

The third installment in my series on the roof system is about the substrate board. (To read my first two articles, “Roofs Are Systems” and “Roof Decks”, see the January/February issue, page 52, and the March/April issue, page 54, respectively.) For the purpose of this article, we will define the substrate board as the material that is placed upon the roof deck prior to the placement of thermal insulation. It often is used in part to support vapor retarders and air barriers (which will be discussed in my next article in the September/October issue).

The type of substrate board should be chosen based on the roof-deck type, interior building use, installation time of year and the cover material to be placed upon it.

The type of substrate board should be chosen based on the roof-deck type, interior building
use, installation time of year and the cover material to be placed upon it.

Substrate boards come in many differing material compositions:
• Gypsum Board
• Modified Fiber Reinforced Gypsum
• Plywood
• High-density Wood Fiber
• Mineral Fiber
• Perlite

Substrate boards come in varying thicknesses, as well: 1/4 inch, 1/2 inch, 5/8 inch and 1 inch. The thickness is often chosen based on the need for the board to provide integrity over the roof deck, such as at flute spans on steel roof decks.

TOUGHNESS

The type of substrate board should be chosen based on the roof-deck type, interior building use, installation time of year and the cover material to be placed upon it. For example, vapor retarder versus thermal insulation and the method of attachment. Vapor retarders can be adhered with asphalt, spray foam, bonding adhesive, etc. The substrate board must be compatible with these. You wouldn’t want to place a self-adhering vapor retarder on perlite or hardboard because the surface particulate is easily parted from the board. Meanwhile, hot asphalt would impregnate the board and tie the vapor-retarder felts in better. The substrate board must have structural integrity over the flutes when installed on steel roof decks. The modified gypsum boards at 1/2 inch can do this; fiberboards cannot. If the insulation is to be mechanically fastened, a substrate board may not be required.

It should be more common to increase the number of fasteners to prevent deformation of the board, which will affect the roof system’s performance.

It should be more common to increase the number of fasteners to prevent deformation of the board, which will affect the roof system’s performance.

The substrate board should be able to withstand construction-generated moisture that may/can be driven into the board. Note: In northern climates, a dew-point analysis is required to determine the correct amount of insulation above the substrate board and vapor retarder, so condensation does not occur below the vapor retarder and in the substrate board.

Substrate boards are often placed on the roof deck and a vapor retarder installed upon them. This condition is often used to temporarily get the building “in the dry”. This temporary roof then is often used as a work platform for other trades, such as masonry, carpentry, glazers and ironworkers, to name a few. The temporary roof also is asked to support material storage. Consequently, the substrate board must be tough enough to resist these activities.

The most common use of a substrate board is on steel and wood decks. On steel roof decks, the substrate board provides a continuous smooth surface to place an air or vapor retarder onto. It also can provide a surface to which the insulation above can be adhered. Substrate boards on wood decks (plywood, OSB, planking) are used to increase fire resistance, prevent adhesive from dripping into the interior, provide a clean and acceptable surface onto which an air or vapor retarder can be adhered, or as a surface onto which the insulation can be adhered.

PHOTOS: HUTCHINSON DESIGN GROUP LTD.

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Roof Decks: Don’t Underestimate the Backbone of the Roof System

NOTE: This article is intended to provide general information while conveying the importance of the roof deck as an integral part of a roof system. Additional information about specific effects and concerns in regard to roofing can be found in The NRCA Roofing and Waterproofing Manual and various roof-cover manufacturers’ design guides.

Wood plank decks can provide a dramatic exposed roof deck.

Wood plank decks can provide a dramatic exposed roof deck.

The roof deck is the backbone and an integral component of all roofing systems. Its main function is to provide structural support for the roof system and, therefore, is a building element that needs to be designed by a licensed design professional because proper support of the roofing above is critical to the roof system’s success.

Roof decks also add thermal performance and fire resistance and ratings, provide slope for drainage and enhance wind-uplift performance. They must accommodate building movement and often determine the attachment method of the vapor retarder, insulation and membrane.

Roof Deck Types

There are many types of roof decks being installed today:

  • Steel
  • Precast concrete panel
  • Structural concrete
  • Cementitious wood fiber
  • Wood planking
  • Plywood/OSB
  • Poured gypsum

Some decks are covered with topping fills that become the base for the roof system and may also be an integral structural component:

  • Concrete
  • Lightweight insulation concrete topping
  • Lightweight aggregate concrete topping

Other deck toppings are available, such as poured gypsum and lightweight concrete with integral insulation, but these are considered substrate covers and not roof decks.

The most prevalent roof deck in the U.S. for commercial buildings is steel. On the West Coast, plywood/OSB is very popular. In addition to the roof decks already mentioned, in the course of roof-replacement work the designer may come in contact with the following:

While the “plate” test is not a preferred method, it can quickly and inexpensively give an indication of retained moisture in lightweight aggregate concrete roof deck covers.

While the “plate” test is not a preferred method, it can quickly and inexpensively
give an indication of retained moisture in lightweight aggregate
concrete roof deck covers.

  • Book tile
  • Lightweight precast concrete planks
  • Precast gypsum planks
  • Transite

Collaboration with the Structural Engineer

Because a roof deck is the foundation for the roof system, the designer needs to coordinate the roof system design requirements for the roof deck with the structural engineer to ensure the performance of the roof system. For example, the roof deck may need to extend to the roof edge. In this example, the roof deck may not need to extend to the roof edge for structural concerns but is needed to support the roof system; the roof designer must address this. If the roof deck is structurally sloped, the designer and engineer must determine whether the low point is a potential drain location. Are there steel beams in the way of the drain location? The roof deck must be attached to the structure to prevent uplift. And the designer and engineer must determine what the deflection of the roof-deck span may be between structural supports. For example, steel deck is sometimes installed with spans of 7 feet between joists and flexes (deflects) under foot traffic. This typically is not a good condition onto which a ridged roof system, such as a bituminous one, should be installed. It cannot be expected to accommodate such deflection. PHOTOS: Hutchinson Design Group Ltd. [Read more…]

Attention Roof System Designers: Numerous Roof Components Work Together to Affect a Building

There has been a great deal of opinion expressed in the past 15 years related to the roof cover(s), or the top surface of a roof system, such as “it can save you energy” and “it will reduce urban heat islands”. These opinions consequently have resulted in standards and code revisions that have had an extraordinary effect on the roofing industry.

The building type should influence the type of roof system designed. Some spaces, like this steel plant, are unconditioned, so insulation in the roof system is not desired.

The building type should influence the type of roof system designed. Some spaces, like this steel plant, are unconditioned, so insulation in the roof system is not desired.

Let’s say it loud and clear, “A single component, does not a roof make!”. Roofs are systems, composed of numerous components that work and interact together to affect the building in question. Regardless of your concern or goal—energy performance, urban heat-island minimization, long-term service life (in my opinion, the essence of sustainability) or protection from the elements—the performance is the result of an assembled set of roof system components.

Roof System Components

Energy conservation is an often-discussed potential of roofs, but many seem to think it is the result of only the roof-cover color. I think not. Energy performance is the result of many factors, including but not limited to:

Building use: Is the building an office, school, hospital, warehouse, fabrication facility, etc.? Each type of building use places different requirements on the roof system.

Spatial use and function be low the roof deck: It is not uncommon in urban areas to have mechanical rooms or interstitial spaces below the roof—spaces that require little to no heating or cooling. These spaces are typically unconditioned and unoccupied and receive no material benefit from the roof system in regard to energy savings.

Roof-deck type: The type of roof deck—whether steel; cast-in-place, precast and post-tensioned concrete; gypsum; cementitious wood fiber; or (don’t kill the messenger) plywood, which is a West Coast anomaly—affects air and moisture transport toward the exterior, as well as the type of roof system.

Roof-to-wall transition(s): The transition of the roofing to walls often results in unresolved design issues, as well as cavities that allow moisture and vapor transport.

Meanwhile others, like this indoor pool, require extreme care in design and should include a vapor retarder and insulation.

Meanwhile others, like this indoor pool, require extreme care in design and
should include a vapor retarder and insulation.

Roof air and/or vapor barrier: Its integration into the wall air barrier is very important. Failure to tie the two together creates a breach in the barrier.

Substrate board: Steel roof decks often require a substrate board to support the air and vapor barrier membranes. The substrate board also can be the first layer of the roof system to provide wind-uplift resistance.

Insulation type: Each insulation type—whether polyisocyanurate, expanded polystyrene, extruded polystyrene, wood fiber, foam glass or mineral wool—has differing R-values, some of which drop with time. Many insulation types have differing facer options and densities.

The number of insulation layers: This is very important! A single layer of insulation results in a high level of energy loss; 7 percent is the industry standard. When installing multiple layers of insulation, the joints should be offset from layer to layer to avoid vapor movement and thermal shorts.

Sealing: Voids between rooftop penetrations, adjacent board and the roof-edge perimeters can create large avenues for heat loss.

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