The Stud Wall and the Roof

Photo 1. With a stud wall parapet, inappropriate wall substrate and base anchor screws into a material with low pull-out resistance, this roof blew off in what would be considered moderate winds. Images: HUTCHINSON DESIGN GROUP LTD. 

How do I start an article on a topic that is so problematic, yet it’s not being addressed by designers, roof system manufacturers, FM, SPRI, NRCA or any other quality assurance standard? Like many transitions in the building industry, the use of metal studs in exterior wall construction and roofing in new construction developed out of the twin concerns of value engineering and cost reduction. It has crept silently forward without any real consideration of the possible effects this less robust construction method would have on roof system performance. 

Photo 2. When the base anchors pull out of the substrate, the membrane becomes unsecured and will lift up. Here the membrane was observed lifting to heights of 3 to 4 feet, at which point it popped the coping off.

You would think that someone along the line would say, “Hmm, I wonder how strong, effective or appropriate a screw fastener through a modified gypsum board sheathing would be?” Let me answer that question: Worthless. (See Photos 1-3.) 

There are many issues with metal stud wall construction as it relates to roofing: air drive, moisture, interior pressures, and membrane adhesion to substrate, just to name a few. This article will address only one concern: The base anchor attachment horizontally into steel stud walls, most often clad with a modified gypsum substrate board. (See Photo 4.)

Why Is This a Concern?

Photo 3. All the base anchor screws pulled out of the substrate except one that was into the stud, which just tore away when the rest of the membrane lifted.

Problems often begin in the design phase when the condition is not detailed appropriately. (See Figures 1 and 2.) The architect/engineer/ designer shows some lines and figures that the roofing contractor or manufacturer will make it work — and specifies a 20-year warranty. The designer’s first mistake is to think that contractors and manufacturers design. They do not.If I were a betting man, I would guess that 99 percent of the specified wall substrate for roof-side metal stud walls is a product that is unacceptable for roofing base flashing application. You’re smiling now, aren’t you? Been there, huh? Designers often have little knowledge as to how a roof system, or even a roof membrane, is installed, and thus don’t even realize the errors of their ways. If they did, they might realize that at the very least a base anchor attachment is at 12 inches on center, and at some time a screw is going to have to go horizontally into the inappropriate sheathing substrate. Concept 1: Architects design. I know this is scary.

Figure 1. This is a common architectural stud wall parapet detail. No base anchor is even being acknowledged, nor is the concern with vertical vapor drive in the stud wall cavity. This type of detailing, in my opinion, is below the standard of care of the architect.

Architects and designers who do not prepare project-specific details seem to love manufacturers’ standard details, which are provided as a baseline for developing appropriate project-specific details. They are not an end all, and thinking they are is a huge mistake. Another common mistake is not realizing that manufacturers do not have a standard detail for base anchor attachment into metal stud walls. This is probably because they never imagined that anyone would really try to anchor into such a poor substrate. Concept 2: Manufacturers produce products that can be assembled in a roof system; they do not design.

Oh, but the contractor will make it work. Yeah, right. Concept 3: Contractors install materials provided by the manufacturer, as specified by the designer; they do not design. Are you starting to see a trend here?

You can now see the conundrum of the blind leading the blind. 

So, to be clear:

  • Architects: Design
  • Manufacturers: Produce products
  • Contractors: Install materials

To say it a bit clearer:

  • Architects: Design
  • Manufacturers: Do Not Design
  • Contractors: Do Not Design

Read it again and see where the responsibility lies. Of course, the manufacturer needs to produce quality materials, which sometimes does not occur, and contractors need to install the materials correctly, which sometimes does not occur.

Pull-Out Strength

So that we can get this detail correct, let’s look at pull-out strengths of various materials. But let’s start with trying to determine what pull-out resistance is required. For our example, let’s use 60-mil TPO, a common roofing membrane on new construction projects. 

Figure 2. This parapet detail has been well thought out in regard to thermal drive and concerns with condensation within the stud wall cavity, but ignores how the roof membrane will be attached to the wall. The insulation thickness will result in an unbraced section of the screw and allow rotation before it pulls out of what is assumed to be a gypsum base sheathing.

Manufacturers report on their data sheets for 60-mil TPO tear strength of around 130 pounds of force (lbf). The test for this isn’t pulling the membrane out from base anchors, but it’s a good start for our discussion. I suspect that if base anchors are attached at 9 inches or 12 inches on center that the series of fasteners will elevate this value.

Given that we know that the tear resistance of TPO with a series of fasteners is greater than the ASTM D751 Tearing Strength test, I will suggest that we need a substrate with a pull resistance greater than 260 lbf, or twice the tear strength value. After that the membrane will tear itself out from around the fastener plate. 

To determine the pull-out resistance of various sheathing materials, I had the pull-out resistance of a base anchor screw tested on several materials by Pro-Fastening Systems, a specialty distributor focusing on commercial roofing in the Midwest that provides certified pull-out testing. Three pull-out tests were performed on each material. (See Photo 5.) The mean resistance values are as follows:

Photo 4. This exterior view gives a good idea of how inadequate gypsum-related products are in regard to providing a pull-out resistance. A 16-, 18- or 20-gauge plate should have been placed at the stud wall from the concrete deck up above the anchor point.

1/2” plywood: 422 pounds

5/8” plywood: 402 pounds

1/2” glass-faced gypsum: 13.3 pounds

1/2” integral fiber reinforced gypsum: 110 pounds

22-gauge steel deck: 646 pounds

22-gauge acoustical steel deck: 675 pounds

18-gauge steel stud: 1,086 pounds

26-gauge metal stud: 646 pounds

16-gauge steel plate: 1,256 pounds

18-gauge steel plate: 978 pounds

20-gauge steel plate:724 pounds

22-gauge steel plate:625 pounds

So as a starter we eliminate all the typical gypsum-based sheathing materials from being used at the base of the roof. I’m not keen on plywood either, as over time, as the plywood dries, the pull-out strength lessens. Additionally, gluing to wet plywood never works well. 

Designing the Base Anchor on Metal Stud Walls

Photo 5. Various materials were tested to determine their pull-out resistance. The results confirmed what intuitively most roofing contractors would know — that gypsum-based products have very little holding power.

The concept is simple — provide a substrate with a pull-out resistance greater than the tear strength of the roofing membrane attached in series. So, let’s pretend you’re drafting. Come on now, get your paper out, a number 2H pencil, a parallel rule and triangle to get the feel of the detail — no CAD for you today. For our example, assume you’re in the Chicago area, minimum R-value of 30, tapered insulation and 24 feet from the drain to the wall. 

First, draft and show the roof deck and your wall, roof edge and studs. Now you’re ready to start your detail. First go to your roof plan, where you have shown all the tapered insulation, and calculate what the thickness will be at your studs. Remember, code requires thickness within 4 feet of the drain. For our detail, you’re near Chicago and thus the height of a tapered insulation layout might be as follows. For the R-30 at the roof drain with a substrate board, insulation and cover board, let say for simplicity it’s 6.5 inches (1/2-inch cover board + 5.4 inches of code-required insulation + 1/2-inch cover board). Now you need to calculate the tapered insulation. For our example, the distance from drain to wall is exactly 24 feet. With a taper of 1/4-inch per foot tapered that is 6.5 inches (1/4 inch x 24 feet = 6 inches, plus the 1/2-inch starting thickness of the tapered). If you plan to use foam adhesive, add 3/8 inch per layer of foam, and be sure you understand all the layers in a tapered system. So, at the wall, the insulation will be approximately 13 inches. With the screw and plate anchor say, 2 inches above the insulation surface, we have a height of 15 inches. So, let’s say we need a substrate capable of pull-outs at least 18 inches in height from the roof deck.

Figure 3. Design of a stud wall parapet includes delineating all the components and tells the contractor what is expected. Burying such information in the specification does no one any good, as the architect most likely will not know to review the shop drawings to those requirements.

Now, I know you are thinking, “OMG, 18 inches — I can cut in a little 6-inch strip at the top of the insulation.” Don’t do it. The strip will not have any continuity or strength and will often buckle under load. Additionally, this continuous substrate piece needs to be placed on the stud. 

Back to your drafting board. Draw in against your stud a continuous 16-, 18- or 20-gauge galvanized steel plate. Depending if the membrane is to be taken up and over the stud wall or terminated some distance above the roofing, the rest of the wall can be clad in less robust materials. Pick any substrate that is roofing membrane compatible and place it over the continuous steel plate and studs above. Tell the contractor how often you want the substrate anchored.

Figure 4. We often find that a simple isometric drawing showing the construction of stud wall parapets is helpful in informing all the related trades how their work interrelates.

Draw in your substrate board, vapor retarder, insulation (and don’t forget to show and call out the spray foam seal between the insulation and wall, as there is often a void). Bring your membrane to the wall, turn it up 3 inches fully adhered to the substrate and show a plate and screw. Call this plate and screw out and note the spacing on the drawing; I’ve never seen a spec up on the roof. The base flashing can now be delineated coming down over the anchors and out onto the flat. Depending on the material, show a weld or seam tape. Now compare your detail to Figures 3 and 4. Who has properly designed the condition?

Remember

There are many issues and concerns with steel stud walls and roofing. This issue with substrate cladding in regard to the interface with the roofing system is only one that I see again and again on projects that have wind damage issues. By carefully designing the roof termination conditions, taking into account all the possible impacts and then detailing the conditions properly, your standard of care can be met and the owner well served.

About the author: Thomas W. Hutchinson, AIA, FRCI, RRC, CSI, RRP, is a principal of Hutchinson Design Group Ltd. in Barrington, Illinois. For more information, visit www.hutchinsondesigngroup.com

Polyiso Wall Insulation Product Line Meets New Model Energy Codes

EnergyShield CGF Pro, glass faced polyiso insulation for commercial exterior walls, helps protect the integrity of the continuous insulation layer.

EnergyShield CGF Pro, glass faced polyiso insulation for commercial exterior walls, helps protect the integrity of the continuous insulation layer.

EnergyShield CGF Pro and EnergyShield Ply Pro are the newest members of the Atlas Roofing Corporation’s commercial polyiso wall insulation line.

EnergyShield CGF Pro, glass faced polyiso insulation for commercial exterior walls, helps protect the integrity of the continuous insulation layer by resisting jobsite damage, particularly in masonry, brick veneer and metal panel assemblies. Additionally, the product offers more vapor permeability than foil-faced insulation, has multiple NFPA fire tested assemblies and is engineered for incorporation into commercial wall assemblies.

EnergyShield Ply Pro is a Class A polyiso wall insulation bonded to plywood for commercial continuous wall insulation systems. The single component provides insulation, together with a fire-treated plywood substrate that can be mechanically fastened to various cladding systems, resulting in fast installations and labor savings. EnergyShield Ply Pro offers the highest R-value per inch of any rigid insulation.

“One of our key priorities is to make Polyiso products easy for designers and installers to use in commercial applications,” said Tom Robertson, EnergyShield bsiness unit manager. “These products are intended to bring design flexibility, R-value and NFPA fire tested assemblies advantages of Polyiso to a wider audience.”

EnergyShield CGF Pro and Ply Pro are available for ordering though an Atlas representative. The EnergyShield line of high performance insulation provides continuous insulation boards for all design, code and efficiency requirements. EnergyShield products are designed and manufactured in eight locations throughout the US and Canada by Atlas Roofing Corporation.

Cover Boards: The Membrane and Insulation Protector

Continuing on our roof system component analysis—after discussion of the roof deck, substrate board, vapor retarders and insulation—we now have worked our way up to the cover board. For the purpose of this discussion, the cover board is defined as the board placed upon the insulation as the final substrate to which the roof cover will be placed.

The purpose of the cover board is multifaceted; it can include:

    Insulation Protection: Placed to protect the thermal layer from the often deleterious effects of repeated foot traffic, which can result in insulation crushing, loss of roof-cover adhesive, inability to resist wind uplift and mechanical- fastener puncture through the membrane.

    Asphaltic core boards are very flexible and will conform to irregular surfaces and offsets without fracture. Here crews work to install the cover board in bead-foam adhesive in preparation for the three-ply modified bitumen roof cover. PHOTO: Clark Roofing

    Asphaltic core boards are very flexible and will conform to irregular
    surfaces and offsets without fracture. Here crews work to install the cover board in bead-foam adhesive in preparation for the three-ply modified bitumen roof cover. PHOTO: Clark Roofing

    Enhanced Roof-cover Adhesion: Cover boards can enhance the bond between the roof cover to the substrate.

    Enhanced Resistance to Wind Uplift: Cover boards and their ability to enhance the bond of the roof cover to the underlying substrate can result in an increased wind-uplift rating above and beyond that which can be provided with organic-faced insulations. They reduce the possible effects of facer-sheet delamination.

    Enhanced Fire Resistance: Many cover boards will enhance the fire resistance of the assembly.

    Hail Protection: Numerous studies show the value of cover boards in enhancing a roof cover’s ability to resist damage by hail.

    Provides Separation: A cover board provides separation between a roof cover and insulation that may not be compatible or the attachment adhesive of the roof membrane is not compatible with the insulation.

    Reduces Thermal Shorts (Energy Loss): Thermal insulation is often attached to the roof deck with mechanical fasteners, which results in conductive heat loss, up to 7 percent according to the Rosemont, Ill.-based National Roofing Contractors Association. This is a large value when some roof covers, which utilize mechanical attachment, purport to provide energy savings. Furthermore, when only one layer of insulation is used (a cardinal sin in my opinion) an additional 7 to 8 percent energy loss can occur. Placing a cover board above mechanically attached insulation and/or a single layer of insulation will enhance the energy performance of the roof system.

    Enhanced Roof-system Performance: I firmly believe the use of a roof cover board in a roof system improves the overall performance of the roof system and increases the probability of the roof attaining a long-term service life, which is the essence of sustainability. NRCA agrees; the organization recommends the use of cover boards in all low-slope assemblies.

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ECHOTape Repair Tape Now Sold by The Home Depot via HomeDepot.com

Pressure-sensitive tape supplier, ECHOtape’s full repair line will be sold online by a home improvement retailer, The Home Depot via HomeDepot.com. Launched in 2014 at the beginning of the third quarter, the repair line provides contractors with an alternative to duct tapes, and is designed to deliver solutions for repairs, sealing and waterproofing.

“We are very excited to be working with such a trusted name in home improvement like The Home Depot,” says Risa Edelstein, director of marketing for ECHOtape. “We dedicate our business to providing the ultimate tape solutions for a variety of applications and now contractors, remodelers, retrofitters and builders across the nation can purchase our performance-based repair tapes.”

ECHOtape’s comprehensive repair line is geared towards building contractors. In total, seven tapes are available now on HomeDepot.com. The products include three types of repair tapes with different color options:

  • All Purpose Repair Tape: This tape leaves little residue in comparison to a duct tape and is thick and flexible. Ideal for stretching and wrapping, this tape can be used for temporary repairs as well as for rips, tears, gashes and holes. This tape is available in clear and white.
  • All Weather Repair Tape: This tape is made with a butyl-based adhesive, which makes it sticky enough for applications to concrete, stone, wood, glass, metal, plastic, cement, plywood, and damp fabrics, and is ideal for sealing holes and cracks. It is puncture- and tear-resistant, waterproof, and will not crack in temperatures as low as -30 F or fail in temperatures as high as 200 F if applied correctly. The tape is available in white, silver and black.
  • All Leak Repair Tape: Also made with butyl-based adhesive, it is considered an extreme adhesive tape with double the stickiness of the All Weather Repair Tape. It shares many of the same qualities, including being waterproof, but is also resistant to corrosion. Because of its high level of adhesive, it can be used for repairing leaks in roof joints, skylights, RVs, pools and ponds. This tape is available in black and white.

“We are committed to making our products widely available to contractors in the U.S.,” says Edelstein. “This is an important step in increasing convenience for purchasers, and we look forward to continuing to expand our reach and product availability.”

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…]