It’s All about Design

There Are No Shortcuts When Designing Low-slope Cool Roof Systems in ASHRAE Climate Zones 3 Through 7

By Thomas W. Hutchinson, AIA, CSI, RRC, FRCI

Concerns for the environment in concert with legislative and code mandates have been the drivers for massive changes in the roofing industry. Consequently, the use of cool roof membranes, which are defined as roof covers with an initial solar reflectance of 0.65 or greater according to the EPA Energy Star Program, has become the preferred choice of many architects when designing low-slope roof systems.

As with any roof-cover material, the appropriate design and use of the material is required to achieve long-term success and a truly sustainable roof system. (See Photo 2 below.) Unfortunately, major proponents of cool roof systems have failed to inform and/or educate the design community about appropriate cool roof system design, which has resulted in condensation, resultant interior moisture penetration, mold and loss of thermal value. (See Photo 1.)

I believe roof-system design is equal in importance to structural, mechanical, plumbing and electrical design. Therefore, it is imperative that designers who utilize cool roof systems in ASHRAE Climate Zones 3 through 7 take extra care to achieve a properly functioning and sustainable low-slope roof system. (See Photo A.)

Challenges
Roof system design, in general, brings with it many challenges, including code compliance, construction sequencing, thermal-efficiency attainment, reduction of the building’s carbon footprint and achievement of long-term service life (defined as at least 30 years). Designing to use a cool roof membrane also includes the challenge of the unknown. Although the use of cool roof membranes has grown rapidly, they have not yet weathered the test of time, which often leads to unintended consequences.

Today’s multiple varieties of roof systems and components, code requirements and cool roofing concepts require that the architect/roof consultant or roofing contractor have full knowledge and understanding of the existing building structure and reroofing options. The designer also must investigate and analyze climatic conditions and restraints, impinging building components and systems, environmental factors, review of potential roof system solutions and construction sequencing to properly detail the project.

The design and ultimate installation of a roof system without giving consideration to the results of the investigational analysis, code review and the owner’s goals may lead to roof system selection that does not provide a long-term, sustainable design solution and results in premature failure.

Investigate and Analyze
Prior to the design of a cool roof system, a complete investigation of the climatic conditions and related building components and systems that affect the roof system must be undertaken. The intended roof and interior building use must be understood, and the roof system designer should also have solid knowledge of the HVAC system and its impacts on the roof system. The designer should understand these conditions and be designing to prevent potential problems.

For example, when designing a roof system over a high humidity space, a prudent designer would understand the potential for reaching dew point within the roof system exists and would incorporate a vapor retarder into the roof system.

The following components and related questions must be considered when designing new or replacement low-slope roof solutions:

Roof drains: What is their size, type and location? Are there sufficient roof drains to adequately and quickly remove water from the roof? Can additional drains be installed and/or moved to facilitate efficient tapered-insulation layouts?
Parapets and adjacent masonry walls: How is through-wall flashing integrated with the roofing? Do the parapet coping stones have through-wall flashings below? Will the roofing be integrated into the coping?
Gutters: If utilized, determine their size, expansion joint and downspout locations, and select their material type and gauge.
Roof deck: Metal, concrete or wood? Which is appropriate for the expected wind-uplift pressures, anticipated dead and live loads, and interior building conditions?
HVAC systems: The type and operation of HVAC systems, as well as their location in relationship to the roof is critical. Recent emphasis on “positive building pressure” has tremendous implications in regard to the roof systems. It is wise to provide roof details to the HVAC engineer for inclusion into his or her drawings/specifications.
Interior conditions: Determine the interior occupancy, climate conditions and space use to understand how they will affect the roof’s design and/or performance. Will increased rooftop energy efficiency affect the interior HVAC air balancing?
HVAC equipment, curbs, associated piping types and penetrations: Are there multiple pipes penetrating through a single location? Can any roof penetrations be located in a wall? How is the HVAC unit supported? Who is providing the curb? Are the curb heights correct for the intended tapered insulation? Are there any “hot” pipes? (See Photo 3.)
Building and roof expansion joints: Are the expansion joints properly located? Is the height proper? How will expansion joints terminate at walls and roof edges?
Penthouse walls and base flashings: Metal siding on penthouses often is low to the roof membrane. Because the new roof system may rise higher than the bottom of the siding, the siding support system must be identified.
Cooling towers: How are they supported? Will any associated piping penetrate the roof membrane? Is the piping grouped together?
Snow loads and dropping icicles: In ASHRAE Climate Zones 3 through 7, snow and ice are common occurrences that can negatively affect the roof system performance. (See Photo A.)
Roof access: What is the access to the roof from the building’s interior? Do scuttles or ladders provide access to the higher roofs?
Cables, conduit and pipes: Will IT cabling, tower conduit and refrigerant piping cross the roof? Are they properly supported? If the roof slope is greater than 2:12, will sliding snow affect the piping and supports? Can piping be brought up through roof curbs and separated from the roof?
Traffic: What will rooftop maintenance traffic patterns be? Will there be any skylights? If so, will traffic patterns be near them? How often will there be traffic? Is it foot or mechanical?
Air intakes: Will fumes during installation of the new roof system enter the building’s air circulation systems? Note: Low-VOC and water-based adhesives are available, but weather and construction conditions may affect their use. Moreover, their long-term performance has not been confirmed.
Roof use: Beyond water repelling, will the roof provide a working surface or be used by tenants/owners for recreational activities?
Geography/climatic conditions: What is the climate? Is the roof located in an urban or rural area? What types of wind can be expected? What substances will come in contact with the roof system from adjacent buildings or airplanes?
Building-construction-sequencing considerations: When will the roof be installed in relation to adjacent windows and/or metal panels? How are the roof areas laid out? Will any specific roof areas require priority over other areas, such as high roofs before low roofs?

Selecting Roof System Components
After the results of the climatic investigation and building-element and system-impact assessment, an analysis of the potential roof system components can be undertaken. This decision must be based on sound engineering principles so long-term roof system service life, which is the essence of a sustainable roof system, can be attained. This will place a burden upon architects to become more acquainted with roof-system design. Although manufacturers can provide a great deal of information about their products, it ultimately is within the architect’s “standard of care” to design a roof system appropriate for the specific building. Effectively, manufacturers produce, architects design and contractors install. Failure to understand and follow this truism can result in costly litigation, ruined reputations and unhappy clients.

Effectively, manufacturers produce, architects design and contractors install. Failure to understand and follow this truism can result in costly litigation, ruined reputations and unhappy clients.

When selecting components, a roof-system designer must consider the following:

Which cool roof system assembly should be specified: fully adhered, mechanically fastened or ballasted? Recent concerns with condensation in ASHRAE Climate Zones 3 through 7 (See Photo A.) with mechanically attached cool roof membranes suggest they are not an appropriate roof system for most buildings and the fully adhered system is the system of choice. [To read more about this, see RCI Interface, “Condensation Problems in Cool Roofs,” August 2009, page 11.]
The appropriate insulation type and method of attachment must be investigated. What are the owner’s energy-efficiency requirements? The required R-value should be accomplished in a minimum of two layers to prevent thermal shorts, energy loss and air infiltration to the underside of the membrane.
The dew point must be calculated to determine whether to use a vapor retarder. It is almost mandatory to include a vapor retarder in cool-roof assemblies in ASHRAE Climate Zones 3 through 7 (See Photo A.) to prevent the migration, pushing up and/or suction of warm moist air into the roof system where it will condensate below the membrane. (See Photo 4.)
The roof is part of the building envelope and should be considered the fifth façade after the walls. The interface of the walls and roof system must be understood. For example, is there an air barrier in the wall construction into which the new roof air or vapor retarder should be incorporated? How will the roof vapor barrier transition at roof edges, penetrations and roof drains and roof curbs?
All roofs require maintenance. When designing a roof replacement, an opportunity exists for minimizing the required maintenance. This means the installation of maintenance-free components, such as prefinished sheet metal, should be considered.
All roof base flashings at walls and curbs should be fully adhered to prevent air inflow and moisture accumulation, as well as damage caused by wind. (See Photo 6.)
Plan for future reroofing with copings and counter flashings that can be screw fastened and facilitate future removal and reinstallation.
Think about the time of year in which construction will occur and how climatic conditions will impact the roof installation. Using water-based adhesives during a winter installation is counterproductive in most of the continental U.S. Are the materials available during the time of the year when the roofing will be done?
What is the availability of local, knowledgeable installers? This may determine what type of roof system is installed.
What is the building’s expansion and contraction requirement across the roof? Are expansion joints required?
Do the roof-drain locations facilitate the layout of tapered insulation? If not, you may wish to relocate some drains. Consideration should also be given to the type of appurtenances required by the drains, including sump pans, extension rings and under-deck clamps. I typically provide the plumbing engineer roof-to-plumbing element details for inclusion on his or her drawings.
Consider how the disbursement of water at grade will affect the surrounding area during the various seasons. Do not install downspouts to sidewalks or driveway locations when icing may be an issue. The appropriate method of downspout support should be determined and proper downspout materials should be specified to protect against vandalism.
If gutters are utilized with roof slopes above ½:12 in areas that receive snow, be cognizant that snow and ice will slide off the cool roof membrane. In this design, snow fences at the eave may be required.
In snowy geographic locations, the use of low-profile gravel stops may be a concern because high winds can blow large sections of snow and ice off the roof.
Be aware of insulation types and densities. If the roof deck is structurally sloped, tapered insulation saddles should be designed; the slope of the taper must be twice the roof deck slope to direct water to the roof drains. If the structure is flat, consider using tapered insulation to remove water from the roof quickly. Foam insulations have a propensity to shrink, opening up joints and facilitating air movement. Stone-wool insulation is dimensionally stable and can result in tight joints. Additionally, consider the use of high-density coverboards to protect the thermal insulation integrity. The use of hydroscopic insulations or coverboards (that absorb water) is discouraged. (See Photo 5.)
Consideration should be given to the required sheet-metal details. I prefer the use of prefinished or natural metals, such as copper, instead of bare, galvanized materials.
Consider construction sequencing. Will heavy equipment be utilized on the roof to install other building components? If so, a temporary roof, such as a vapor retarder (See Photo 4.), should be designed so the roof proper can be installed after all other construction work is complete.
Review the “Tenets of Sustainable Low Slope Roofing” by the CIB/RILEM Joint Committee on Roofing Materials and Systems (CIB publication no. 271, July 2001).

Design
Once appropriate decisions to these considerations have been made, the architect or roof consultant may proceed to the design of the roof system. Beginning with the preliminary design and roof plan, the designer should lay out the new roof system and insulation systems, indicating drainage paths. Under no circumstances should a flat roof be designed; codes often do not permit them and good standard of care practices do not allow for this design. If the roof deck is not structurally sloped, tapered insulation should be incorporated into the design. In addition to ensuring water quickly leaves the roof, it provides for high levels of thermal insulation. All building components that impinge upon the roof should be located in the roof plan, given consideration and detailed accordingly. (See Figures 1 and 2.)

Once the overall roof plan has been conceived and thought has been given to other building components, the designer produces a final set of construction documents. This is the most critical phase of the design process. Attempts to specify a roof system without quality contract documents often results in roof systems that prematurely fail. The goal of the construction documents is to clearly communicate to the roofing contractor your design intent. Thirty-year roof system designs are not accomplished with generic details from a manufacturer’s Web site. They are created with carefully designed and detailed drawings and corresponding specifications.

All details should be drawn to the largest scale possible to provide lots of information. (See Figure 3.) Each detail should be thoroughly noted to leave no question about what is required. Once general details are complete, attention should be given to the details that will make or break your roof-replacement system design. They should include the following:

Elevation drawings of terminations, such as when a roof edge terminates into a masonry wall.
Counter flashing.
Intersection of gravel stops, copings and masonry as they abut adjacent structures.
Drainage details, such as roof edge scuppers-conductor box-downspout drainage systems. (See Figure 5.)
HVAC equipment penetration details. (See Figure 4.)
Tie-in details between two differing roof systems, etc.

The general thought in this phase of detailing is to do whatever is necessary to properly communicate to the roofing contractor what is required. This may require three or four isometrics in a series that build on each other to indicate exactly how a concern is to be handled. Details pertaining to roof replacement should indicate existing components to be removed, existing components to remain, and new roof systems and their attachment.

Sharing and coordinating roof drain, roof curb and other details with their system designers also is required. Work always proceeds more efficiently and with better quality when the roof curbs are the correct height, HVAC equipment and associated piping appropriately pass through pipe curbs, roof drains have the appropriate extension rings and both are correctly located.

Following a final review of the construction drawings, the specifications should be prepared specifically for the project in question. These specifications should relate the technical sections of all building components involved in the roof system construction, including roof drains, HVAC, curtainwalls, wood blocking, masonry, etc.

To arrive at a set of construction documents for bidding purposes, the architect/roof consultant designer has gone through a painstaking and comprehensive investigation of existing building components, roof system options, materials, causes and effects, as well as has been involved in a multitude of decisions and an evaluation of costs and results. I firmly believe the architect/roof consultant must continue to be involved during the bidding process to answer questions, issue addenda, hold pre-bid conferences and inform the roofing contractors of the specific requirements of the project.

I firmly believe the architect/roof consultant must continue to be involved during the bidding process to answer questions, issue addenda, hold pre-bid conferences and inform the roofing contractors of the specific requirements of the project.

Construction
Prior to the commencement of the roof installation, pre-construction meetings must be held and submittals verified. I recommend the roofing foreman and superintendents of trades affecting the roof, such as plumbing, mechanical, carpentry, masonry and glazing, attend so roof intersection and coordination can be discussed.

Site visits will verify compliance with the contract documents and allow for the answering of questions and review of unknown conditions. Continued communication between the roof system designer and the contractors during all phases of construction is invaluable to the success of the roof system and achievement of a sustainable long-term roof system.

Knowledge Leads to Success
The successful completion of any roof system design with a sustainability goal is not easy. It involves investigation, analysis, design, decision making, continued communication and problem solving, as well as construction observation. There are no shortcuts or inexpensive long-term solutions. A well thought-out systematic procedure will lead to successes. Owners will be happy with results, the built environment will be enhanced, and the roof designer will obtain the self satisfaction of knowing that he or she has been involved in one of the most important comprehensive construction processes that a building owner can undertake.

Thomas W. Hutchinson is principal of Hutchinson Design Group, Barrington, Ill.

Photo 1: Inappropriate roof system design when utilizing cool roof membranes can result in unintended results, such as condensation, mold growth and interior moisture penetration.
Photo 2: Appropriate roof system design utilizing cool roof membranes can result in roof systems whose potential to achieve sustainable results is high.
Photo 3: All HVAC-related items, such as flues and curbs, need to be designed into the roof system.
Photo 4: It is recommended that roof system designers thoroughly investigate and determine whether a vapor retarder (right) is required to ensure cool roof system performance. It has been this author’s experience that in ASHRAE Climate Zones 3 through 7 a vapor retarder will enhance the long-term performance of the cool roof system.
Photo 5: The use of high-density coverboards to protect the thermal insulation integrity and enhance the boundary of the cool roof membrane to the roof substrate is recommended.
Photo 6: Fully adhering roof base flashing to roof curbs and walls, such as this parapet wall, prevent the inflow of air behind the cool roof membrane and potential for condensation.
Figure 1: Roof plans should indicate drainage paths, tapered insulation slopes and detail tags for all conditions, including all rooftop equipment, locations of walkways, accesses, ladders, mechanical equipment and penetration. Roof plans also always should include dimensions, scale and a directional arrow.
Figure 2: Roof plans should indicate drainage paths, tapered insulation slopes and detail tags for all conditions, including all rooftop equipment, locations of walkways, accesses, ladders, mechanical equipment and penetration. Roof plans also always should include dimensions, scale and a directional arrow.
Figure 3: Roof details should be drawn for all conditions at the largest possible scale to communicate clearly as much detail as possible. Note the vapor retarder extends to the precast and is turned up, creating a seal.
Figure 4: Roof penetrations often are a concern with cool roof systems and should be designed and detailed to act independently from the roof system. Note the inside of the curb is required to be insulated and sealed to prevent the formation of condensation.
Figure 5: Drainage components, such as this roof edge scupper-conductor box-downspout, need to be thoroughly detailed to ensure coordination of trades and performance, as well as prevent ponding water in front of the scupper.
Photo A: ASHRAE Climate Zones

Recommended Further Reading
Building Design & Construction, “Repair, re-cover or replace?”
September 1988, pg. 82-85.

Exteriors, “Sorting out complexity of options in reroofing.”
Autumn 1988, pg. 24-29.

Hutchinson, T.W. 2007 "Keys to Sustainable Roofing," Proc. of RoofTech 2007, Calgary, Canada, 4-6 June, 2007.

Hutchinson, T.W. 2004 "Sustainable Low-Slope Roof Systems Designed for the Long-Term – Getting it Right the First Time," Proc. of the 2004 CIB World Building Congress, Toronto, 1-7 May, 2004.

Professional Roofing, “Reroofing: An opportunity for improvement.”
September 1988, pg. 15-17.

RCI Interface, “Condensation Problems in Cool Roofs”
August 2009, pgs. 11-16.

Roof Design, “A conservative approach to reroofing,”
September/October 1985, pg. 24-33, 51.

“Towards Sustainable Roofing”, CIB W083 / RILEM 166-MRS Joint Committee on Roofing Materials and Systems; CIB Publication No. 271; July 2001

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It’s All about Design

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