IBC and FM—What's the Difference When it Comes to Wind Design?

A common question being asked in the roofing industry is whether or not the 2016 version of ASCE 7 is going to increase the design wind pressures acting on a building. The answer is "yes" in many cases. So, the follow up question is "by how much?" And, that leads to the next question, "how much more capacity will roof systems be required to have when wind design follows ASCE 7-16?"This blog looks at 2 buildings of different sizes and heights in two cities in the U.S., one centrally located and one along the Gulf coast, and compares design wind pressures based on ASCE 7-10 and ASCE 7-16. By assessing the required roof-system capacity for all roof zones in conjunction with the increases in the size of roof zones, we can begin to put some real numbers to the question "by how much will ASCE 7-16 affect the low-slope roofing industry?"While analysis of two building types in two cities doesn't represent a significant study, this study offers an example of how the 2016 version of ASCE 7 is going to affect installed roofing systems over the next decade.Case StudiesDesign wind pressures (DWP) were determined for two building types in two cities. DWPs were based on varying the Risk Category and Exposure. The following table shows the building types, cities, variables, and constants.The big box store is 290' long x 169' wide x 24' tall, and the apartment building is 100' long x 40' wide x 55' tall. Wind speeds were determined using an online tool.DWPs can be determined by using proprietary third party methods (e.g., RoofNav), simplified calculation tools (e.g., RoofWindDesigner), or hand calculations following the methods within the ASCE 7 standard.For this case study, DWPs were determined using hand calculations via an Excel spreadsheet. The DWPs are slightly more refined since there was no rounding of wind speeds for ASCE 7-16. Both RoofNav and Roof Wind Designer round to the nearest 5 or 10 mph. The maps in ASCE 7-16 have been revised, not only from a wind speed perspective but from the number of wind isobars on the maps. The ASCE 7-16 maps are more refined which allows for more exact wind speed values.ASCE 7-10 Map, Risk Category II, showing a limited number of wind-speed isobars.ASCE 7-16 Map, Risk Category II, showing many additional wind-speed isobars relative to the ASCE 7-10 wind map shown in the above figure.In total, 72 different sets of DWPs were calculated. Buildings over 60' tall are designed the same in ASCE 7-16 as in ASCE 7-10; therefore, this article focuses on buildings less than 60' tall.Design Wind Pressure ResultsThe following charts provide the DWPs for each of the scenarios that were analyzed for this article.Design wind pressures based on ASCE 7-10 and ASCE 7-16 for a big box store in Kansas City, MO that is 24' tall.Design wind pressures based on ASCE 7-10 and ASCE 7-16 for a big box store in Mobile, AL that is 24' tall.Design wind pressures based on ASCE 7-10 and ASCE 7-16 for a 5-story apartment building in Kansas City, MO that is 55' tall.Design wind pressures based on ASCE 7-10 and ASCE 7-16 for a 5-story apartment building in Mobile, AL that is 55' tall.Comparing the 2010 and 2016 versionsThe primary intent of this article is to start generating data to help answer the question "how much did the DWP increase from the 2010 version to the 2016 version of ASCE 7?" Using the DWP calculated for each roof zone within each of the four scenarios, we calculated the percent increase in DWP per zone. The percent increases in DWP were calculated by dividing the ASCE 7-16 value with the corresponding ASCE 7-10 value for each roof zone. However, because there are 4 roof zones in ASCE 7-16 (and 3 roof zones in ASCE 7-10), both Roof Zones 1' and 1 were divided by the value for ASCE 7-10 Roof Zone 1 to determine the percent increase. Values in the following charts that are below 100% (shaded gray) indicate a reduction in DWP from ASCE 7-10 to 7-16.Percent increases in DWP per roof zone for a big box store in Kansas City, MO that is 24' tall, focusing on Exposure C values.Percent increases in DWP per roof zone for a big box store in Mobile, AL that is 24' tall, focusing on Exposure C values.Percent increases in DWP per roof zone for a 5-story apartment building in Kansas City, MO that is 55' tall, focusing on Exposure C values.Percent increases in DWP per roof zone for a 5-story apartment building in Mobile, AL that is 55' tall, focusing on Exposure C values.For the big box store scenarios, the percent increases in DWP per roof zone varies from 84% to 178%. The percent reductions (values less than 100%) are all in Roof Zone 1' while Roof Zones 1, 2, and 3 have increased DWP. As seen in the Figures, Roof Zone 1 percent increases are greatest while Roof Zone 3 increases are smallest.For the 5-story apartment building scenarios, the percent increases in DWP per roof zone varies from 84% to 177%. The percent increases in Roof Zone 1 are the largest. The percent reductions are all Roof Zone 1'.It is worth noting that the increases in Pressure Coefficients from ASCE 7-10 to 7-16 are, in fact, relatively indicative of the increases in DWP. For more on Pressure Coefficients and the wind design process.The case studies in this blog also show that the 2016 version of ASCE 7 imposes higher DWPs in Roof Zones 1, 2, and 3. However, given some of the substantial reductions in Roof Zone 1', it could be expected that installed roof system capacity may be lower. While that is hopeful, the roofing industry has a minimum-capacity backstop of 60 psf. This means that any calculated DWP at or below 60.0 psf (even for DWP values as low as 23 psf) will have a 60-psf-capacity roof assembly installed to meet building code requirements. The reality is many buildings are required to use a 60-psf-capacity roof system when in fact '60 psf' is more capacity than needed based on calculated DWPs.Let's take a closer look at one subset for each of the 4 scenarios.Example 1: Big Box; Central USThe big box store in KC, MO (Risk Category II, Exposure C) saw the DWP increase from 38 psf to 56 psf in Roof Zone 1 per ASCE 7-10 vs ASCE 7-16; and reduce to 32 psf in Roof Zone 1'. Regardless of the increases or decreases, a 60-psf-capacity roof should be installed in both Roof Zone 1 and Roof Zone 1' per 7-16, which is unchanged relative to ASCE 7-10.Roof zones and pressures for a 24' tall big box store in Kansas City, MO based on ASCE 7-10 using Risk Category II and Exposure C.Roof zones and pressures for a 24' tall big box store in Kansas City, MO based on ASCE 7-16 using Risk Category II and Exposure C.For Roof Zones 2 and 3, the DWPs are not only increased, but the sizes of the roof zones that require higher capacity are also increased. Roof Zone 2 per ASCE 7-10 required a 75-psf-capacity roof, and a 75-psf-capacity roof will be needed per ASCE 7-16. However, approximately 9% of the roof area will need a higher capacity because of the increase in roof zone size. The following chart shows roof area percentages based on the required roof system capacity for this scenario.Chart showing required roof capacity and the roof area (%) where each is required.Example 2: Big Box; Gulf CoastA big box store in Mobile, AL (Risk Category II, Exposure C) saw the DWP increase from 69 psf to 109 psf in Roof Zone 1 per ASCE 7-10 vs ASCE 7-16; and reduce to 63 psf in Roof Zone 1'. A 75-psf-capacity roof should be installed in Roof Zone 1', and higher-capacity roofs will be required in Roof Zones 1, 2, and 3 relative to ASCE 7-10.Roof zones and pressures for a 24' tall big box store in Mobile, AL based on ASCE 7-10 using Risk Category II and Exposure C.Roof zones and pressures for a 24' tall big box store in Mobile, AL based on ASCE 7-16 using Risk Category II and Exposure C.For Roof Zones 1, 2, and 3, the DWPs are not only increased, but the size of the roof zones that require higher capacity are also increased. A larger perimeter roof zone (combining Roof Zones 1 and 2) with higher required capacity is created (relative to 7-10). Approximately 30+% of the roof area will need a higher capacity. The following chart shows roof area percentages based on the required roof system capacity for this scenario.Chart showing required roof capacity and the roof area (%) where each is required.Example 3: 5-story; Central USThe 5-story apartment building in KC, MO (Risk Category II, Exposure C) saw the DWP increase from 45 psf to 66 psf in Roof Zone 1 per ASCE 7-10 vs ASCE 7-16; and reduce to 38 psf in Roof Zone 1'.However, because the method used to determine the sizes of roof zones in ASCE 7-16 is changed, Roof Zones 1' and 1 do not exist on this building! Decreases of DWP in Roof Zone 1' are rendered meaningless, and the lack of a Roof Zones 1' and 1 means the entire roof consists of only Roof Zones 2 and 3, the perimeter and corner roof zones.Roof zones and pressures for a 55' tall apartment building in Kansas City, MO based on ASCE 7-16 (top) and ASCE 7-10 (bottom) using Risk Cat II and Exposure C.The increases in DWPs for the roof zones significantly increase the roof system capacity requirements for this scenario. Almost the entire roof area will need a higher capacity roof. The following chart shows roof area percentages based on the required roof system capacity for this scenario.Chart showing required roof capacity and the roof area (%) where each is required.Example 4: 5-story; Gulf CoastThe 5-story apartment building in Mobile, AL (Risk Category II, Exposure C) saw the DWP increase from 81 psf to 129 psf in Roof Zone 1 per ASCE 7-10 vs ASCE 7-16; and reduce to 74 psf in Roof Zone 1'. Similar to Example 3, due to the size of the roof zones relative to the size of the roof, Roof Zones 1' and 1 do not exist in this scenario.Roof zones and pressures for a 55' tall apartment building in Mobile, AL based on ASCE 7-16 (top) and ASCE 7-10 (bottom) using Risk Cat II and Exposure C.The increases in DWP for the roof zones significantly increase the roof system capacity requirements for this scenario. The entire roof area will need a higher capacity. The following chart shows roof area percentages based on the required roof system capacity for this scenario.Performance versus PrescriptiveThe case studies in this analysis are presented using a performance method for wind design. That is, a specific DWP is determined for each roof zone and a roof system with appropriate capacity is selected for use in the associated roof zone. Another method that is often used in the roofing industry is a prescriptive method for wind design. The DWP for Roof Zone 1 (or Roof Zone 1' in certain localities) is used to select a roof system with an appropriate capacity for Roof Zone 1 (similar to a performance method), and then Roof Zones 2 and 3 use an installation method/layout that is prescriptively increased relative to Roof Zone 1 installation methods. Prescriptive installation methods for Roof Zones 2 and 3 are regularly used with mechanically attached (MA) roof systems, and for MA roofs, Roof Zone 2 increases are commonly 50% and Roof Zone 3 increases are commonly 100%.ConclusionASCE 7-16 DWPs are generally increasing relative to ASCE 7-10, but this impact is better understood by comparing the required roof system capacity for different building types and dimensions in different cities. Looking only at DWPs does not provide enough information; a comparison of required roof system capacities is needed. Not only did the DWPs change, but the methodology for determining the size of the perimeter and corner roof zones changed significantly in the new ASCE 7-16 version; the amount of roof area contained by the perimeter and corner roof zones increased. The case studies—of two buildings less than 60 ft tall in two cities—and corresponding analysis in this blog are just one additional small step to understanding how the ASCE 7-16 will affect wind design over the next decade.

In part 1, Retrofit Single Ply Roof Systems: An Assessment of Wind Resistance, we provided information about the following: Four (4) methods to re-cover a metal panel roof The many options for attaching a single-ply system to a metal panel roof An example calculation for wind uplift design pressures and appropriate fastener patterns that provide the necessary resistance capacity Industry concerns about wind uplift when not attaching the retrofit single-ply system into every purlin In this blog, we will discuss and analyze the four full-scale physical tests that were performed to determine their wind-uplift capacity. Physical Testing There have been no publically available validation studies or data supporting any particular approach to the installation of retrofit single-ply roof systems (RSPRS). Non-validated attachment methods could result in failures during wind events. Therefore, the objective of GAF's physical testing program was to determine the wind-uplift resistance of RSPRS fastened directly into purlins. A variety of fastening patterns and fastener densities were tested in order to provide a better understanding of the effect of wind loads on these systems. The physical testing was performed at the Civil, Architectural and Environmental Engineering Department of the Missouri University of Science and Technology in Rolla, MO (MS&T). Test Roof Construction Four full-scale test roofs were constructed and tested in a 10 ft. wide x 20 ft. long chamber. The test roofs were installed by Missouri Builder Services, Jefferson City, MO, with oversight by GAF. After the test roofs were constructed, the MS&T research team instrumented the assemblies for data collection. The four test roofs consisted of 24 in. wide, 24-gauge structural metal roof panels attached to 16-gauge Z-purlins with concealed expansion clips and purlin screws. The purlins were connected to and supported by horizontal steel channels; the purlin/channel construction was supported by four vertical steel columns. To complete the test specimen, flute fill polyisocyanurate insulation, flat stock polyisocyanurate insulation, and a mechanically attached 60 mil TPO membrane were installed. Prior to membrane installation, the insulation was mechanically attached with minimal fasteners to prevent shifting during the testing. The cross-section shows a graphical representation of the completed RSPRS over the structural metal panel roof system. For Tests #1, #2, and #3, purlin fasteners and 2 3/8 in. barbed fastener plates were used to secure the membrane, simulating a "strapped" installation. The fasteners and plates were not stripped in. For Test #4, purlin fasteners and 3 in. specially-coated induction weld fastener plates were used. The purlins in all tests were attached to C-channels. This did not allow for data collection at a purlin-to-mainframe connection. When an RSPRS is mechanically attached to every other purlin, the load path is altered significantly. This raises a question about the effect on the wind-uplift capacity of the existing metal building when the load path is altered. More information on that topic can be found here. Therefore, it is recommended to engage a structural engineer when altering the load path of an existing structure. Results and Discussion The table shows the ultimate loads achieved, tributary area and load per fastener, as well as fastening method. The term "ultimate load" refers to the point of failure of the roof system during physical testing. Test #1 The fastening pattern for Test #1 was 5 ft. o.c. fastener rows and 12 in. fastener spacing within the row. Test #1 failed when the membrane ruptured simultaneously at seven fastener locations in the center purlin. The system successfully completed 160.1 psf and then failed as the pressure was being increased to 174.5 psf. The membrane pulled over the five center fastener plate locations in an essentially circular pattern along the outer edges of the fastener plates. The outer two failure locations resulted in L-shaped tearing of the membrane, which was attributed to the boundary conditions of the test chamber. The fastener plates were deformed upward. There were many locations of permanent upward membrane deformation. Photo of the outcome of Test #1. The permanent upward membrane deformation was evident along the edges of the rows of fasteners, as can be seen in the upper row of fasteners in the photo. There was very little permanent upward membrane deformation at the centerline between fasteners within a row. This pattern of deformation leads to the belief that the load within the membrane is being transferred from fastener row to fastener row, and not significantly from fastener to fastener within a row. Test #2 The fastening pattern for Test #2 was 5 ft. o.c. fastener rows and 24 in. fastener spacing within the row. Test #2 failed when the membrane ruptured simultaneously at the three central fastener locations in the northern quarter-point row of fasteners. The system successfully completed 116.9 psf and then failed as the pressure was being increased to 124.1 psf. The membrane pulled over the three center fastener plate locations within the row. The center rupture was circular at the fastener plate. The outer two ruptures were "D" shaped; the straight-line edges were attributed to the boundary conditions of the test chamber. Photo of the membrane rupture at the center fastener plate location for Test #2. The tributary area for each fastener for Test #2 was double that of Test #1. This led to the hypothesis that the ultimate load for Test #2 would be one-half of that from Test #1, or 81.4 psf. However, the ultimate load was 119.5 psf, which is approximately 73% of that from Test #1. This is believed to indicate that the membrane transitioned from one-way loading to a more efficient two-way loading. The load was not only distributed across the 5 ft. purlin-to-purlin span (as was the case in Test #1), but was also distributed between fasteners within a row. The uplift loads were pulling on the fastener and fastener plates from all sides (two-way loading) instead of just two sides (one-way loading). During the test, the membrane deflected up approximately 4 in. between fasteners within a row. The loads were more equally distributed within the membrane and around the fastener plate, and therefore, the load per fastener increased from 813.5 lbs. (Test #1) to 1195 lbs. (Test #2). The membrane resisted the uplift loads in two generalized directions: between fastener rows and between fasteners within a row, which aligns with the machine direction (MD) and cross-machine direction (XMD) reinforcement yarns within the membrane, respectively. The membrane had permanent upward deformation between rows and between fasteners within a row because of this two-directional loading. The permanent upward membrane deformation was circular around fasteners. Test #3 The fastening pattern for Test #3 was 5 ft. o.c. fastener rows and 36 in. fastener spacing within the row; fasteners were staggered row to row. Test #3 failed when the membrane ruptured at a single fastener location in the southern quarter-point row of fasteners. The system successfully completed 59.3 psf and then failed as the pressure was being increased to 66.5 psf. The membrane pulled over the center-most fastener plate within the row (at the red circle). The photo shows a close up of the failure location for Test #3. The failure was "D" shaped, similar to failure locations in Test #2. The flat edge was on the boundary edge of the test roofs; the rounded edge is towards the center of the test roof. Similar to Test #2, there was circular upward permanent membrane deformation at fastener locations for Test #3 as shown in Figure 10. This shows that the membrane is being loaded in the MD and XMD. This is due to the relatively wide spacing of the fasteners (2 ft. and 3 ft.) relative to Test #1, which had 1 ft. spacing of fasteners within a row. The tributary area for each fastener for Test #3 was 50% greater than Test #2. This led to the hypothesis that the ultimate load would be 2/3 of Test #2, or about 79.7 psf. However, the ultimate load was 61.9 psf which is approximately 52% of that from Test #2. Comparing Test #3 to Test #1, traditional assumptions based on tributary area would lead to an expected ultimate load for Test #3 to be 1/3 of Test #1. The ultimate load from Test #1 was 162.7 psf, so the expected ultimate load for Test #3 was 54.2 psf. The actual ultimate load for Test #3 was 61.9 psf which is approximately 38% of that from Test #1. While two-direction membrane loading appears to increase the expected ultimate load of a roof system relative to the traditional linear expectation of failure load, it appears there is a limit to this increase. For this series of tests, the limit seems to be 5 ft. o.c. for fastener rows with 24 in. fastener spacing within each row. Test #4 The fastening pattern for Test #4 was 5 ft. o.c. fastener rows and 24 in. fastener spacing within the row; fasteners were staggered row to row and induction welded. Test #4 failed in two locations—a fastener plate pulled over the fastener head and the membrane separated at the reinforcement layer at the adjacent welded fastener plate. The system successfully completed 59.3 psf and then failed as the pressure was being increased to 66.5 psf. The failures occurred in the southern quarter-point row of fasteners. The photo shows the 2 failure locations for Test #4. Test #4 used induction welded fasteners, which means the fastener plates were under the membrane. Therefore, the membrane was cut in order to evaluate each failure. Based on audible observation at the time of failure, the two failures occurred "simultaneously." It was difficult to determine from visual examination which occurred first: the fastener plate pulling over the fastener head or the delamination of the membrane at the fastener plate. Test #4 and Test #2 have the same tributary area per fastener location—10 square feet. However, Test #2 achieved a 119.5 psf ultimate load and Test #4 achieved a 64.8 psf ultimate load. All components were identical for both test roofs except for the fastener/plate combination and that Test #4's fasteners were staggered row-to-row. The above-membrane fastener (e.g., an in-seam fastener) is 2 3/8 in. in diameter. An induction welded fastener plate is 3 in. in diameter and is constructed such that a raised 'ring' surface adheres to the membrane, not the entire fastener plate. The area of a standard 2 3/8 in. above-membrane fastener plate is approximately 4.4 square inches. The area of the attachment surface for an induction welded fastener plate is approximately 3.3 square inches. Therefore, an induction welded fastener plate has approximately 75% of the surface area of a traditional mechanically attached fastener plate to restrain the membrane. Individual fastener load for Test #2 (with the same tributary area as Test # 4) was 1195 lbs. Direct extrapolation to the induction welded fastener plate (at 75%) leads to the predicted value of the fastener load for Test #4 to be 896 lbs. This prediction assumes the reinforcement is the weak link, but the test clearly shows the cap-to-core connection to be the weak link, and therefore, it makes sense that the failure load per fastener for Test #4 was less than 896 lbs. In fact, it was 648 lbs per fastener. The analysis of these two different types of fastening methods and failure modes supports the result that Test #4 has lower wind uplift resistance than Test #2 even though the tributary area for each fastener is the same for Tests #2 and #4. Conclusions and Recommendations Review and analysis of the four full-scale physical tests of retrofit single-ply roof systems installed over structural metal panel roof systems resulted in a number of conclusions. They are as follows: Uplift resistance of RSPRS and individual fastener loads in an RSPRS are based on the membrane's reinforcement strength and one-directional versus two-directional loading of reinforcement. Reducing the overall fastener density increases the tributary area for each fastener. As expected, the ultimate load is reduced with larger tributary areas. Two-directional membrane loading increases the expected ultimate load of a roof system relative to linear extrapolation based on fastener tributary area. However, it appears there is a limit to this expected increase. For this series of tests, the ultimate load exceeded expectations for the Test #2 fastening pattern, but the ultimate load was more in line with traditional linearly extrapolated expectations for the Test #3 fastening pattern. This work emphasizes the limitations of extrapolation and validates the use of physical testing to determine uplift resistance of roof systems. Permanent deformation of the membrane was observed in all four physical tests at the end of testing and was not seen to be a water-tightness issue. The test procedure performed did not determine what pressure during the test cycling the membrane deformation began. This observation may provide an explanation for "wrinkles" observed in mechanically attached membranes that have experienced high wind events. For additional information about this topic, here is the GAF paper that was presented at the 2020 IIBEC Convention and Trade Show, and here is a webinar presented in early 2020.

In the first Value Engineering post about the attributes of high performing roof designs, value engineering is defined as "a concept that states there are less expensive ways to get equivalent performance," and the post described the performance attributes that make for a long-lasting, high-performing roof system. These performance attributes include: imagesEnergy efficiency (reflectivity, thermal resistance, and air leakage)Impact resistanceWind resistanceCondensation preventionHigh heat (and UV) resistancePositive drainageWear resistanceThese performance attributes manifest themselves in certain aspects of the overall design of the roof system. What type of membrane, membrane thickness, cover board or no cover board, adhered or mechanically attached cover board, insulation layout (including tapered), insulation attachment method(s), vapor retarder / air barrier or not, and attachment to the roof deck are some of the questions that drive the performance of a roof system.And the roof design is…After considering all of the above performance attributes, your design strategies for a long-lasting, high performing, and durable roof include:Adhered reflective roof membrane with high heat-resistanceAdhered HD polyiso coverboard2+ layers of adhered insulation, staggered and offset, with tapered insulation and crickets and saddlesAir barrier (over a substrate board) at the deck levelThird-party-tested edge metal detailsWalkway padsEnergy Efficiency – reflectivity and thermal resistanceThe attachment method is integral to a roof system's energy efficiency. Consider, for example, a roof that is designed to have the first layer of insulation fastened to the deck and the remainder of the components above are to be adhered. There is a cost for fasteners as well as adhesives. One way to "value engineer" this roof is to use fasteners through the entire system thereby removing the cost of the adhesive and labor to install the adhesive. While eliminating the adhesive is a cost reduction strategy, it is possible that longer fasteners will be needed. Sometimes something that is eliminated creates the need for a greater cost elsewhere. This is often not mentioned or discussed, so ask the question!Value engineering the removal of the adhesive also, unfortunately, can result in a roof system with a 15 to 30% reduction in R-Value relative to the as-designed roof system. If the roof was designed to have an R-30, the actual R-value as installed would be about R-21 to R-25! It's been shown that over the life of a roof, roof systems that use adhered membranes and an adhered top layer of insulation can offset the cost of adhesives when the initial cost of 'lost' insulation performance and the additional annual cost for increased heating and cooling are factored into the analysis.One of the key issues with value engineering of a roof system is that proponents of the reduction of costs are generally only looking at the initial cost to install a roof. However, the operational costs, the energy efficiency, the wind resistance, are commonly compromised. Operational costs, which are tied directly to a roof system's energy efficiency, are often much higher over the life of a roof. Also, roof systems with different wind-resistance ratings may need to be selected after value engineering revises the attachment method.Also, is the mechanical system design and whole-building energy-use analysis predicated on a roof system R-value of 30? Determining the effect on mechanical unit sizing and cost, including differences in whole building energy use may show that the savings from value engineering the roof will have a negative effect on annualized energy use and the ability of the HVAC system to maintain occupant comfort. Whole building energy simulation modeling, such as EnergyPlus™ (EnergyPlus.net), can be used. EnergyPlus™ is funded by the U.S. Department of Energy's Building Technologies Office, and managed by the National Renewable Energy Laboratory.Each "dot" is a fastener and plate; fasteners and plates are thermal bridges that reduce the R-value of the insulation layer.Impact ResistanceUsing any type of coverboard will improve the impact-resistance of a roof system because of the toughness of coverboards in general. However, a key issue with impact resistance is the location of the plate and fastener relative to the roof membrane. If a coverboard is mechanically attached (so the plates and fastener head are immediately beneath the membrane), impacts at fastener heads and plates have been shown to result in damage to the membrane. Adhered coverboards remove this concern, but do require the use of some type of adhesive. It's not just the use of a coverboard, but the installation method that matters.Ice-ball impact above fasteners always punctures the membrane. Left to right: increasing damage from hairline crack to complete puncture.Impacts come from hail as well as rooftop use by occupants and anyone performing work on rooftop mechanical units or to rising walls, for example. Reducing membrane thickness, changing from a fleece-back membrane to a smooth-back membrane, as well as changing to a more commodity-type membrane versus one that is shown to have a long service life are value engineering choices that can reduce cost, yet also reduce the impact resistance.Wind ResistanceDesigners make wind-design decisions that are intended to reduce risk (e.g., choosing Partially Enclosed versus Enclosed, selecting Exposure Category C instead of Exposure Category B). If a designer and owner have determined to reduce the potential risk of high-wind-event damage by increasing the design wind loads and subsequently the capacity of a roof system, that high capacity could be value engineered out. Coordinating wind design assumptions and documenting owner expectations and decisions can be critical pieces of information that alleviate the reduction of wind load capacity based on cost. Owners have their reasons; value engineering should not undermine owner expectations. By including design wind pressures for each roof zone as well as providing design loads for edge metal, and parapets and coping, minimum performance requirements are established in the construction documents.For more on wind design, read our blog here.Condensation preventionRoofs that include air barriers above the deck and below insulation provide the best protection against the development of condensation in a roof system. Your roof design took into consideration the interior and exterior design temperatures and calculations were performed to determine potential dew point locations, which resulted in the correct placement of the air barrier.This roof design is "above code." While the energy code allows the roof membrane to be the air barrier, air can still move up into the roof system. This is called air intrusion and can bring moisture into the roof system, potentially causing condensation issues.Placing the air barrier layer on the top surface of a rigid board fastened to the roof deck is one option. However, with "first layer fastened and the upper layers (insulation, cover board, membrane) adhered" systems, there is an opportunity to include a vapor retarder / air barrier between the insulation layers without fasteners penetrating the vapor / air barrier layer. Reducing air intrusion and the potential for condensation is achieved using both designs; this is a real value engineering opportunity! The costs for materials and labor for each design can be determined, and the least expensive design that provides condensation control can be implemented, instead of eliminating the properly located air barrier.While removing the air barrier and the associated adhesives and components reduces costs, changing to a roof system that uses only mechanical attachment also negatively affects the potential for condensation and thermal performance of the roof system. For example, consider a high-humidity building, such as a natatorium—the use of an air barrier at the deck level reduces air intrusion into the roof system from the interior and the moisture that air carries. This reduces the potential for condensation and damage to the roof system.High Heat (and UV) ResistanceHeat and UV are two environmental factors that affect roof membrane service life. Understanding this and that storms are increasing in severity and frequency, in addition to recognizing temperatures are rising (ASHRAE's climate zone maps are moving northward), your roof design includes a membrane that has been shown to have a long service life. It's likely the specification includes GAF's TPO Everguard Extreme® membrane due to its ability to withstand high heat conditions.It's possible that a high-performing roof membrane will be value engineered down to one of reduced initial cost. The rationale will be that it still keeps water out! But to what long-term cost? You designed a roof to meet your client's expectations for a long service life. By considering annualized costs, not simply initial cost, it may be shown that a longer service life roof is actually a better long-term value for the owner. A key strategy is to use annualized costs when pushing back against value engineering.Positive DrainageMost roof systems are not guaranteed or warranted against ponding water. The combination of UV and water is one mechanism that advances the deterioration of many roof membrane materials. And because water needs to be removed from a roof quickly and efficiently, your roof design includes properly sized drains and scuppers, a tapered insulation layout to efficiently move water to drains and scuppers, and crickets and saddles to prevent localized ponding.Example of a tapered layout, courtesy GAF Tapered Design GroupWhen asked to reduce the cost of the tapered insulation, one possible answer is to install more drains that are closer together. This can reduce the build-up of tapered insulation, and the associated cost. Perhaps the use of lightweight insulating concrete is less costly? Perhaps the use of a TPO membrane, which does not have ponding water limitations in the guarantee/warranty, makes sense?Wear ResistanceThicker membranes, granulated surfaces, and walkway pads are helpful in preventing unwanted wear to roofing membranes. You've coordinated expectations with the owner, your client, that rooftop protection against wear and tear is important during the operation phase of the building. Reducing the thickness of a membrane reduces its potential wear resistance, as well as its impact resistance (as previously noted in this blog). As with many things, none of these issues are stand-alone issues.Perform a cost comparison of a granule-surface modified bitumen membrane and a smooth surface system with a high-solids coating system to see if there are less expensive systems that still meet wear-resistance requirements. Don't just look at initial costs; consider maintenance costs over the life of the roof that affect annualized costs. A relatively small number of walkway pads, strategically placed around rooftop units and systems that will require regular maintenance (e.g., HVAC units and solar arrays), can be an effective and economic solution.Product substitutionsNot all products with the same intended function (and described with the same terminology) are created equal. A vapor retarder is a good example. A self-adhered vapor retarder with a perm rating of 0.03 has a desired performance and was included for a specific reason – to effectively block the diffusion of moisture into the roof assembly. Substituting a less expensive vapor retarder with different properties (e.g., a higher perm rating) and physical characteristics (30 mil asphaltic vs. a 6 mil poly), won't be installed in the same fashion—even though they are both vapor retarders. The least expensive material may be the most costly to install properly.DetailsDetails matter. Without a doubt, the ability for a roof to keep water out is most challenging at the details—penetrations, perimeters, and locations where there is an interruption, end, or change of direction. You designed a two-part counterflashing for ease of maintenance and future reroofing; a single-piece counterflashing is less expensive initially. When it's time to reroof, will the facade material (e.g., masonry, stucco, EIFS) need to be removed and repaired in order to install new counterflashing? Again, you've established with the owner that future efforts for maintenance and reroofing are important.There are ways to value engineer roofing details, especially when it comes to air barrier and vapor retarder constructability. A good example is an overhang. It seems logical to design an air barrier to enclose the overhang (the blue lines in the graphic), but there are many potential dis-continuities (the red lines in the graphic). The air barrier can be inboard of the eave; the wall air barrier could be tied to the underside of the roof deck, and the above deck air barrier (whether at deck level or the roof membrane) is tied to the top of the roof deck making a continuous air barrier system. There likely are labor savings associated with this value-engineered design revision.WeatherA change of seasons is often the impetus for changing from an adhered system to one that strictly uses mechanical attachment. While this may not be value engineering, a change of seasons can mean daily temperatures are below manufacturer recommended minimums for adhesive materials. However, manufacturers have (and are developing) lower temperature adhesives. Regardless of why a roof system design eliminates adhesives in favor of fasteners, there are energy efficiency and an impact-resistance trade offs.Owner expectationsMuch of the prevention of value engineering is having a clear understanding of the owner's performance requirements and expectations. Getting 'buy-in' from an owner for high-performance roof systems is key. During the design phase, explaining the importance of design decisions and aligning them with the owner's performance requirements should be documented. Documented design decisions that are made in conjunction with the owner provide a defense when "it costs too much" is used as the basis for value engineering.The Whole Building Design Guide, from the National Institute of Building Sciences, includes detailed information about value engineering during the design and construction phases of a project.