Low-Slope ("Flat") Roofs

Scope Images
Right – The roof membrane is fully adhered and deck seams are sealed so the membrane will not flutter and fail due to negative pressure from high winds
Right – The roof membrane is fully adhered and deck seams are sealed so the membrane will not flutter and fail due to negative pressure from high winds
Scope

In high wind zones, construct low-slope (“flat”) insulated roof assemblies that resist the wind pressures that can pull off membrane roof coverings during high-wind events such as hurricanes. 

  • Install the roof decking in accord with IBHS Fortified Home sheathing nailing pattern recommendations.
  • Seal the roof deck sheathing by covering the entire plywood or OSB sheathing with a self-adhering “peel and stick” membrane or by sealing the sheathing seams with a compatible tape.
  • Install one or more layers of rigid foam insulation with seams staggered and taped.
  • Install an exterior sheathing layer.
  • Select a roof covering that is appropriate for the maximum design wind uplift pressures expected for the site, following the recommendations of the IBHS Fortified Home program. A fully adhered roof membrane is recommended.

See the Compliance Tab for related codes and standards requirements, and criteria to meet national programs such as DOE’s Zero Energy Ready Home program, ENERGY STAR Certified Homes, and EPA Indoor airPLUS.

Description

In high wind zones, low-slope (“flat”) roof assemblies (slope less than 2:12) with a waterproofing roof membrane cladding need to be able to resist the wind pressures that can act on them during high-wind events such as hurricanes. 

Low-slope roof coverings include built-up bitumen roof systems, modified bitumen roof systems, and single-ply roofing membranes including thermoplastic membranes such as TPO (thermoplastic olefin), PVC (polyvinyl chloride), and KEE (ketone ethylene ester), and thermoset membranes such as EPDM (ethylene propylene diene monomer). This guide discusses single-ply roofing membranes, the most commonly used flat-roof cladding in residential construction today. The IBHS Fortified Home program recommends that the low-slope roofing system and installation method selected is adequate for the highest design pressures expected on the roof at the site. Maximum design wind uplift pressures for low-slope roofs are listed in Table 2-5 of ASCE 7, Minimum Design Loads for Buildings and Other Structures. The International Residential Code 2012, 2015, and 2018, reference the ASCE-7-10. IBHS recommends that builders follow the values in ASCE 7-16, which are based on more recent research on design loads for low-slope roofs. To certify a home to IBHS Fortified Home, IBHS requires that the roofing manufacturer’s tested uplift design pressures must have a factor of safety of 2.0. IBHS notes that the following reports can be used to validate compliance with the project site-specific design pressure requirements because they already have the 2.0 factor of safety applied: an ICC Evaluation Service Report, the Florida Product Approval list, or the Miami-Dade Notice of Acceptance (NOA). For more information see the IBHS Fortified Home Hurricane Standard,

Single-ply roof membranes can be either mechanically attached or fully adhered to the low-slope roof's rigid insulation or cover board. This guide describes problems that have been noted in the field with mechanically attached single-ply membranes and recommends installation methods to increase the roof claddings' resistant to wind uplift pressures.

Low-slope roof assemblies that are insulated above the roof deck are typically constructed from multiple layers (Figure 1), including plywood or OSB deck sheathing, one or more layers of rigid isocyanurate or extruded polystyrene (XPS), and an upper covering board of plywood or OSB. It is common for the single-ply roofing membrane installed over the coverboard to be mechanically attached rather than fully adhered. 

 

Low-slope roof assemblies constructed of two deck sheathing layers sandwiching rigid foam, and topped with mechanically fastened membrane.
Figure 1. Low-slope roof assemblies are typically constructed from multiple layers including OSB or plywood deck sheathings, rigid foam, and membrane attached with fasteners. (Source: Building Science Corporation).

During high-wind events, high localized areas of negative pressure (“suction”) occur above the roof membrane (Figure 2 and Figure 3). Interior spaces can simultaneously experience high positive pressures especially if a window or door is “blown in” or fails. This type of high positive interior pressure increases the risk of roof failure, especially in a multi-layer roof assembly where elements (or layers) are not sealed and where roof membranes are mechanically attached not fully adhered. When air from within the building is allowed to leak into the roof assembly, the membrane can “flutter” and ultimately fail (Figure 4 and Figure 5). This flutter is caused by a combination of the negative pressures above the roof membrane due to wind and the positive pressures beneath the membrane due to air leakage into the assembly.

During high wind events, vortices form along the edges of the roof creating areas of localized negative pressure (“suction”) above the roof
Figure 2. During high wind events, vortices form along the edges of the roof creating areas of localized negative pressure (“suction”) above the roof. (Source: Building Science Corporation).
Low-slope roof assemblies constructed of two deck sheathing layers sandwiching rigid foam, and topped with mechanically fastened membrane.
Figure 3. During high wind events, high localized areas of negative pressure (“suction”) occur above roof membranes. (Source: Building Science Corporation).
Wrong – If roof membrane is not fully adhered, it can flutter and fail due to negative pressure from wind above and positive pressure from air leakage through roof deck below.
Figure 4. Mechanically attached membrane flutters and fails due to negative pressure/suction from wind above and air leakage into the assembly from the building if seams are not sealed. (Source: Building Science Corporation).
Wrong – This roof membrane on a low slope roof is fluttering and may fail because it is not fully adhered and deck seams below are not sealed, allowing air to leak up into the assembly while the membrane is pulled up due to negative pressures from wind.
Figure 5. This roof membrane on a low slope roof is fluttering and may fail because it is not fully adhered and deck seams below are not sealed, allowing air to leak up into the assembly while the membrane is pulled up due to negative pressures from wind. (Source: Building Science Corporation).

P(t) = P(m) + P(d) + P(i) + P(i) + P(d)

Where P(t) = total wind pressure

P(m) = membrane pressure

P(d) = deck pressure

P (i) = insulation pressure

P(d) = deck pressure

 

The most airtight (“tightest”) element experiences the greatest pressure. In most roof assemblies, the tightest element is the roof membrane itself so it, therefore, often experiences the greatest wind load.   

In high-wind zones, fully adhered membranes are recommended. Additionally, a fully adhered air control layer (“air barrier”) should be installed at the lower deck sheathing and joints in the rigid insulation should be sealed or taped (Figure 6). This approach reduces the air leakage into the assembly and thus the air pressure difference across the upper membrane and thereby reduces the risk of membrane failure due to “blow off” and “fluttering.”

Right – The roof membrane is fully adhered and deck seams are sealed, so the membrane will not flutter and fail due to negative pressure from high winds.
Figure 6. In high wind zones, roofing membranes on low slope roofs should be fully adhered and sheathing seams should be sealed to reduce the air pressure difference across the upper membrane and the risk of failure to “blow off” and “fluttering”. (Source: Building Science Corporation).

Fluttering, besides stressing the membrane, leads to the pumping transfer of airborne moisture from the interior into the roof assembly in cold and mixed climates. Historically, this has typically been a cold-climate phenomenon but the problems are migrating south as membranes become white rather than black. Dark membranes get very hot – and the heat drives the moisture back down into the building. In warmer climate zones, mechanically attached dark membranes roofs avoided moisture issues because the moisture that was pumped up was driven back down by the huge temperature gradient. However, with energy conservation and light-colored membranes, many roofs no longer get hot enough to drive flutter-driven moisture back down into the building. Failures that were limited to cold climates now happen in mixed climates.

Air control layers (“air barriers”) control fluttering since any attempt by the membrane to lift off the insulation layers is resisted by suction – if air from the interior is not able to enter the roof assembly due to the presence of the lower control layer (“air barrier”). The lack of an air control layer and a leaky deck will allow replacement air to enter the roof assembly from the interior and the suction resistance is lost (Figure 4).

It is necessary to provide connection between the lower air control layer (“air barrier”) of the roof assembly and the roof membrane with the wall system (Figure 7 and Figure 8). It is also necessary to provide connection and continuity of the water control layer of the roof assembly with the water control layer of the wall assembly.

Right – This low-slope roof and parapet assembly has continuity of both the air and water barriers.
Figure 7. Parapet to Wall Connection – The roof deck and the upper layer of sheathing are both covered with a fully adhered membrane that is connected to the wall membrane for continuity of the air barrier and water barrier. (Source: Building Science Corporation).
Right – This low-slope flat roof assembly has continuity of both the air and water barriers.
Figure 8. Flat Roof to Wall Connection - It is necessary to provide connection between the lower air control layer (“air barrier”) of the roof assembly and the roof membrane to integrate with the wall system.  It is also necessary to provide connection and continuity of the water control layer of the roof assembly to the water control layer of the wall assembly. (Source: Building Science Corporation).

Modified Approach

The fully adhered air control layer (“air barrier”) at the lower deck sheathing can be omitted if the lower roof deck sheathing joints are sealed with tape or membrane strips that are compatible with the plywood or OSB sheathing.

Structural Attachment

Fastener spacing and type for the lower deck sheathing should follow the minimum requirements specified by the Insurance Institute for Building and Home Safety (IBHS) Fortified Home Program.  These are described in the IBHS Fortified Roof Standard Detail Set for Fortified Roof (2019).

For the upper deck sheathing, the fasteners should be epoxy-coated steel screws installed on 6-inch centers. Rigid insulation should be limited to a maximum thickness of 10 inches and installed using multiple 2-inch-thick sheets with joints offset horizontally and vertically.

Tapes and Membranes

Tapes and membrane selection for sealing the lower sheathing joints should follow the recommendations of the Insurance Institute for Business & Home Safety as described in their guide Choosing the Right Tape. Tapes for sealing the joints in rigid insulation should be acrylic based and selection should follow the recommendations of the Insurance Institute for Business & Home Safety in Choosing the Right Tape.

 

Ensuring Success

Provide for a high degree of airtightness at the lower roof deck sheathing either by using a fully adhered membrane air control layer (“air barrier”) or by sealing the joints of the lower roof deck sheathing with tape or membrane strips. Tape should be applied with a roller to ensure consistent pressure.

Connections at roof perimeters are important as uplift forces are greatest.

The IBHS Fortified Home program offers online training for roofers through its Fortified Wise University program.

Climate

Select a roof covering that is appropriate for the maximum design wind uplift pressures expected for the site, following the recommendations of the IBHS Fortified Home program. A fully adhered roof membrane is recommended.

The installation approach given on the Description tab works in all climates to reduce the risk of roof membrane blow off or fluttering.

In cold climates this approach also addresses high interior moisture loads that can lead to condensation within the layers of the roof assembly.

In cold climates, snow loads must be considered in the structural design.

Hail damage is minimized in high hail zones by the use of deck sheathing to support the roof membrane.

See the Compliance tab for more information on the minimum levels of roof assembly thermal resistance required by code in each climate zone.

Videos
Publication Date
Author(s)
Guertin
Organization(s)
Fine Homebuilding
Description
Video presentation explaining proper roof membrane and shingle installation, including key weak points where moisture can get in during a disaster weather event.
Publication Date
Description
Video discussing wind-resistant roof types and providing recommendations on how to choose the right roof type for your home per FLASH Strong Home guidelines.
CAD

Compliance

The Compliance tab contains both program and code information. Code language is excerpted and summarized below. For exact code language, refer to the applicable code, which may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.

ENERGY STAR Certified Homes, Version 3/3.1 (Rev. 09)

ENERGY STAR Certified Homes requires that builders comply with the National Water Management System Builder Requirements which specifies water management details for roofs, walls, foundations, sites, and building materials.

Please see the ENERGY STAR Certified Homes Implementation Timeline for the program version and revision currently applicable in in your state.

DOE Zero Energy Ready Home (Revision 07)

Exhibit 1 Mandatory Requirements.
Exhibit 1, Item 1) Certified under the ENERGY STAR Qualified Homes Program or the ENERGY STAR Multifamily New Construction Program.

Exhibit 1, Item 2) Ceiling, wall, floor, and slab insulation shall meet or exceed 2015 IECC levels and achieve Grade 1 installation, per RESNET standards. See the guide 2012 or 2015 IECC Code Level Insulation – DOE Zero Energy Ready Home Requirements for more details.

Exhibit 1, Item 6) Certified under EPA Indoor airPLUS. See the EPA Indoor airPLUS checklist for additional building and site water management requirements.

2009, 2012, 20152018, and 2021 International Residential Code (IRC)

2018 IRC R301.2.1 Wind design criteria. Buildings shall be constructed in accordance with the wind provisions of this code using the ultimate design wind speed in Table R301.2(1) as determined from Figure R301.2(5)A. Where not otherwise specified, the wind loads listed in Table R301.2(2) adjusted for height and exposure using Table R301.2(3) shall be used to determine design load performance requirements.

Retrofit:

2015 and 2018 IRC

Section N1101.3 (Section N1107.1.1 in 2015 and 2018, N1109.1 in 2021 IRC). Additions, alterations, renovations, or repairs shall conform to the provisions of this code, without requiring the unaltered portions of the existing building to comply with this code. (See code for additional requirements and exceptions.)

Appendix J regulates the repair, renovation, alteration, and reconstruction of legally existing buildings and is intended to encourage their continued safe use. Note that provisions contained in this appendix are not mandatory unless specifically referenced in the adopting ordinance.

This Retrofit tab provides information that helps installers apply this “new home” guide to improvement projects for existing homes. This tab is organized with headings that mirror the new home tabs, such as “Scope,” “Description,” “Success,” etc. If there is no retrofit-specific information for a section, that heading is not included.

Existing Homes

An existing low-slope roof can be retrofitted to improve the water resistance and wind resistance of the roof as described in the Description tab of this guide. The thermal performance of the roof can also be improved by adding insulation under the roof deck, or by adding insulation above the roof deck as described in this guide and also in the guide Water Managed Roof – Re-roofing and Adding Insulation over a Flat Roof.

For more on roof/wall connections, see the U.S. Department of Energy’s Standard Work Specifications.

More Info.

Access to some references may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.

References and Resources*
Author(s)
IBHS Fortified Home
Organization(s)
IBHS Fortified Home
Publication Date
Description
Website offering an online training that teaches disaster-resistant best practices for roofers.
Author(s)
Insurance Institute for Business & Home Safety
Organization(s)
IBHS
Publication Date
Description
Webpage providing video links and Spanish/English handouts for installing roofs to meet the hurricane and high wind resistant guidance in the IBHS Fortified Home criteria.
Author(s)
Insurance Institute for Business & Home Safety
Organization(s)
IBHS
Publication Date
Description
Checklist providing recommendations for the right tape in order to reduce the risk of leaks during storms with wind-driven rain.
*For non-dated media, such as websites, the date listed is the date accessed.
Contributors to this Guide

The following authors and organizations contributed to the content in this Guide.

The following authors and organizations contributed to the content in this Guide:

Building Science Corporation.

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