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Water Managed Roof – Re-roofing and Adding Insulation over a Flat Roof

Scope

Retrofit an existing flat roof to improve thermal, water, and air control performance as follows:

  • Remove the existing roofing membrane and inspect the roof for any deficiencies.
  • Make the necessary corrections to the roof framing and decking prior to beginning the new work.
  • Air seal wall cavities made accessible by roof demolition and install insulation in the roof cavity.
  • Provide a continuous air control layer within the roof assembly. Ensure the roof assembly air control layer is connected to the wall (and other roof elements) air control layer.
  • Install polyisocyanurate rigid foam insulation over the air control layer. Ensure that the ratio of rigid foam (“air impermeable insulation”) to cavity insulation (“air permeable insulation” meets the code requirements to avoid condensation.
  • Install insulation cover board and roofing membrane with flashing as the water control layer.
  • Flash around all roof penetrations, including blocking added for PV racks.
  • Slope the roof deck to a drain or scuppers by installing either tapered sleepers below the structural sheathing, or tapered insulation.
  • Address combustion safety and controlled mechanical ventilation as needed, given the increased airtightness associated with this retrofit.

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

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 Indoor airPLUS.

Description

Controlling rainwater is the single most important factor in the design and construction of durable roof assemblies. The fundamental principle of water management is to shed water by layering materials in such a way that water is directed downwards and outwards out of the building or away from the building. The key to this fundamental principle is drainage.

When a flat roof is retrofitted by adding rigid foam insulation above the existing roof sheathing, the new air and water control layers must be carefully integrated with the existing roof structures.

The assembly shown here is an unvented roof assembly and must meet relevant code requirements (Table R806.4 Insulation for Condensation Control of the 2009 IRC (ICC 2009a) and Table R806.5 Insulation for Condensation Control of the 2012 IRC (2012a). The ratio of air-impermeable insulation (polyisocyanurate) to air-permeable insulation (cavity fill) must be sufficient to avoid condensation problems. Assemblies with greater depth of cavity fill insulation require more rigid board insulation. Per 2012 IRC Section806.5, the interior of the roof (gypsum board or plaster) can have a Class II or Class III vapor retarder; a Class I vapor barrier (polyethylene) is prohibited as it will eliminate drying of the assembly. For further explanation, see IRC FAQ: Conditioned Attics.

Polyisocyanurate is a suitable rigid foam for installing above the roof deck. The rigid foam can be installed over new sheathing coated with an integrated water-resistive barrier as shown in Figure 1 or the insulation can be installed over the existing roof sheathing, which is first covered with a fully adhered air control membrane that extends up the parapet sides above the flat roof as shown in Figure 2. If foil-faced polyisocyanurate rigid foam insulation is used, a protective layer of insulation cover board should be installed over it prior to installing the roofing membrane water control layer, as shown in Figures 1, 2 and 3. Glass fiber-faced polyisocyanurate can be installed without an insulation cover board.

“Flat” roofs should never be flat. All low-slope roofs must be sloped to drains (as shown in Figure 4) or scuppers at a minimum slope of ¼ inch per foot (ASTM 2009). It is vital that any roof penetrations (drains, skylights, or mechanical curbs) are properly flashed to prevent water entry. The materials that form the water control layer (in this case the roof membrane) should overlap each other in shingle fashion or be sealed in a watertight manner (in the field of the roof if the slope is insufficient to rely on shingle-lapped water shedding).

Skylights, mechanical curbs, and other roof penetrations must be integrated into the roof’s drainage plane(roofing membrane) (see Figures 5 and 6). Membranes or formable flashings that line these curbed openings are all elements of the roof water control layer. These approaches work best when they are sloped toward the roof drain, so that rainwater is directed off of the building.

Existing flat roof and brick masonry walls are retrofitted with polyisocyanurate rigid foam insulation plus new OSB sheathing coated with an integrated water-resistant barrier that serves as an air control layer
Figure 1. Existing flat roof and brick masonry walls are retrofitted with polyisocyanurate rigid foam insulation plus new OSB sheathing coated with an integrated water-resistant barrier that serves as an air control layer.

Existing flat roof and brick masonry walls are retrofitted with a new fully adhered air barrier membrane plus polyisocyanurate rigid foam insulation and a roofing membrane water control layer
Figure 2. Existing flat roof and brick masonry walls are retrofitted with a new fully adhered air barrier membrane plus polyisocyanurate rigid foam insulation and a roofing membrane water control layer.

Existing flat roof and wood-framed walls are retrofitted with a new fully adhered air barrier membrane plus polyisocyanurate rigid foam insulation and a roofing membrane water control layer
Figure 3. Existing flat roof and wood-framed walls are retrofitted with a new fully adhered air barrier membrane plus polyisocyanurate rigid foam insulation and a roofing membrane water control layer.

A roof drain is installed in an existing flat roof retrofitted with above-deck rigid foam insulation that is integrated with new air and water control layers
Figure 4. A roof drain is installed in an existing flat roof retrofitted with above-deck rigid foam insulation that is integrated with new air and water control layers.

The mechanical curb for a skylight is integrated with the water management and air control layers on a flat roof retrofitted to include new above-deck rigid foam insulation
Figure 5. The mechanical curb for a skylight is integrated with the water management and air control layers on a flat roof retrofitted to include new above-deck rigid foam insulation.

The blocking for a new PV roof-mounting system is integrated with new rigid foam and the air and water control layers installed over an existing flat roof
Figure 6. The blocking for a new PV roof-mounting system is integrated with new rigid foam and the air and water control layers installed over an existing flat roof.

How to Re-Roof a Flat Roof

  1. Inspect the structural integrity of the roof. Pooling water (seen in Figure below) is a sign that more slope and drains or scuppers are needed. Remove the existing roof membrane, insulation, and sheathing (where needed) and check the roof framing for any deficiencies, rot, insect damage, etc. (see Figure below). Do not proceed if any repairs need to be performed. Based on the findings, revise the roof assembly and review specific detailing as needed. Follow the minimum requirements of the current adopted building and energy codes.
    The existing flat roof before removal of membrane shows lack of slope allowing water to pool on the surface
    The existing flat roof before removal of membrane shows lack of slope allowing water to pool on the surface. (Image courtesy of Building Science Corporation).

    Sheathing is removed from a flat roof to retrofit with air sealing, insulation, and water control layers
    Sheathing is removed from a flat roof to retrofit with air sealing, insulation, and water control layers. (Image courtesy of Building Science Corporation).

  2. Remove two or three roof sheathing boards near the perimeter of the roof, leaving one or two boards at the parapet (Figure below). Mechanically fasten a strip of pressure-treated plywood to the interior vertical face of the parapet to allow for attachment of the strip of fully adhered air barrier membrane. Install a strip of OSB sheathing with an integrated water-resistive barrier at the roof perimeter adjacent to the parapet on top of the remaining board sheathing (Figure below). Provide a continuous bead of sealant between the existing board sheathing and a strip of new roof sheathing.
    The existing sheathing boards are removed near the perimeter of the flat roof and pressure-treated plywood is installed at the vertical face of the parapet
    The existing sheathing boards are removed near the perimeter of the flat roof and pressure-treated plywood is installed at the vertical face of the parapet. (Image courtesy of Building Science Corporation).

    A strip of OSB sheathing is installed along the perimeter when retrofitting a flat roof with a parapet
    A strip of OSB sheathing is installed along the perimeter when retrofitting a flat roof with a parapet. (Image courtesy of Building Science Corporation).

  3. Spray 2 inches of closed-cell spray foam in the roof cavity at the wall perimeter to create an air barrier connection between the wall and the roof, and to provide adequate thermal resistance to prevent condensation (Figures below). The area should be free of debris and dust prior to spraying for adequate adhesion. Install fibrous insulation (e.g., cellulose) in the rafter cavities beneath the existing board roof sheathing (Figures below). Re-install the roof sheathing boards.
    Closed-cell foam is sprayed into roof cavities along the masonry parapet wall to form a continuous air barrier between the wall and the sheathing of the flat roof
    Closed-cell foam is sprayed into roof cavities along the masonry parapet wall to form a continuous air barrier between the wall and the sheathing of the flat roof. (Image courtesy of Building Science Corporation).

    Closed-cell spray foam fills the roof joist cavities forming an air barrier between the masonry parapet wall and the roof sheathing
    Closed-cell spray foam fills the roof joist cavities forming an air barrier between the masonry parapet wall and the roof sheathing. (Image courtesy of Building Science Corporation).

    The base of the plywood parapet is air sealed with spray foam and fibrous insulation is installed in the rafter cavities in this flat roof retrofit
    The base of the plywood parapet is air sealed with spray foam and fibrous insulation is installed in the rafter cavities in this flat roof retrofit. (Image courtesy of Building Science Corporation).

    Roofing paper protects the top of the new plywood parapet while the base of the parapet is air sealed with spray foam and fibrous insulation is installed in the rafter cavities in this flat roof retrofit
    Roofing paper protects the top of the new plywood parapet while the base of the parapet is air sealed with spray foam and fibrous insulation is installed in the rafter cavities in this flat roof retrofit. (Image courtesy of Building Science Corporation).

  4. If needed to create a slope, install sleepers (tapered wood furring strips) on the roof rafters prior to installing the new roof sheathing (Figure below). Install OSB sheathing with an integrated water-resistive barrier over the entire field of the roof (Figure below). Seal all of the sheathing seams with appropriate tape that is applied to a clean surface and pressed on with a roller to ensure good adhesion. This coated, taped OSB is the air control layer at the field of the roof. Install a strip of fully adhered air barrier membrane on top of the new roof sheathing at the roof perimeter adjacent to the parapet and extend it to the top of plywood to form an air barrier connection to the parapet wall. Seal all penetrations, such as drains, skylights, and mechanical curbs, in an airtight and durable manner to the air control layer.
    ALTERNATELY, instead of the coated OSB, install a fully adhered air barrier membrane over the existing roof sheathing and extend up to the top of the plywood at the parapet to form a continuous air control layer. Seal all penetrations, such as drains, skylights, and mechanical curbs, in an airtight and durable manner.
    Sleepers (tapered wood furring strips) are installed over the existing board sheathing to slope the new sheathing toward the drain in this flat roof retrofit
    Sleepers (tapered wood furring strips) are installed over the existing board sheathing to slope the new sheathing toward the drain in this flat roof retrofit. (Image courtesy of Building Science Corporation).

    New coated OSB roof sheathing is installed over the existing sheathing of the flat roof and taped at the seams to provide a continuous air barrier
    New coated OSB roof sheathing is installed over the existing sheathing of the flat roof and taped at the seams to provide a continuous air barrier. (Image courtesy of Building Science Corporation).

  5. Install polyisocyanurate rigid foam insulation over the roof sheathing (Figure below). The ratio of air-impermeable insulation (polyisocyanurate) to air-permeable insulation (cavity fill) must be sufficient to avoid condensation problems, per Section R806.4 of the 2009 IRC (ICC 2009a) and Section R806 of the 2012 IRC (2012a). If tapered sleepers are not installed, provide tapered insulation to ensure the roof is sloped to drain. Fit the joints of the insulation together tightly. When installing multiple layers, offset the seams in two directions, add a continuous bead of sealant at the perimeter of the roof between each layer, and tape the seams of each layer. Install wood blocking for future photovoltaic (PV) panel installation (Figure below). The surface of the blocking must stand proud of the roof deck to reduce water penetration risks at fastener penetrations (Figure below). Install insulation cover board (typically fiberglass-faced gypsum board) over the polyisocyanurate rigid foam insulation. Ensure compatibility of the cover board with the roof assembly.
    Polyisocyanurate rigid foam insulation is installed in multiple layers with staggered, taped seams over the flat roof. Note the continuous bead of sealant at the roof perimeter between each layer
    Polyisocyanurate rigid foam insulation is installed in multiple layers with staggered, taped seams over the flat roof. Note the continuous bead of sealant at the roof perimeter between each layer. (Image courtesy of Building Science Corporation).

    Wood blocking for future PV panel installation is installed in the layers of polyisocyanurate rigid foam insulation during a flat roof retrofit
    Wood blocking for future PV panel installation is installed in the layers of polyisocyanurate rigid foam insulation during a flat roof retrofit. (Image courtesy of Building Science Corporation).

    The wood blocking for future PV panel installation extends above the surface of the top layer of polyisocyanurate rigid foam insulation installed as part of a flat roof retrofit
    The wood blocking for future PV panel installation extends above the surface of the top layer of polyisocyanurate rigid foam insulation installed as part of a flat roof retrofit. (Image courtesy of Building Science Corporation).

  6. Install roofing membrane over the insulation cover board and lap it over the parapet to provide a continuous water control layer (Figure 20). Seal all penetrations, such as drains, skylights, mechanical curbs, and PV blocking against water leaks per new construction best practices (Figures 21, 22, and 23). Install metal cap flashing at the parapet over the roofing membrane. Provide drip edges on either side of the cap flashing to prevent staining of the building façade. (See Figure below and BSI-050: Parapets—Where Roofs Meet Walls.)
    Roofing membrane is installed over polyisocyanurate rigid foam insulation and insulation cover board that has been cut to fit around locations for blocking for the PV system rack
    Roofing membrane is installed over polyisocyanurate rigid foam insulation and insulation cover board that has been cut to fit around locations for blocking for the PV system rack. (Image courtesy of Building Science Corporation).

    Blocking installed on a flat roof for a PV system rack is sealed around the edges with sealant then will be covered with self-adhering roof membrane to prevent water leakage
    Blocking installed on a flat roof for a PV system rack is sealed around the edges with sealant then will be covered with self-adhering roof membrane to prevent water leakage. (Image courtesy of Building Science Corporation).

    The blocking is completely flashed with roofing membrane before the PV rack hardware is attached on a flat roof
    The blocking is completely flashed with roofing membrane before the PV rack hardware is attached on a flat roof. (Image courtesy of Building Science Corporation).

    Strips of roofing membrane are used to flash around a skylight on a flat roof retrofit.
    Strips of roofing membrane are used to flash around a skylight on a flat roof retrofit. (Image courtesy of Building Science Corporation).

    Metal cap flashing is installed over the roof parapet and extends down over the roofing membrane (which extends up the sides of the parapet wall)
    Metal cap flashing is installed over the roof parapet and extends down over the roofing membrane (which extends up the sides of the parapet wall). A drip edge on each side of the flashing directs water to drip away from the walls, to minimize the likelihood of staining. (Image courtesy of Building Science Corporation).

  7. Install walking mats to protect the membrane surface when accessing the flat roof (Figure below).
    The retrofitted flat roof has PV panels and walking mats installed over the roofing membrane
    The retrofitted flat roof has PV panels and walking mats installed over the roofing membrane. (Image courtesy of Building Science Corporation).

Ensuring Success

Provide the water control layer in the roof assembly and install it in a continuous manner. Ensure it is connected to the water control layer of the adjacent assemblies and roof elements, such as skylights, mechanical curbs, and drains.

Provide the air control layer in the roof assembly and install it in a continuous manner. Ensure it is connected to the air control layer of the adjacent assemblies and roof elements, such as skylights, mechanical curbs, and drains.

Slope the roof deck to a drain or scupper (with a minimum slope of .25 inch per foot).

The ratio of air-impermeable insulation (polyisocyanurate) to air-permeable insulation (cavity fill) must be sufficient to avoid condensation problems, per Section R806.4 of the 2009 IRC (ICC 2009a) and Section R806 of the 2012 IRC (2012a).

Remediate any hazardous conditions that will be affected (e.g., exposed or aggravated) by the planned work. Examples of hazardous materials that may be found in roof assemblies of existing structures include (but are not limited to) lead, asbestos, mold, animal dropping/remains, etc. Follow applicable laws and industry procedures for mitigation of hazardous materials. Engage the services of a qualified professional when needed.

Provide minimum combustion safety by providing direct-vent sealed-combustion equipment or forced draft equipment (see Direct Vent Equipment). When furnaces, boilers, and water heaters are installed within the home’s pressure boundary, ideally this equipment would be direct-vent sealed-combustion equipment. If existing equipment is not direct-vent sealed-combustion or forced draft, the homeowners must decide whether to

  • replace equipment with direct vent or forced draft equipment, or
  • retrofit forced-draft to existing equipment.

Provide whole-house and local exhaust (source control) mechanical ventilation complying with Section M1507 of the 2012 International Residential Code. Mechanical ventilation may be installed as part of the larger attic/roof retrofit project.

Climate

The roof assembly should be designed for a specific hygrothermal region, rain exposure zone, and interior climate. The climate zones are shown on the map below, which is taken from Figure C301.1 of the 2012 IECC.

IECC climate zones
IECC Climate Zone Map

The insulation levels should be based on the minimum requirements for vapor control in the current adopted building code and the minimum requirements for thermal control in the current energy code. Table 1 provides the minimum thermal resistance (R-value) requirements specified in the 2009 IECC (ICC 2009b) and the 2012 IECC (ICC 2012b) based on climate zone for roof assemblies.

Attic Insulation Requirements per the 2009 and 2012 IECC
Table 1. Attic Insulation Requirements per the 2009 and 2012 IECC

It is important to maintain a sufficient ratio of exterior insulation to total roof assembly insulation. In colder climate zones, the amount of exterior (rigid) insulation needed to avoid condensation problems increases. Table 2 provides information on minimum levels of air impermeable insulation for condensation control specified in Table R806.4 Insulation for Condensation Control of the 2009 IRC (ICC 2009a) and Table R806.5 Insulation for Condensation Control of the 2012 IRC (2012a).

Insulation required for Condensation Control per the 2009 and 2012 IRC.
Table 2. Insulation required for Condensation Control per the 2009 and 2012 IRC.

Training

Right and Wrong Images

None Available

Presentations

None Available

Videos

None Available

CAD Images

None Available

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

[Guidance for Version 3.0, Rev 08 is coming soon.]

ENERGY STAR Certified Homes is a voluntary high-performance home labeling program for new homes operated by the U.S. Department of Energy and the U.S. Environmental Protection Agency. Builders and remodelers who are conducting retrofits are welcome to seek certification for existing homes through this voluntary program.

The ENERGY STAR Certified Homes (Version 3.0, Rev. 07) Water Management Checklist specifies:

Water-Managed Roof Assembly

3.1 Step and kick-out flashing at all roof-wall intersections, extending ≥ 4 inches on wall surface above roof deck and integrated with drainage plane above.

3.2 For homes that don’t have a slab-on-grade foundation and do have expansive or collapsible soils, gutters & downspouts provided that empty to lateral piping that deposits water on sloping final grade ≥ 5 ft. from foundation or to underground catchment system ≥ 10 ft. from foundation.

3.3 Self-sealing bituminous membrane or equivalent at all valleys and roof deck penetrations.

3.4 In 2009 IECC Climate Zones 5 and higher, self-sealing bituminous membrane or equivalent over sheathing at eaves from the edge of the roof line to > 2 ft. up roof deck from the interior plane of the exterior wall.

DOE Zero Energy Ready Home

The DOE Zero Energy Ready Home Program is a voluntary high-performance home labeling program for new homes operated by the U.S. Department of Energy. Builders and remodelers who are conducting retrofits are welcome to seek certification for existing homes through this voluntary program.

The U.S. Department of Energy Zero Energy Ready Home National Program Requirements specify as a mandatory requirement (Exhibit 1, #2.2) that, for all labeled homes, whether prescriptive or performance path, ceiling, wall, floor, and slab insulation shall meet or exceed 2012 IECC levels. See the guide 2012 IECC Code Level Insulation – DOE Zero Energy Ready Home Requirements for more details.

200920122015, and 2018 IRC

Section R801.3 Roof drainage.

Section R907 (R908 in 2015 and 2018 IRC) Reroofing.

Section R901 Roof Assemblies. This section outlines the design, materials, construction, and quality of roof assemblies.

Retrofit: 200920122015, and 2018 IRC

Section N1101.3 (Section N1107.1.1 in 2015 and 2018 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 existing buildings and is intended to encourage their continued safe use.

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.

Case Studies

  1. Author(s): PNNL
    Organization(s): PNNL
    Publication Date: September, 2013

    Case study of a DOE Zero Energy Ready Home in west Connecticut that scored HERS 39 without solar PV. The 3,000 ft2 two-story home has R-33 double-walls, R-72 flat roof with closed-cell foam and blown cellulose, an ERV, and LED lighting.

  2. Author(s): PNNL
    Organization(s): PNNL
    Publication Date: September, 2013

    Case study of a DOE Zero Energy Ready Home in Seattle, WA, that scored HERS 34 without PV. This 2,000 ft2 system home has R-45 double-stud walls, an unvented flat roof with 2 inches of spray foam plus 18 inches blown cellulose, R-42 XPS under slab, triple-pane windows, and a ductless mini-split heat pump.

References and Resources*

  1. Author(s): International Code Council
    Organization(s): ICC
    Publication Date: January, 2009

    Code for residential buildings that creates minimum regulations for one- and two-family dwellings of three stories or less. It brings together all building, plumbing, mechanical, fuel gas, energy and electrical provisions for one- and two-family residences.

  2. Author(s): International Code Council
    Organization(s): ICC
    Publication Date: January, 2012

    Code for residential buildings that creates minimum regulations for one- and two-family dwellings of three stories or less. It brings together all building, plumbing, mechanical, fuel gas, energy and electrical provisions for one- and two-family residences.

  3. Author(s): Lstiburek
    Organization(s): Building Science Corporation
    Publication Date: January, 2006

    Book presenting the best techniques for energy and resource efficient residential construction in the colder climates of North America.

  4. Author(s): Lstiburek
    Organization(s): Building Science Corporation
    Publication Date: December, 2010

    Information about dense pack, cellulose insulation.

  5. Author(s): Pettit, Neuhauser, Gates
    Organization(s): Building Science Corporation
    Publication Date: July, 2013

    Guidebook providing useful examples of high performance retrofit techniques for the building enclosure of wood frame residential construction in a cold and somewhat wet climate.

  6. Author(s): Loomis, Pettit
    Organization(s): Building Science Corporation
    Publication Date: May, 2015

    This Measure Guideline provides design and construction information for a deep energy enclosure retrofit solution of a flat roof assembly.

  7. Author(s): National Roofing Contractors Association
    Organization(s): National Roofing Contractors Association
    Publication Date: December, 2006

    NRCA's premier technical publication gives you the most current and useful technical information in the roofing industry.

Contributors to this Guide

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

Building Science Corporation, lead for the Building Science Consortium (BSC), a DOE Building America Research Team

Last Updated: 12/28/2015