Flashing of Penetrations in Existing Roofs

Scope Images
Provide flashing and sealing integrated with the air and water control layers for vents and other roof penetrations
Provide flashing and sealing integrated with the air and water control layers for vents and other roof penetrations
Scope

When reroofing a home or doing a renovation that creates a new penetration in the roof, inspect and replace or add flashing and sealing around all roof penetrations, such as vents, flue stacks, chimneys, sky lights, mounting brackets for solar panels, etc., to provide a continuous water control layer across the roof assembly, as follows:

  • Inspect all of the penetrations through the roof to determine if proper flashing and other water management details are in place to prevent water leaks.
  • Install flashing where missing around all new and existing roof penetrations through the roof.
  • Install the water control materials in shingle fashion . 

For more on roofs, 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

Vents, flue stacks, chimneys, sky lights, mounting brackets, etc., are all penetrations through a roof’s water control layer, which is the roof covering together with the roofing felt, synthetic underlayment, or other weather-resistant barrier installed over the roof decking. Any of these penetrations represent a weak spot in the roof’s armor where water could leak into the building, with the potential to cause significant damage over time. Whenever a new hole is cut in the roof, for example to install a flue, a skylight, an exhaust fan duct, or a brackets for solar panels, make sure that installation of proper flashing is part of the job. When the roofing on an older home is replaced, take the opportunity to inspect all of the flashing and repair or replace missing, damaged, or poorly installed flashing at all roof penetrations as well as at roof-wall joints and at valleys.

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 such that water is directed down and out away from the building. The key to this fundamental principle is drainage.

Roofs should slope to drain water away from the top of buildings - the steeper the slope, the better. Any roof penetrations must be properly flashed to prevent water entry. The materials that form the water control layer should overlap each other in shingle fashion or be sealed so that water drains down and does not collect on the roof.

Membranes or formable flashings around roof penetrations should be integrated into the roof’s water control layer (or drainage plane) in a continuous manner and minimize the risks of water intrusion.

“Flat” roofs should never be truly flat—they should have some slope. A minimum slope of ¼:12 is recommended. Therefore, they could more accurately be called “low-slope roofs.” All low-slope roofs must be sloped to roof drains, scuppers, or edge details that carry rainwater off the roof.

Figures 1 through 4 show examples of water management details around four common roof penetrations – a plumbing vent pipe through an insulated sloping roof, a roof drain, a skylight curb, and a solar panel mounting bracket through a low-slope roof. The figures show the water management details integrated with rigid foam insulation that has been installed above the roof deck and beneath the cladding as part of a roof insulation upgrade. However, in most aspects, these details also apply to roofs that do not have rigid foam installed on the roof deck.

Further details for sealing around a pipe in a sloping asphalt shingle roof are described in the how-to steps.

Water management details are properly integrated with the roof insulation, underlayment, and cladding around a vent pipe

Figure 1. Water management details are properly integrated with the roof insulation, underlayment, and cladding around a vent pipe.

Water management details for a roof drain installed along with rigid foam on a flat roof

Figure 2. Water management details for a roof drain installed along with rigid foam on a flat roof.

Water management details at a mechanical curb for a skylight

Figure 3. Water management details at a mechanical curb for a skylight.

Water management detail for a solar panel rack mounting block installed in rigid foam that was installed over an existing roof

Figure 4. Water management detail for a solar panel rack mounting block installed in rigid foam that was installed over an existing roof.

How to flash around a vent pipe roof through a sloping roof with rigid foam installed above the decking.

This sequence of steps shows a plumbing vent pipe penetration through a sloping roof with insulation retrofitted on top of the roof deck.  The attic in this detail is conditioned interior space.

  1. Inspect the integrity of the roof system (roofing membrane and/or cladding). Check for any deficiencies, water damage, active leaks, etc. Proceed with re-flashing only if required repairs have been performed.
  2. Inspect the structural integrity of the roof. Check the roof framing for any deficiencies, rot, insect damage, etc. Proceed with re-flashing only if required repairs have been performed. Based on the findings, review specific detailing, and revise the roof assembly plans as needed. Follow the minimum requirements of the current adopted building code regarding the wood roof framing construction.
  3. Cut a hole through the roofing that is 0.5 inches larger than the pipe and install the pipe. Extend the pipe 2 feet above the roof sheathing. Install closed-cell foam backer rod and urethane sealant in the hole around the pipe as an air control layer.

    Cut a hole through the roofing that is 0.5 inches larger than the pipe and fit in the pipe. Extend the pipe 2 feet above the roof sheathing. Install closed-cell foam backer rod and urethane sealant in the hole around the pipe as an air control layer.

  4. Install insulating sheathing in multiple layers with joints staggered and taped. Install closed-cell foam backer rod and urethane sealant at the pipe perimeter.

    Install insulating sheathing in multiple layers with joints staggered and taped. Install closed-cell foam backer rod and urethane sealant at the pipe perimeter.

  5. Install 5/8 in. plywood roof sheathing over the layers of rigid foam using H-clips and screw fasteners that extend through the foam to the structure. Cut the plywood to fit snuggly around the pipe.

    Install 5/8" plywood roof sheathing over the layers of rigid foam using H-clips. Cut the plywood around the pipe penetration.

  6. Cut two sections of peel-and-stick roof membrane that will each extend out from the vent pipe for 18 inches. Cut a half-circle notch out of each section so the membrane will lay flat around the pipe. Install the section of membrane on the downhill side of the pipe first, to fit around the pipe penetration and lay flat.

    Cut two sections of peel and stick roof membrane that will each extend out from the vent pipe for 18 inches. Cut a notch out of each section so the membrane will lay flat around the pipe. Install the section of membrane on the downhill side of the pipe first, to fit around the pipe penetration and lay flat.

  7. Install a piece of formable self-adhered flashing membrane around the pipe like a collar, This collar will overlap the down-slope piece of fully adhered roof membrane.

    Install the membrane flashing collar. This collar will overlap the down-slope piece of fully adhered roof membrane.

  8. Install the upper layer of the fully adhered roof membrane. Cut the membrane around the pipe penetration and lap it over the pipe collar and over the edge of the lower layer of membrane. A flexible-type membrane is ideal for flashing around pipes.

    Install the upper layer of the fully adhered roof membrane. Cut the membrane around the pipe penetration and lap over the membrane collar and over the edge of the lower layer of membrane.

  9. Install asphalt shingles, stopping just below the pipe.

    Install asphalt shingles, stopping just below the pipe.

  10. Install the metal/rubber flashing collar over the membrane flashing collar. Overlap the metal collar onto the lower layer of the asphalt shingles.

    Install the metal flashing collar over the membrane flashing collar. Overlap the metal collar onto the lower layer of the asphalt shingles.

  11. Continue installing shingles around and above the pipe. These shingles will cover the upper edge of the metal flashing collar. Cut the shingles around the pipe penetration.

    Continue installing shingles around and above the pipe. These shingles will cover the upper edge of the metal flashing collar. Cut the shingles around the pipe penetration.

Regarding the metal pipe flashing, the Insurance Institute of Building and Home Safety (IBHS) Fortified Home program recommends the following in its General Flashing Guidelines for Steep-Sloped Roofs:

  1. Use corrosion-resistant metal flashing with a thickness of not less than designated in local building code or the metal flashing material table, Table 1 (excerpted from the IBHS Fortified General Flashing Guidelines for Steep-Sloped Roofs).
  2. Unless otherwise noted to be more restrictive, fasten all metal flashing at a maximum of 6 in. on center at the edges with approved compatible corrosion-resistant fasteners.
  3. Prime metal surfaces receiving approved flashing cement with ASTM D41 primer.

Use approved flashing cement in compliance with the roof system manufacturer’s installation instructions.

Metal Flashing Material Table

 

Ensuring Success

Inspect the existing roof framing for any deficiencies and make any corrections if necessary.

Ensure that the water control layers of the roof system overlap each other in shingle fashion.

Climate

Hurricane-Prone Regions

The IRC does not have additional requirements for sealing and flashing roof penetrations in hurricane-prone regions or other high-wind areas. Some local jurisdictions may have additional requirements or require specific product approval. Building codes establish minimum requirements, but products must also be installed in accordance with manufacturer’s instructions. This is important because codes typically do not provide all the detailed information for a durable installation. Assessments by FEMA after hurricanes commonly find that water intrusion and structural building failures are due to improper installation of building components. So, even where the IRC does not require additional measures, proper installation is more critical in hurricane-prone regions. See flashing details in the Insurance Institute of Building and Home Safety (IBHS) Fortified Home program General Flashing Guidelines for Steep-Sloped Roofs.

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
Author(s)
Weather Proofing America
Organization(s)
Weather Proofing America
Description
Video explaining how to correctly install wall-to-roof flashing and kick-out flashing to protect walls from rain water damage.
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)

Water Management System Builder Requirements

3. Water-Managed Roof Assembly.
3.1 Step and kick-out flashing at all roof-wall intersections, extending ≥ 4 on wall surface above roof deck and integrated with drainage plane above.12
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 discharges water on sloping final grade ≥ 5 ft. from foundation, or to underground catchment system not connected to the foundation drain system that discharges water ≥ 10 ft. from foundation. Alternatives & exemptions in Footnote.3, 13, 14
3.3 Self-adhering polymer-modified bituminous membrane at all valleys & roof deck penetrations.3, 15
3.4 In 2009 IECC Climate Zones 5 & higher, self-adhering polymer-modified bituminous membrane 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. 3, 15

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

200920122015, and 2018 International Residential Code (IRC)

2018 IRC G2427 Venting of appliances:

2018 IRC P2606 Penetrations: P2606.1 requires the roof penetration to be sealed.

2018 IRC P2607 Waterproofing of openings: P2607.1 requires the roof penetration to be flashed.

2018 IRC M2301 Solar thermal energy systems: M2301.2.9 requires roof and wall penetrations to be flashed and sealed in accordance with Chapter 9 to prevent entry of water, rodents, and insects.

2018 IRC Section R903.2 Flashing.

Retrofit: 

2009, 2012, 20152018, and 2021 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.

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)
Pettit,
Neuhauser,
Gates
Organization(s)
Building Science Corporation
Publication Date
Description
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.
Author(s)
Lstiburek
Organization(s)
Building Science Corporation
Publication Date
Description
Book presenting the best techniques for energy and resource efficient residential construction in the colder climates of North America.
Author(s)
National Roofing Contractors Association
Organization(s)
National Roofing Contractors Association
Publication Date
Description
Manuals describing design, materials, and installation techniques applicable to almost all types of roof systems; four-volume set includes Metal Panel and SPF Roof Systems, Membrane Roof Systems, Architectural Metal Flashing and Condensation and Air Leakage Control, and Steep-Slope Roof Systems.
Author(s)
Home Innovation Research Labs
Organization(s)
Home Innovation Research Labs
Publication Date
Description
Factsheet providing homeowners with an overview of approaches for flashing and sealing roof penetrations to help discussion with a roofing contractor.
*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.

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