Chimneys Connected to Roof Structure

Scope Images
Right - Step flashing along a chimney is integrated in a layered manner with asphalt shingle roofing and topped with counterflashing that is embedded into brick mortar joint above
Right - Step flashing along a chimney is integrated in a layered manner with asphalt shingle roofing and topped with counterflashing that is embedded into brick mortar joint above
Scope

Design or retrofit the chimney for adequate support and water protection.

  • Provide adequate connection of the chimney to roof framing to make the chimney more resistant to toppling in strong winds.
  • Install flashing that is properly integrated with the roof flashing to keep water out of the chimney and roof assemblies.

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

One of the largest penetrations in a residential roof assembly is a chimney. Chimneys typically have numerous requirements beyond that of venting products of combustion. They must be adequately connected to the roof structure to resist wind loads based on site design wind speed and exposure category. Chimneys must also be waterproofed to resist rainwater entry during high wind events such as hurricanes.

This guide provides guidance for the waterproofing of chimneys to resist rainwater entry during high wind events such as hurricanes. The guidance is applicable to both new construction and the re-roofing of existing roof assemblies.

It is common for chimneys to significantly extend or protrude above roof surfaces in order to provide sufficient “draft” for the venting of products of combustion. This tends to expose the chimney surfaces to higher wind pressure differences than those acting on the field of the roof assembly. These forces can be further increased when the chimneys are also located near roof eaves and roof rakes. For these reasons, the chimney must be properly anchored and waterproofing of the joint between the roof assembly and the chimney is significant in controlling rainwater entry during high wind events such as hurricanes.

In its Fortified Home program, the Insurance Institute for Building and Home Safety (IBHS) requires that the chimney be adequately connected to the roof framing to prevent the chimney from collapsing or breaking away during high winds. See the CAD tab of this guide for a detail from IBHS that specifies a retrofit method for providing adequate anchorage of an existing chimney to the roof members. IBHS requires that if the chimney is more than 5 feet above the roof, support detailing must be specified by an engineer or licensed design professional. See the IBHS Fortified Home Hurricane Standards (2019) for additional details.

Waterproofing of the connection between the roof assembly and the chimney is also important for controlling rainwater entry during high wind events such as hurricanes. The fundamental principle of waterproofing the connection between the roof assembly and the chimney is to connect the water control layer of the roof assembly to the water control layer of the chimney.

In high wind zones, a fully adhered roof membrane underlayment should be installed over the roof deck sheathing. This underlayment water control layer needs to be connected to the water control layer of the chimney.

The water control layer of a chimney is most commonly the brick layer or other exterior cladding comprising the structural enclosure supporting the chimney flue. The following description pertains to brick chimneys. If the chimney is enclosed with lumber framing and siding see the guides Step and Kick-Out Flashing at Roof-Wall Intersections and Flashing of Roof-Wall Intersections in Existing Homes for additional guidance on flashing and water sealing.

The rainwater management of the brick layer is based on the rainwater shedding of individual bricks and the mortar connecting them to each other. This brick layer comprised of individual bricks and mortar needs to be connected to the fully adhered roof membrane underlayment.

Metal flashing typically serves as a water control layer transition between the brick layer and the fully adhered roof membrane underlayment. The metal flashing consists of three elements – a base flashing, a step flashing, and a counterflashing.

The base flashing should be sealed with a full bed of mastic to the top surface of the fully adhered roof membrane underlayment. A step flashing is then integrated in a layered manner with the asphalt shingle roofing. Counterflashing is embedded into the mortar joints of the brick layer and overlaps the base flashing and the step flashing, completing the waterproofing transition connecting the brick layer to the fully adhered roof membrane underlayment (Figure 1 and Figure 2).

Right - Step flashing along a chimney is integrated in a layered manner with asphalt shingle roofing and topped with counterflashing that is embedded into brick mortar joint above.
Figure 1. Step flashing along a chimney is integrated in a layered manner with asphalt shingle roofing and topped with counterflashing that is embedded into the brick mortar joint above. (Source: Building Science Corporation).
Right - Counterflashing tops a layer of step flashing which comes down above the asphalt shingle and a layer of L-shaped base flashing which comes down and extends below the shingle; the base flashing is adhered to the roof underlayment with mastic, shown in blue.

Figure 2. Counterflashing tops a layer of step flashing which comes down above the asphalt shingle and a layer of L-shaped base flashing which comes down and extends below the shingle; the base flashing is adhered to the roof underlayment with mastic, shown in blue. (Source: Building Science Corporation).

If a fully adhered roof membrane is not installed over the entire surface area of the roof deck, alternative roof deck water control requirements are necessary to provide acceptable performance and acceptable risk in high wind zones. Alternative roof deck air sealing and water control requirements can be the IBHS Fortified Home Hurricane Technical Summary (2019).

If an alternative roof deck water control approach is used, the base flashing should be sealed to the alternative roof deck water control layer with a full bed of mastic and step flashing and counterflashing should be installed as described above and in the guide Step and Kickout Flashing.

Where sloping roofs intersect the upper vertical surface of the brick layer enclosing the chimney flue, a cricket or saddle flashing should be installed to redirect rainwater that is draining down the roof slope around the chimney (Figure 3).

Right – A chimney cricket is installed and flashed to direct rainwater around the chimney.
Figure 3. A chimney cricket or saddle flashing is installed to direct rainwater out and around the chimney. (Source: Building Science Corporation).

 

Ensuring Success

The chimney must be adequately tied to the roof framing for structural support in high winds.

The fundamental principle of waterproofing the connection between the roof assembly and the chimney is to connect the water control layer of the roof assembly to the water control layer of the chimney.

Metal flashing typically serves as a water control layer transition between the brick layer and the fully adhered roof membrane underlayment. Metal flashing typically has three elements - a base flashing, a step flashing, and a counterflashing. The flashing elements need to be layered in a manner to shed and drain incident rainwater.

Where sloping roofs intersect the upper vertical surface of the brick layer enclosing the chimney flue, a cricket or saddle flashing should be installed to redirect rainwater that is draining down the roof slope around the chimney.

Climate

In its Fortified Home program, the Insurance Institute for Building and Home Safety (IBHS) requires that the chimney be adequately connected to the roof framing to prevent the chimney from collapsing or breaking away during high winds. See the CAD tab of this guide for a detail from IBHS that specifies a retrofit method for providing adequate anchorage of an existing chimney to the roof members. IBHS requires that if the chimney is more than 5 feet above the roof, support detailing must be specified by an engineer or licensed design professional. See the IBHS Fortified Home Hurricane Standards (2019) for additional details.

Waterproofing of the connection between the roof assembly and the chimney is also important for controlling rainwater entry during high wind events such as hurricanes. The fundamental principle of waterproofing the connection between the roof assembly and the chimney is to connect the water control layer of the roof assembly to the water control layer of the chimney.

In high wind zones, a fully adhered roof membrane underlayment should be installed over the roof deck sheathing. This underlayment water control layer needs to be connected to the water control layer of the chimney. The water control layer of a chimney is most commonly the brick layer or other exterior cladding comprising the structural enclosure supporting the chimney flue.

Videos
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.
Publication Date
Author(s)
Ivy Tech ivyVILOS
Organization(s)
Ivy Tech ivyVILOS
Description
Video from Ivy Tech ivyVILOS giving an in-depth tutorial on how to properly flash a chimney, focusing on pre-construction measurements. Part 1 of 11.
Publication Date
Author(s)
Ivy Tech ivyVILOS
Organization(s)
Ivy Tech ivyVILOS
Description
Video from Ivy Tech ivyVILOS giving an in-depth tutorial on how to properly flash a chimney, focusing on material preparation before construction. Part 2 of 11.
Publication Date
Author(s)
Ivy Tech ivyVILOS
Organization(s)
Ivy Tech ivyVILOS
Description
Video from Ivy Tech ivyVILOS giving an in-depth tutorial on how to properly flash a chimney, focusing on measuring and bending flashing before installation. Part 3 of 11.
Publication Date
Author(s)
Ivy Tech ivyVILOS
Organization(s)
Ivy Tech ivyVILOS
Description
Video from Ivy Tech ivyVILOS giving an in-depth tutorial on how to properly flash a chimney, focusing on installing the base front apron. Part 4 of 11.
Publication Date
Author(s)
Ivy Tech ivyVILOS
Organization(s)
Ivy Tech ivyVILOS
Description
Video from Ivy Tech ivyVILOS giving an in-depth tutorial on how to properly flash a chimney, focusing on installing step sides. Part 5 of 11.
Publication Date
Author(s)
Ivy Tech ivyVILOS
Organization(s)
Ivy Tech ivyVILOS
Description
Video from Ivy Tech ivyVILOS giving an in-depth tutorial on how to properly flash a chimney, focusing on installing the top pan. Part 6 of 11.
Publication Date
Author(s)
Ivy Tech ivyVILOS
Organization(s)
Ivy Tech ivyVILOS
Description
Video from Ivy Tech ivyVILOS giving an in-depth tutorial on how to properly flash a chimney, focusing on installing the front cap flashing. Part 7 of 11.
Publication Date
Author(s)
Ivy Tech ivyVILOS
Organization(s)
Ivy Tech ivyVILOS
Description
Video from Ivy Tech ivyVILOS giving an in-depth tutorial on how to properly flash a chimney, focusing on installing the left side cap flashing. Part 8 of 11.
Publication Date
Author(s)
Ivy Tech ivyVILOS
Organization(s)
Ivy Tech ivyVILOS
Description
Video from Ivy Tech ivyVILOS giving an in-depth tutorial on how to properly flash a chimney, focusing on installing the right side cap flashing. Part 9 of 11.
Publication Date
Author(s)
Ivy Tech ivyVILOS
Organization(s)
Ivy Tech ivyVILOS
Description
Video from Ivy Tech ivyVILOS giving an in-depth tutorial on how to properly flash a chimney, focusing on installing the top cap flashing. Part 10 of 11.
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 6) Certified under EPA Indoor airPLUS. See the EPA Indoor airPLUS checklist for additional building and site water management requirements.

2009 2018 International Energy Conservation Code (IECC) and 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 f height and exposure using Table R301.2(3) shall be used to determine design load performance requirements.

Retrofit: 

2009, 2012, 2015, 2018, and 2021 IECC

Section R101.4.3 (Section R501.1.1 in 2015, 2018, and 2021 IECC).

Section R501.1.1 in 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.)

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

In its Fortified Home program, the Insurance Institute for Building and Home Safety (IBHS) requires that the chimney be adequately connected to the roof framing to prevent the chimney from collapsing or breaking away during high winds. See the CAD tab of this guide for a detail from IBHS that specifies a retrofit method for providing adequate anchorage of an existing chimney to the roof members. IBHS requires that if the chimney is more than 5 feet above the roof, support detailing must be specified by an engineer or licensed design professional. See the IBHS Fortified Home Hurricane Standards (2019) for additional details.

Ensuring Chimneys are Adequately Anchored (The following is excerpted from FEMA P-804, 2010, Wind Retrofit Guide for Residential Buildings.)

If a wood-frame chimney on a home collapses during a high-wind event, significant damage can occur to the home as well as surrounding buildings. Therefore, if a home undergoing a wind retrofit project has a chimney framed with wood, it should be anchored to the structure as part of the continuous load path retrofit so that loads applied to it are transmitted through the load path and adequately resisted. Wood-frame chimneys that are on the interior of the roof and extend 5 feet or less from the roof deck can be retrofitted using prescriptive requirements. The following retrofits should be performed on a qualifying chimney: 1. Tensions straps with a minimum tension capacity of 700lbs. at each end should be fastened to the stud at each corner of the chimney and to the roof framing members below the chimney. 2. Wood structural panels with a minimum nominal thickness of 7/16 inch should be applied to the chimney framing on all sides. 3. Continuous wood blocking supports should be fastened to roof framing members around the base perimeter of the chimney framing using joist hangers. Wood blocking should have minimum dimensions of 2 inches x 4 inches and should be continuous around the chimney framing.

For more information on this type of solution, consult retrofit programs such as the IBHS FEH program for prescriptive details. If the wood-frame chimney is on the interior of the roof, the entire chimney structure is supported by the roof framing members, which allows the use of the prescriptive solution described above. If the chimney is located along the roof edge, the chimney structure is supported by both roof framing members and a load-bearing wall on the exterior of the building, making a prescriptive retrofit more difficult. Similarly, larger chimneys may require a more detailed and less generic solution to adequately anchor the chimney to the structure. If a chimney exists on the home that extends farther than 5 feet above the roof deck or extends along the roof perimeter, a professional engineer should be engaged to develop a detailed solution. The solution should address the following:

  • Chimney wall framing adequacy
  • Overturning stability and base shear requirement
  • Adequacy and bracing requirements for roof support members
  • Attachment schedule of chimney structure to the roof structure.

A professional engineer will typically be required to complete the continuous load path retrofit portion of the Advanced Mitigation Package. An engineered solution may involve installing additional metal connectors at the roof level if the side wall framing members are continuous from the bearing wall framing. It may include altering the member size and spacing of roof framing members to support the load from the chimney, installing wood posts at each end of the wall if the chimney side wall framing members start from the top of the supporting wall, or setting posts at the interior side of the wall framing at each corner. A retrofit solution for a masonry chimney will be more difficult than for a wood-frame chimney, and would likely require rebuilding the chimney (at least for the portions above the roof line) (FEMA P-804, 2010).

Inspect the chimney to verify there is flashing where the chimney intersects with the roof. If flashing does not exist, install flashing as described in this guide, ensuring that it is properly connected to the connection between the roof assembly and the chimney is important for controlling rainwater entry during high wind events such as hurricanes. The fundamental principle of waterproofing the connection between the roof assembly and the chimney is to connect the water control layer of the roof assembly to the water control layer of the chimney. If the exterior cladding of the chimney is brick, step flashing, base flashing, and counter flashing should be installed as described in the Description tab.

In high wind zones, a fully adhered roof membrane underlayment should be installed over the roof deck sheathing. This underlayment water control layer needs to be connected to the water control layer of the chimney.

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)
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
Guide describing the requirements by FORTIFIED Home™ for improving the home's resistance in severe thunderstorms, straight-line wind events, and high winds at the outer edges of tornadoes.
Author(s)
Kapur,
Mahadevia,
Park,
Passman,
Perotin,
Reeder,
Seitz,
Sheldon,
Tezak
Organization(s)
FEMA
Publication Date
Description
Report providing guidance on how to improve the wind resistance of existing residential buildings in Mississippi and across the Gulf Coast.
*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.

Building Science Corporation

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