Reroofing with Steep-slope Metal Panel Roof System Over an Existing Low-slope Roof: Part 2

Let’s continue the discussion about converting low-slope roofs to steep-slope metal roofs. Part 1 discussed attachment of framing, the new attic space, ventilation and condensation issues, and drainage.

Before After Retrofit

Reroofing Code Requirements 

Converting a rooftop is a specialized type of reroofing.  The codes specifically allow this via an exception that says “complete and separate roof systems, such as standing-seam metal roof panel systems, that are designed to transmit the roof loads directly to the building’s structural system and that do not rely on existing roofs and roof coverings for support, shall not require the removal of existing roof coverings.”

To meet this code requirement—and to not have to remove the existing roof system—the loads must bypass the existing roofing system. This is critical to create a load path from the new structure to the existing structure for dead loads, snow loads, rain loads, and uplift (e.g., wind) loads.

Structural Loads & Wind Resistance

IBC’s Chapter 16, Structural Design, includes all the required information and design methods to determine the dead, snow, wind, and rain loads acting on the building.  The new framing members and their connections, as well as the new metal panels and their attachments to the new framing, must be able to resist the loads acting on the building.  The resistance must exceed the loads.  Most often, wind resistance loads control the design.  Manufacturers and structural engineers should be consulted for material specifics and fastener requirements.

Fire Resistance

Fire resistance for a converted roof needs to meet the requirements of the model codes.  Check with manufacturers for fire classification of the system installed, and ensure it meets the minimum class (A, B, or C) required in the project location.  See the blog “Fire resistance of metal panel roof systems” for more information.

Insulation

For all types of reroofing, the most recent insulation requirements need to be met.  In most cases, additional insulation will be necessary.  Insulation can be placed at the attic floor (i.e., on top of the existing low-slope roof) or directly under the new metal panels.  Where the new roof meets the wall is very important for continuity of the overall building envelop insulation; lack of continuity is energy inefficient and may be a point of condensation.  The location of the new insulation needs to be coordinated with the ventilation plan and condensation potential should be considered. See Part I for more information.

While reroofing with metal can be an aesthetic improvement and solve leak issues, structural loads and wind resistance, fire resistance, and insulation requirements are necessary considerations when converting from a low-slope roof to a steep-slope metal panel roof system.  Don’t overlook the basic code requirements, or the need to deal with heat, air, and moisture issues of the new attic space.

Reroofing and the Building Code

Reroofing is and always will be the predominant project type in the roofing industry.  Roughly 70-90% of all roofing projects (depending on the year) are performed on existing buildings.  Understanding the reroofing requirements in the building code is critical to proper design and construction.  And fortunately, the reroofing requirements are not all that complicated.International Building Code

The 2015 International Building Code, Section 1511, Reroofing provides the building code requirements when reroofing.  Reroofing projects are divided into two types: recovering and replacement (which includes full removal of the existing roof).

Metal panel reroofing projects must meet the same fire, wind, and impact requirements for roof systems for new construction; however, they do not need to meet the minimum slope requirements (¼:12 for standing seam; ½:12 for lapped, nonsoldered and sealed seams; 3:12 for lapped, nonsoldered, non-sealed seams) if there is positive drainage.  Also, reroofing projects do not need to meet the secondary drainage requirements (i.e., installation of emergency overflow systems is not required).

The requirements for metal panel and metal shingle roof coverings are in Section 1507.4, Metal roof panels and Section 1507.5, Metal roof shingles of the 2015 IBC.  These apply for new construction and reroofing, and include information about decks, deck slope, materials, attachment, underlayment and high wind, ice barriers, and flashing.  The 2012 IBC has the same requirements; the 2015 IBC added new language about deck slope and attachment requirements for metal roof panels.  Nothing was changed for metal roof shingles.

In general, recovering is only allowed if there is one existing roof in place, except if a recover metal panel roof system transmits loads directly to the structural system (bypassing the existing roof system).  This provides a great advantage for metal panel roofs!  The existing roofs do not need to be removed, but new supports need to be attached through the existing roof (typically a metal panel roof) directly into existing purlins.

If metal panels or metal shingles are installed over a wood shake roof, creating a combustible concealed space, a layer of gypsum, mineral fiber, glass fiber, or other approved material is required to be installed between the wood roof and the recover metal roof system.

Good roofing practice is codified in the reroofing section of the IBC; contractors who design and install a recover or replacement metal roof are legally required to follow locally adopted code requirements.  And, of course, all metal roofs must be installed according to the manufacturer’s approved instructions.

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