5 Tips for Retrofitting with Metal Roofing

August 11, 2010

Why choose a metal roof when retrofitting your building? According to Mark James, a former independent roofing consultant and, more recently, general manager of Houston-based MBCI NuRoof Retrofit Solutions, metal roofing is a durable, energy efficient, and low-maintenance product with an average life expectancy of more than 40 years.

Based on an AIA/CES course he gives to AEC professionals, here are five points James says you must consider before undertaking a reroofing project using a metal roofing system.

1 Identify all technical issues, geometry, and conditions of the existing roof.

You need to have the existing roof thoroughly inspected because rooftop conditions will dictate the new roof's geometry.

The inspector's survey will provide the contractor and the metal roofing system manufacturer with a roof plan. The survey must identify:

Roof perimeter construction, including the gravel stop/fascia and whether or not it will be concealedParapets, noting their height, coping size, and styleLocation of load-bearing and interior firewalls (firewalls will have to be extended through the new roof)Existing drainage systemsLocation of all rooftop equipment (electrical, plumbing, and HVAC) and other obstructions that will have to be worked around or relocated

There are two options for working around existing roof equipment, says James: 1) build over the equipment (when adding a new high-slope roof), or 2) raise the equipment to the new roof level.

Most roofing retrofits are built over the existing equipment, says James. He recommends getting your mechanical engineer involved early on in the project to determine what airflow ventilation is required in the space between the old roof and the new one. And don't forget to provide access to the equipment for maintenance, repair, and replacement.

2 Analyze the existing roof system.

Prepare a checklist of all existing conditions on the old roof. That includes inspecting the underside of the roof and taking note of the type, size, spacing, and span of the existing joists and supports. You also have to be aware of the existing structural support system's ability to take on imposed positive and negative loads, and the building's ability to withstand lateral wind forces created by the new retrofit framing and cladding system. Finally, you must know what collateral loads have been added since the building was first constructed.

3 Know what type of roof is right for your project.

There are four different types of metal retrofit roofs: through-fastened, functional standing seam, structural architectural standing seam, and a pure architectural nonstructural system.

Through-fastened roofs are generic, pre-engineered metal roof systems. These days, the material is more likely to be replaced than installed new, says James.

Functional standing-seam roofs work for low-cost, low-slope applications (low-slope roofs typically range in slope from ½:12 to 2:12). They have trapezoidal-shaped major ribbed seams and install over an open frame system designed for greater rainwater carrying capacity for low-slope applications. As their name implies, these roofs are more utilitarian than aesthetic.

Structural architectural panel systems are the looker, says James. Designed for higher slope applications (typical roof pitch greater than 2:12) over an open framing system, this vertical-ribbed standing seam roof type is specified in applications where aesthetics are important and the roof will be used as an important design element.

Pure architectural nonstructural roofing systems are also designed for high-slope applications, but are specified for the sole purpose of appearance. They look good, but the beauty is skin deep, as the system offers virtually no structural strength and has to be installed over solid wood or metal decking.

4 Make sure you understand the framing and anchoring systems.

Nearly all existing roofs are constructed using a primary and secondary structural grid layout regardless of the support method or type of system employed. Retrofit framing systems use a structural grid that imposes a series of concentrated point loads onto the existing roof. These two grid systems must be matched to ensure the existing roof's structural integrity is maintained.

Adding a new roof on top of an old roof obviously increases the dead weight on the existing roof. To avoid exceeding the design load of the building, you should bring in your structural engineer during the design phase to determine if the existing framing structure needs beefing up. Don't wait until you're into actual construction. Most buildings facing a roof replacement require remedial work, says James. This is especially true for flat roofs, which are designed to consistently and uniformly carry a load, that are being replaced by sloped roofs, and especially in areas with snow loading requirements.

Another word of caution from James: Pay careful attention to the roof valleys. Drift loads increase significantly in the valleys.

With regard to anchoring systems used to secure the new roof system to the existing structure, “It really comes down to the design of the system,” he says. Anchors must be selected based on rooftop pull-out testing, wind uplift values (the recommended calculation is to multiply negative pressure value by a factor of 2.5), and manufacturer specs. Anchor types are based on framing type (wood or steel), thickness of structural concrete support, and moisture content in the existing substrate (anchors should be corrosion resistant).

5 Insulate the heck out of your new roof.

While insulation requirements are code issues (based on ASHRAE 90.1, which 29 states have adopted, according to James), it's important to install insulation over the old roof. Most older buildings have roofs with insulation values of R6-R10, he says. James recommends insulating to at least R-30.

A final note of caution from Mark James: Don't use a vapor barrier in the upgrade because it can trap moisture within the existing roof.