By Dale Nelson and S. Mark James
Replacement of existing metal roofs with a metal-over-metal retrofit roof assembly (instead of removing the existing roof) can provide major structural and sustainable benefits.
With the recent catastrophic damage from Hurricane Michael at Tyndall AFB and elsewhere on the Florida Panhandle, just 82-mi away, a facility at Hurlburt Field that had been given a metal-over-metal roof retrofit suffered no damage—even with Michael’s documented peak wind speed of 157-mph. The existing metal building had been constructed some years ago and needed a new metal roof due to the harsh environment in the locale of Hurlburt. In lieu of removing the existing roof and replacing it, and to minimize disruptions of ongoing activities within the building, the Base Facility Construction Department elected to do a metal-over-metal retrofit.
Metal-over-metal is a re-roofing technique where a new roof is installed directly over a new engineered structural sub-framing system that attaches directly to the existing roof’s support system, without removing the existing metal roof. The most important benefit when using this method is increasing wind resistance of the new roof. Additional benefits include not exposing the building contents to the elements, greater insulation, and safer working conditions.
Extensive Service Life
The U.S. Air Force and other military branches are no strangers to this type of re-roofing approach. The high-performance metal roofs being manufactured today have an extensive service life of 50 or more years, adding to their long-term benefits. Once the metal-over-metal option was approved, Hurlburt Field leadership also knew that it was possible to engineer the new retrofit assembly to be in accordance with the minimum wind uplift design criteria for the area. This “hardening” of the existing roof can apply to walls as well.
Hardening of building roofs is common on metal-over-metal retrofit roofs in coastal states. When existing metal roofs reach the end of their reasonable service life, many older buildings that were engineered for 90-mph to 100-mph wind speeds must be upgraded to minimum public building code requirements. Most buildings in coastal states are required to withstand 120-mph inland and 130-mph for coastal areas. Some parts of Florida and Texas have requirements of 155-mph or greater. U.S. government facilities typically specify criteria that exceed locally adopted codes.
In addition to upgrading the building with the increase in wind uplift resistance, the base also chose to include 3-in of unfaced fiberglass insulation installed between the existing roof and the underside of the new metal roof.
A Proven Approach
The retrofit work was performed by CCI Mechanical LLC and Royster Contracting LLC of Shalimar and Fort Walton Beach, Fla., respectively. (Since 1977, Royster has completed several metal-over-metal retrofit projects using sub-framing products manufactured and tested by Roof Hugger LLC of Lutz, Fla.) Roof Hugger provided 2,700-ft of standard factory-notched sub-purlin system manufactured to fit over 12-in on center purlin bearing ribbed roof panels. Additional sub-framing was required at the roof’s edge and corner zones to satisfy higher wind uplift forces. Atop the sub-purlin system, a white 24-in wide trapezoidal standing seam roof in 24-gauge steel was installed.
The sub-framing/sub-purlin products are designed to “nest” over existing metal roof panels regardless of their configuration and major rib spacing. This design provides for a low-profile retrofit roof assembly. Total depth is dependent of the height of the major ribs of the existing metal roof but can be increased to accommodate greater insulation thicknesses.
Rigid insulation can be employed instead of using fiberglass, as was the case on a larger 11,000-ft2 project at Goodfellow AFB, Texas, which yielded a total roof assembly thermal resistance increase from R-19 to R-50. These systems have withstood hurricane force wind loads from Katrina, Ike, Michael and other storms without a single failure since first introduced nearly 30 years ago. Over 90-million-ft2 of these sub-purlin products have been installed since 1991, with more than 3-million-ft2 of that on U.S. military installations domestically and offshore.
Applying Best Practices
There are several ways that existing metal roofs have been and are still being retrofitted. Many of these methods should not be considered structurally correct sub-framing systems. They simply lack the ability to withstand wind uplift and therefore are subject to failure.
To help building owners, including federal and state governments, withstand severe weather conditions, there are several considerations that should be contemplated for other techniques used in retrofitting metal roofs without removing the existing roof.
- Never install hat sections on top of the major ribs of an existing thru-fastened metal roof. The hat section width will allow only one screw to be installed into the existing purlin while an adjacent screw will be attached to the existing 26-gauge to 29-gauge metal. This does not provide sufficient pull-out strength. Sitting on top of the existing ribs requires a long attachment screw and when exposed to thermally induced movement, the hat sections will “rock” back and forth causing fastener back out and ultimate failure.
- When using wood purlins over existing metal roof ribs, the quantity of fasteners for the new metal roof will have to be increased due to the much-reduced screw pull-out strength into wood, increasing installation labor and material costs. Also, pressure treated wood is corrosive when placed in contact with existing and new metal roofs.
- Stand-off clip system designs over existing metal roof purlins require special attention. The manufacturer of the stand-off clips must have a system to provide required additional sub-framing in the edge and corner zones of the existing roof. These systems are challenged to withstand the increased wind loads in the zones where additional clips may be required between the existing purlins to properly secure the new metal roof panels.
- Sprayed-on coatings are preparation sensitive quick fixes—temporary solutions that fail to permanently remedy the existing roof issues. Coatings provide nothing toward compliance with new stringent building code for wind speeds or snow loads.
When done correctly, a metal-over-metal retrofit that transfers the loads directly to the existing structural members, will increase the strength, energy efficiency and even aesthetics, all while significantly extending the useful life of the building.