The Resilient Roof Curb

Photo 1: Roof damage after a storm. Thank goodness the conduit is still attached to the RTU so it didn’t blow off the roof. Images: Hutchinson Design Group Ltd.

Resiliency is the buzzword for this decade. Designing resilient roof systems, in my estimation, will become a standard and make its way into the codes by 2030 or before. This is the second in a series of articles based on experience and observations following extreme climatic events on how I have designed resilient roofs and/or how I would suggest various components of the roof be designed for resiliency. In this article we will look at roof exhaust curbs, typically used to support mechanical equipment. The goal is to prevent the units and/or curb from being blown out of place and across the roof. (See Photos 1 and 2.)

What are the qualities that make a resilient roof curb? This is the first question you are now thinking, so I will tell you. Resilient roof curbs should:

  • Be tall enough to be at least 4 inches above the top of the highest point of overflow drainage.
  • Be of solid and robust construction.
  • Be anchored to the roof structure.
  • Secure the unit to the curb.

There you go, go to it.

For those of you who wish a little more information, let explain.

Appropriate Height

The reason for the height is based on experience. The best way to explain this is by example. A client remained in the building during Hurricane Maria. During the storm, she opened the roof hatch and took a photo of the roof, which she sent to me. Upon viewing the photo, I thought it was the ocean. There was water as far as I could see, and there were waves and whitecaps. The drains and small roof edge scuppers had clogged with palm fronds and other debris. The water was over 10 inches in depth. Seeing that visual, I couldn’t believe the roof structure didn’t collapse. (The building was designed for Class 5 hurricanes and was very robust.) Perhaps it would have collapsed had it not been for the low roof curb height and the fact that all the curbs acted as drains once the water gained enough height. The water damaged high-value products in the building’s interior.

Photo 2: Units that leave the curb not only allow water into the interior, but units cartwheeling across the roof will damage the roof with every corner.

The scuppers should have been much larger to prevent blockage, but if the curbs had been higher than the roof edge, the millions of dollars of destroyed goods could have been saved.

Note: With so much damage to surrounding buildings, there is some thought that the water depth on this particular roof provided ballast weight to the roof and prevented wind-related roof damage from occurring. Something to ponder as a defensive option to storms with high winds.

Robust Construction

The construction of the curb is important, in that it not only needs to support the equipment on top but also to take the loads imposed on it by wind, water, snow, sliding ice, etc. The curb is recommended to be of 16-gauge metal, of fully welded construction. It should be insulated and have a metal liner of the same gauge as the exterior of the curb. For long curbs, internal reinforcing is recommended. We recently stopped specifying curbs with wood blocking at the top, an apparent holdover from BUR that needed to be nailed off. The advancement in self-tapping screws make deleting this weak link possible.

Anchorage to the Roof Structure

Keeping the curb attached to the building during storms seems like an obvious goal. The height of the equipment on the curb will determine its overturning potential; the taller the unit, the greater the overturning moment. Thus, I suggest that the curb opening be framed in steel (on steel roof structure with steel decks) as designed by the structural engineer. Coordination with other professionals involved with the building’s design is critical. The curb should be bolted to the steel framing and nuts and washers used; I suggest 16 inches on center. If a linear void exists between the steel framing and the steel deck, it should be infilled with solid dimensional lumber and sandwiched when bolted.

Securing the Unit to the Curb

Rooftop equipment blows off curbs all the time, and often part of the units, typically hoods, blow off. Sharp metal objects blowing across the roof possess a threat to the integrity of the roof and those who may be on the roof. When it is carried over the roof edge, it becomes a life safety threat!

To prevent these failures, the units need to be well secured to the curb and often strapped down. This is a major reason for the robust curb. Exhaust fans typically arrive at the construction site with predrilled pilot holes in the side flanges — often only one per side. When the curbs are 2 feet or greater in length, additional pilot holes should be drilled so that the fasteners are approximately 10 inches on center. The screws should be self–tapping stainless steel, 1/4 inch with stainless steel-clad EPDM washers.

In very high wind conditions such as hurricane-prone regions, it might also be prudent to strap the unit with 1/4-inch stainless steel stranded/twisted aircraft control cable and secure to the unit and curb with stainless steel through bolts, lock washers and bolts with the interior threads deformed to prevent harmonic vibration from loosening the nuts. (See Figure 1.)

Figure 1: At a minimum, roof curbs should be bolted to structural steel and the units above strapped to the heavy-gauge metal curb.

It is hoped that in the near future, manufacturers of the curbs will have these additional support items available as an option.

Achieving Resiliency

Roofs are holistic and their surface is the sum of all their parts. Keeping the roof equipment in place during climatic events is needed to prevent the roof’s failure and interior damage. Roof system designers are encouraged to detail roof curbs and unit attachment — and then specify the correct materials and execution.

This is one more step as we build the resilient roof.

About the author: Thomas W. Hutchinson, AIA, CSI, Fellow-IIBEC, RRC, is a principal of Hutchinson Design Group Ltd. in Barrington, Illinois. For more information, visit www.hutchinsondesigngroup.com.