SpecTopics: Rooftop Mechanical Equipment Securement and the Building Code

A typical rooftop can have several types of equipment present, such as HVAC units, telecommunications equipment, or solar photovoltaic (PV) arrays. This equipment can also be fed by electrical conduits, natural gas service lines, or signal cabling which runs across the roof instead of inside the building. This results in the use of permanent curbs, ballasted arrays/supports, and floating sleepers. The acceptability of floating sleepers and ballasted arrays has always been one of caution. Use slip sheets to protect the roof from the movement of the sleeper or array. The movement can cause abrasions and possible cuts in membranes during high wind events or maintenance. Storms may shift equipment completely off sleepers or move arrays several feet across the roof, or even worse, off the roof entirely.

To address lack of standards on the roof, the 2018 International Building Codes (I-Codes), which includes the International Mechanical Code (IMC), have new requirements for wind resistance of rooftop equipment. The 2018 IMC governs the design, construction, erection, and installation of mechanical appliances and states:

International Mechanical Code
301.15 Wind resistance
Mechanical equipment, appliances, and supports that are exposed to wind shall be designed and installed to resist the wind pressures determined in accordance with the International Building Code.

The 2018 IMC also addresses seismic resistance stating:

301.18 Seismic resistance
Where earthquake loads are applicable in accordance with the International Building Code, mechanical system supports shall be designed and installed for the seismic forces in accordance with the International Building Code.

Additionally, it states that all wind loads are determined by American Society of Civil Engineers (ASCE) ASCE 7-16 calculations for loads. ASCE 7-16 has incorporated rooftop structures and equipment into their calculations.

Along with IBC, Factory Mutual (FM) has also created FM 4481 – “Anchors for Roof Mounted Equipment” which is their version of a rooftop attachment standard, to be used on FM Insured Buildings.

Why the Change?

Ballasted Arrays

• Did not allow for fixed securement, resisting load not always calculated properly to resist wind pressures.
• Friction from movement of array can cause abrasions and possible cuts/leaks in roofing materials.

Equipment Sleepers (Wood/Rubber/Plastic)

• Sleepers did not always last the service life of the unit being supported.
• Can block free flow of water for proper roof drainage. May block snow drifts on the roof.
• Friction from movement of sleepers can cause abrasions and possible cuts/leaks in roofing materials.

Pipe Supports (Wood/Rubber/Plastic)

• Treated wood not always compatible with fasteners or pipe materials being used (i.e. corrosion).
• Can block free flow of water for proper roof drainage. May block snow drifts on the roof.
• Not always sized to prevent upward or lateral movement.
• Friction from movement of sleepers can cause abrasions and possible cuts/leaks in roofing materials.

Benefits of Mechanically Attached Anchors and Anchoring Devices

• Positive wind resistance has been calculated for the equipment being supported.
• Mechanically attached anchors mean no movement along the roof during high winds or storms.
• Mounting anchor service life matches roof membrane and allows for better flashing and tie-in to roofing membrane.

When specifying your next roof, utilize fixed curbs and pre-manufactured equipment supports for positive connection to the roof structure. There are several manufacturers that offer rooftop anchor systems that come with pre-applied flashing materials. The flashing material is manufacturer-specific to ensure compatibility with the roof system and membrane type. When installed by an Authorized Applicator, the flashing is often eligible for coverage under the roofing warranty, providing added peace of mind.

For more information on anchors for solar PV mounting, check out Carlisle’s Solar Ready Roof Brochure.