Installing a new rooftop curb on an existing commercial roof requires careful flashing to prevent leaks and preserve the roof system’s long-term performance. Whether the curb is for an HVAC unit, exhaust fan, skylight, or other rooftop equipment, the flashing must create a continuous, watertight transition between the new curb and the existing roofing membrane.
The exact installation method depends on the membrane type, curb design, and manufacturer requirements, but the following principles apply to most commercial roofing systems.
1. Inspect and Prepare the Existing Roof
Before installing the new rooftop curb, inspect the existing membrane and surrounding roof assembly. The area should be free of ponding water, loose membrane, damaged insulation, dirt, and other conditions that could interfere with adhesion or flashing.
The curb should be positioned so that it does not obstruct existing drainage paths. If the existing roof has tapered insulation or other slope considerations, the curb location should be evaluated carefully to prevent water from collecting around it.
2. Install the Curb Correctly
The new curb should be properly secured to the structural roof assembly according to the equipment manufacturer’s and roofing system requirements. The curb must be level, stable, and designed to provide sufficient height above the finished roof surface.
A common mistake is installing a curb too low. Adequate curb height helps reduce the risk of water entering the equipment opening during heavy rain or drainage problems. Local building codes and the roofing manufacturer’s requirements should be followed when determining the appropriate curb height.
3. Prepare the Membrane Around the Curb
The existing membrane around the new curb should be cleaned and prepared according to the membrane manufacturer’s instructions. Depending on the roofing system, this may involve removing contaminants, lightly cleaning the surface, applying primer, or preparing the membrane for heat welding or adhesive bonding.
The goal is to create a clean, compatible surface that allows the new flashing materials to bond securely with the existing roof membrane.
4. Install Compatible Flashing
The curb should be flashed using materials compatible with the existing roofing system. For example, a TPO roof generally requires TPO-compatible flashing, while PVC, EPDM, and modified bitumen systems require their respective approved flashing methods and materials.
For thermoplastic membranes such as TPO and PVC, flashing is commonly heat-welded to the existing membrane to create a continuous watertight seam. EPDM systems may use compatible uncured or cured flashing materials and approved adhesives or tapes, depending on the application.
Flashing should extend sufficiently onto the existing membrane to provide a reliable bond or weld. Corners are particularly important because they are common areas for stress and potential water entry.
5. Detail the Corners and Transitions
The curb corners should receive careful detailing rather than relying on a simple flat piece of membrane. Prefabricated corners or manufacturer-approved corner patches can help provide consistent waterproofing.
Any fasteners, seams, penetrations, or transitions around the curb should be sealed or flashed according to the roofing system specifications. The finished flashing should have no gaps, fishmouths, open seams, or areas where water can collect.
6. Inspect and Test the Finished Flashing
After installation, the flashing should be inspected carefully. On heat-welded membranes, seams should be checked for proper weld quality. Depending on the membrane system and project requirements, seam testing or other quality-control procedures may be appropriate.
The finished curb flashing should form a continuous waterproof transition from the curb to the existing membrane.
Key Takeaway
A new rooftop curb should not simply be placed on top of an existing commercial roof and sealed around the edges. Proper installation requires compatible membrane flashing, properly prepared surfaces, secure curb attachment, reinforced corners, watertight seams, and thorough inspection.
Because roofing systems differ significantly, the flashing method should always follow the existing membrane manufacturer’s specifications and applicable building-code requirements. For commercial properties, having a qualified roofing professional perform or inspect the work can help prevent premature leaks, membrane damage, and costly roof repairs.
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