Roof Curb — Raised Support for Rooftop Equipment Guide

A roof curb is a raised framed support that lifts rooftop equipment or skylights above the roof surface on a flat or low-slope roof.

Roof Curb diagram — labeled parts, dimensions, and installation context

What It Is

The curb creates a stable, elevated mounting base so equipment can be flashed properly and kept above standing water. It is part structural support, part flashing interface, and part weatherproofing detail.

Types

Common types include equipment curbs for rooftop HVAC units, pipe and fan curbs, skylight curbs, and tapered curbs built to match roof slope. Some are factory-made insulated curbs, while others are site-built to fit the opening.

Where It Is Used

Roof curbs are used on commercial-style flat roofs and some residential low-slope roofs with skylights, rooftop units, large vents, or hatches. They are most often seen where the roof needs to support and waterproof around a heavy or bulky penetration.

How to Identify One

Look for a boxed-up raised frame projecting above the roof field with metal counterflashing or a membrane wrapped up its sides. Soft wood, loose metal flashing, open seams, or ponding water around the curb suggest the support or waterproofing detail needs repair.

Replacement

Replacement may involve rebuilding the curb, reflashing it, or replacing the mounted equipment at the same time. Because the curb interfaces with both the roof membrane and the supported unit, piecemeal repairs need careful coordination.

Roof Curb — FAQ

Why does rooftop equipment sit on a curb instead of directly on the roof?

The curb raises the unit above water and gives the roof membrane a proper vertical surface to flash against. That improves drainage and reduces the chance of leaks around the opening. It also provides a more stable, level mounting base.

Can a leaking roof curb be patched from the outside only?

Sometimes for a short-term repair, but many leaks involve failed membrane details, loose counterflashing, or rotted curb framing beneath the metal skin. If the curb itself is soft or out of square, surface sealant alone usually does not last.

Do skylights use roof curbs too?

Many do, especially on low-slope roofs. The curb raises the skylight above the roof surface so water sheds away from the opening more reliably. Factory skylights often specify minimum curb heights for that reason.

When should a roof curb be replaced?

Replacement is common when the curb framing has rotted, the metal shell has failed, the opening size changes for new equipment, or reroofing exposes poor original construction. If the equipment is being upgraded, it is often smart to evaluate the curb at the same time.