A header curb means two different things depending on the trade. On a roof, the header curb is the upslope (high) side of a roof curb, the raised metal frame that supports rooftop equipment, skylights, and hatches and keeps water out of the opening. In concrete and paving work, a header curb is a formed edge curb cast across the end of a pavement run. This guide covers the roofing meaning in full, then draws a clean line to the concrete version so you land on the right answer.
What is a header curb?
In roofing, a header curb is the top, upslope edge of a roof curb: the side that faces uphill and takes the brunt of running water. The roof curb itself is a raised frame, usually galvanized steel or aluminum, that surrounds a roof opening and gives rooftop equipment a level, watertight base. The header side is the leak-critical side, which is why detailing there matters most.
- Roofing sense: the upslope face of a roof curb (also called the back or high side), where water is diverted around the opening.
- Concrete sense: a curb formed across the end of a pavement section, acting as an edge restraint. This is site and paving work, not roofing.
Header curb vs roof curb: where the header sits
A roof curb has four sides. The header curb is the single upslope side, the downslope side is the apron, and the two sides parallel to the slope are the cheeks. Naming the header side matters because roof slope sends water toward it first, so it carries the flashing detail and, on wider units, a diverter. The rest of the curb sheds water more easily.
| Part of the curb | Position | Job it does |
|---|---|---|
| Header (high side) | Upslope | Takes running water first; often needs a diverter or cricket |
| Apron (low side) | Downslope | Sheds water off the curb; laps over the roof panel or membrane |
| Cheeks (sides) | Parallel to slope | Carry side flashing that ties into the field of the roof |
What a roof curb does
A roof curb raises and supports rooftop equipment above the roof surface while keeping the opening watertight. It transfers the load of a heavy unit onto the roof structure, frames the cutout for ductwork or wiring, and gives the flashing a vertical surface to terminate against. Without a curb, a rooftop unit sits directly on the membrane and almost always leaks.
Curbs are standard under packaged HVAC rooftop units (RTUs), exhaust fans, skylights, smoke hatches, and access hatches. On a replacement, the supply and return duct openings in the curb rarely line up with a new unit, which is where a curb adapter comes in.
Roof curb types
Roof curbs are sized and shaped to the equipment and the roof they sit on. The common families are straight-sided, canted, pitched (self-leveling on a sloped roof), vented, and sound-attenuating, plus curb adapters used to mate a new unit to an existing curb. Pitch and profile are the two choices that most affect whether the curb drains and seals correctly.
| Curb type | Best use | Notes |
|---|---|---|
| Straight-sided (flanged) | Flat and low-slope roofs | Level top, integral base flange for membrane roofs |
| Canted | Built-up and single-ply | Sloped base edge eases the membrane transition |
| Pitched (self-leveling) | Sloped and metal roofs | Tapered so the top sits level on a pitched deck |
| Vented | Fans and exhaust equipment | Built-in louvers or dampers |
| Sound-attenuating | Noisy RTUs near occupied space | Insulated and lined to cut vibration and noise |
| Curb adapter | RTU replacement | Transitions old curb dimensions to a new unit |
What roof curbs are made of
Most prefabricated roof curbs are 14-gauge galvanized steel or 0.080-inch aluminum, welded at the corners for a continuous seal. Many come with 1-inch foil-faced rigid insulation rated at about R-6.5 to cut condensation and heat transfer at the opening. The gauge matters because RTUs can weigh several hundred to a few thousand pounds, and the curb carries that load.
Curbs on standing seam metal vs membrane roofs
A curb built for a flat single-ply or built-up roof can leak badly on a standing seam metal roof, so the curb has to match the roof system. On a metal roof, the curb installs under the roof panel on the upslope (header) side and over the panel on the downslope side, which creates a proper watershed. On membrane roofs, an integral base flange is stripped in with the roofing membrane instead.
For a standing seam roof, the curb should be built for the specific panel profile and seam height. Side flanges that nest seam to seam are what make the assembly watertight, so a generic flat-roof curb is the wrong part.
Do you need a cricket behind the header curb?
Yes, on wider curbs. A cricket is a small peaked diverter built on the upslope (header) side of a curb to split water and steer it around the opening. Industry practice, following NRCA and SMACNA guidance, is to add a cricket or saddle on the high side of any curb wider than about 24 inches measured across the slope. This mirrors the IRC R1003.20 rule that requires a cricket for masonry chimneys more than 30 inches wide.
The header side is where ponding and ice tend to start, so on metal and low-slope roofs the diverter is not optional detailing. For sizing, slope, and build sequence, see the dedicated guide on roof cricket design, which applies directly to the header side of a curb.
How a roof curb is installed
A roof curb is set, structurally fastened, then flashed into the roof so the opening is sealed. The order matters: flashing done before the curb is anchored and squared will not stay watertight. A typical sequence looks like this.
- Frame and square the rough opening in the deck, sized to the curb and the ductwork or unit.
- Set the curb level over the opening, checking that a pitched curb reads level on top.
- Fasten the curb to the structural members, not just the deck, to carry the equipment load.
- Flash the header (upslope) side first, lapping under the roof panel or stripping the base flange into the membrane.
- Flash the cheeks and then the apron so every lap sheds downhill.
- Add a cricket on the header side if the curb is wider than about 24 inches across the slope.
- Set the equipment, connect supply and return ducts to the curb openings, and seal penetrations.
The weatherproofing logic is the same as any roof penetration: upslope laps over downslope, and vertical surfaces get counter flashing. The broader system is covered in the guide to roof flashing and the detail-level walkthrough of step and counter flashing.
Roof curb cost
A prefabricated roof curb commonly runs from about $250 to $2,000 depending on size, pitch, and roof type, before labor. Straight-sided curbs for flat roofs sit at the low end, while pitched or standing seam metal curbs and sound-attenuating models cost more. A curb adapter for an RTU swap typically falls in the $400 to $1,500 range. Prices vary by region, fabricator, and whether the curb is custom.
| Item | Typical range (material only) |
|---|---|
| Straight-sided curb (flat roof) | $250 to $900 |
| Pitched or standing seam metal curb | $600 to $2,000+ |
| Sound-attenuating curb | $900 to $2,500+ |
| Curb adapter (RTU replacement) | $400 to $1,500 |
Rooftop equipment curbs sit on flat and low-slope roofs, so the surrounding system choice matters as much as the curb. Compare the assemblies in the overview of low-slope roof systems. For a specific rooftop unit example, the walkthrough of a rooftop evaporative cooler curb and flashing shows the same details on a smaller unit.
The concrete header curb and curb and gutter
In concrete and site work, a header curb is a curb formed across the end of a pavement run or at a transition, acting as an edge restraint that locks the pavement edge in place. It is different from curb and gutter, which is the continuous curb plus a flat gutter pan cast along a roadway to carry stormwater. State transportation standards, such as TxDOT detail CCCG-21 and GDOT and VA curb details, define these profiles.
Roofers do not build concrete header curbs or curb and gutter; that is civil and paving work. If your search was about roadway or driveway edging, a site concrete or DOT standard is the right source, not a roofing guide.
Header curb FAQ
What is a header curb on a roof?
On a roof, the header curb is the upslope (high) side of a roof curb, the raised metal frame that supports rooftop equipment and seals the opening. Because roof slope drives water toward it first, the header side carries the main flashing detail and, on wider curbs, a cricket to divert water around the opening.
Is a header curb the same as a roof curb?
Not exactly. The roof curb is the whole raised frame around a roof opening. The header curb is one part of it: the upslope side. People often use header curb loosely to mean the roof curb assembly, but strictly it names the high side where water is diverted and the primary flashing terminates.
How tall does a roof curb need to be?
A roof curb usually stands at least 8 inches (about 200 mm) above the finished roof, the minimum flashing height most single-ply membrane manufacturers and the NRCA specify. Taller curbs may be required for snow regions or deep insulation. Exact height depends on the roof system, local code, and the manufacturer specification.
Do you need a cricket behind a roof curb?
Usually yes for wider curbs. Practice following NRCA and SMACNA guidance is to add a cricket or saddle on the upslope side of any curb more than about 24 inches across the slope. This diverts water around the opening and prevents ponding and ice on the header side. Narrow curbs may not require one.
What is a curb adapter?
A curb adapter is a transition piece that lets a new rooftop unit sit on an existing roof curb when the dimensions and duct openings do not match. It saves reroofing around the opening during an RTU replacement. Adapters are custom-made to bridge the old curb footprint to the new unit connection points.
What is a concrete header curb?
A concrete header curb is a curb formed across the end of a pavement section, acting as an edge restraint that holds the pavement edge. It is a paving and site-work element, not roofing, and differs from curb and gutter, which combines a curb with a flat gutter pan along a roadway. State DOT standards define the profiles.
Reviewed by The Roofing Brief Team. Last reviewed July 2026.