Curb and gutter is one of the most ubiquitous elements in site civil design, and it is also one of the elements that generates the most questions from architects and developers who do not work with it daily. Rolled curb, barrier curb, mountable curb, Type A, Type B, extruded versus formed — the terminology varies by jurisdiction, and the selection affects drainage, accessibility, parking geometry, and fire access.

This article explains the common curb and gutter types, where each is used, and the design considerations that drive the selection.

Barrier Curb (Vertical Curb)

Barrier curb, also called vertical curb or Type A curb in many jurisdictions, has a vertical or near-vertical face (typically 6 inches high) with a 6- to 12-inch-wide gutter pan at the base. The face is steep enough that a vehicle tire cannot ride over it at normal speeds.

Standard dimensions: 6-inch face height, 18-inch total depth (face plus gutter pan). The face may have a slight batter (1/4 inch per inch or less). The gutter pan is typically 12 inches wide with a 2 to 5 percent cross-slope toward the gutter flow line.

Where it is used:

  • Public streets — the standard curb type for most urban and suburban streets
  • Parking lot perimeters — where vehicles must be kept on the pavement
  • Pedestrian zones — where the curb protects sidewalks from vehicle encroachment
  • Around landscaped medians in parking lots
  • At accessible ramp locations (where the curb returns provide the side flares)

Drainage: The gutter pan collects and conveys stormwater along the curb line to drain inlets. The longitudinal slope of the gutter should be at least 0.5 percent (1 percent preferred) to move water. At sag points (low points in the gutter grade), a drain inlet is required.

Rolled Curb (Type C or Rolled)

Rolled curb has a rounded profile that vehicles can drive over at low speeds. The face height is typically 4 to 6 inches, and the transition from pavement to curb to gutter is a smooth curve rather than a vertical face. Some jurisdictions call this Type C curb or mountable curb, though "mountable" can also refer to a separate profile.

Where it is used:

  • Residential subdivision streets — many jurisdictions specify rolled curb as the standard for residential streets to allow driveway access without a separate curb cut
  • Parking lot drive aisles — where the design allows vehicles to cross the curb into landscape areas for parking or turning
  • Low-speed areas where the aesthetic of a softer curb profile is desired

Advantages: Lower cost to install (can be extruded by machine, whereas barrier curb is often formed and poured). Allows driveway access without formal curb cuts, which reduces construction cost on residential streets. Easier to replace or repair.

Disadvantages: Does not provide a positive barrier to vehicle encroachment. Vehicles can and do drive over rolled curb, which damages landscaping, irrigation, and sidewalk edges. Not appropriate where pedestrian protection is needed. The rounded profile is more difficult to design accessible curb ramps at, because the curb reveal changes gradually rather than at a defined face.

Mountable Curb

Mountable curb is designed to be driven over by specific vehicle types, typically fire apparatus. It has a sloped face (typically a 3:1 or 4:1 slope) with a 4- to 6-inch height. It is low enough and gradual enough that a fire truck can mount it without damage to the vehicle or the curb.

Where it is used:

  • Fire access routes where the fire code requires drive-over access but the site design includes a curb line (for example, a landscaped median in a fire lane)
  • Emergency vehicle access across landscape buffers
  • Medians in cul-de-sacs where fire apparatus must drive over the center island

Key requirement: When used for fire access, the surface behind the mountable curb must be designed to support the fire apparatus loading (typically 75,000 pounds GVW). This means structural pavement or reinforced turf under the landscape, not just topsoil and sod.

Curb and Gutter vs. Curb Only vs. Edge of Pavement

Not every condition requires curb and gutter:

  • Curb and gutter — provides both vehicle containment and stormwater conveyance. Standard for urban and suburban streets, commercial parking lots, and anywhere sheet flow must be collected and directed to inlets.
  • Curb only (no gutter pan) — sometimes used at landscape borders where drainage is handled by sheet flow into landscape areas. The curb provides vehicle containment but no defined flow path. Less common but seen in parking lot medians.
  • Edge of pavement (no curb) — rural roads, some industrial sites, and areas where stormwater is managed by roadside ditches or sheet flow to adjacent vegetated areas. Common in the Pacific Northwest and in agricultural/industrial zones. The pavement edge is typically supported by a compacted shoulder.

Curb Cuts and Driveway Approaches

A curb cut is a depression in the curb that allows vehicle access from the street to a driveway. The design includes:

  • Wing height — the curb height at the transition point, typically ramped down from full height to 1 inch or flush over a distance of 3 to 5 feet.
  • Driveway approach — the concrete apron between the curb line and the property line. Width varies by use: residential driveways are typically 12 to 24 feet; commercial driveways are 24 to 40 feet. The approach cross-slope must not exceed 2 percent at accessible routes.
  • Sight distance — the curb cut must be located where there is adequate sight distance for vehicles entering the street. Minimum sight distance is based on the street's design speed and varies by jurisdiction.

Most public works departments have standard details for driveway approaches that specify every dimension, radius, and slope. Use the local standard. Do not design a custom approach unless the site geometry absolutely requires it, because non-standard approaches trigger additional review and potential rejection.

Construction Methods

Formed and Poured

Concrete is placed in forms (typically steel or wood) and finished by hand. The forms define the curb profile exactly. This method produces a precise, consistent profile and is required for barrier curb in most public works specifications. Slower and more expensive than extruded curb.

Extruded (Slip-Form)

A machine extrudes the concrete curb profile continuously as it moves along the alignment. Faster and less expensive than formed curb. The profile is consistent but less precise. Extruded curb works well for rolled curb, mountable curb, and long straight runs of barrier curb. It does not work well for tight radius curves (less than about 50-foot radius), complex profiles, or conditions requiring integral doweling to adjacent slabs.

Selection Checklist

  • Is pedestrian protection needed? Use barrier curb.
  • Is fire access across the curb required? Use mountable curb rated for fire apparatus.
  • Is this a residential street with individual driveways? Rolled curb may be the local standard and saves cost.
  • Is this a public street improvement? Check the local standard details. You will use whatever they specify.
  • Does the curb need to convey stormwater? Use curb and gutter with adequate longitudinal slope.
  • Is ADA compliance required at the curb? Barrier curb with standard ramp details. Rolled curb requires different ramp geometry.