Raised Face vs Flat Face Flange

Raised face (RF) and flat face (FF) are the two most common flange facing types in piping systems. The difference is simple: an RF flange has a small raised area around the bore where the gasket sits, while an FF flange has a completely flat contact surface extending to the bolt holes. This seemingly minor geometric difference has major implications for gasket selection, bolt loading, and system integrity.

When to Use Each

RF flanges concentrate bolt load on a smaller gasket area, creating higher seating stress. FF flanges distribute bolt load across a larger area, resulting in lower seating stress but more uniform compression.

Feature	Raised Face (RF)	Flat Face (FF)
Raised area	1.6 mm (Class 150/300) or 6.4 mm (Class 400+)	None
Gasket type	Spiral wound, ring, or compressed fiber	Full-face gasket
Gasket seating stress	High (concentrated on raised face)	Lower (distributed across full face)
Default per ASME B16.5	Yes (standard for Classes 150-2500)	Only for Class 150 cast iron
Mating flexibility	RF mates with RF	FF mates with FF
Common applications	Process piping (steel flanges)	Cast iron equipment, FRP, lined piping

Specs and Dimensions

The raised face height adds to the overall flange thickness:

Pressure Class	RF Height	Total Flange Thickness Includes RF?
150	1.6 mm (1/16 in)	No—RF height is additional
300	1.6 mm (1/16 in)	No—RF height is additional
400	6.4 mm (1/4 in)	Yes—included in listed thickness
600	6.4 mm (1/4 in)	Yes—included in listed thickness
900-2500	6.4 mm (1/4 in)	Yes—included in listed thickness

Per ASME B16.5-2020. For Classes 150 and 300, flange thickness is measured exclusive of the raised face. For Class 400 and above, thickness includes the raised face.

Common Mistake

Never bolt an RF flange to an FF flange (e.g., connecting a steel pipe to a cast iron pump). The raised face creates a bending moment on the FF flange, which can crack brittle materials like cast iron. Machine the raised face off the steel flange, or use a full-face gasket that extends to the bolt holes to distribute the load evenly.

Gasket Selection

RF flanges use gaskets that sit within the raised face area:

• Spiral wound gaskets (most common for RF)
• Ring gaskets (compressed fiber, graphite)

FF flanges require full-face gaskets that extend to the bolt holes:

• Full-face rubber gaskets
• Full-face PTFE gaskets
• Full-face compressed fiber sheets

Pro Tip

ASME B16.5 ships all steel flanges with raised faces as the default finish. If you need flat-face flanges (e.g., for cast iron pump connections), you must specify FF explicitly on the purchase order. Alternatively, machine the RF down to create an FF surface on site—but only for Class 150.

The surface finish (Ra value) also differs. RF faces typically have a concentric serrated finish with 3.2-6.3 micrometers Ra, optimized for spiral wound gasket seating. FF faces may have a smoother finish suitable for full-face rubber gaskets.

Read the full guide to flanges