Can You Mate RF and FF Flanges?

If you’ve ever wondered: can you mate RF and FF flanges? 

This is a common question in the field—and the short answer is: technically yes, but rarely recommended. Here’s why.

Why Mixing RF and FF Flanges Is Problematic

RF flanges have a raised sealing surface that compresses the gasket, concentrating pressure over a smaller area. That’s great for high-pressure systems. 

FF flanges, on the other hand, use a flush surface that spreads gasket contact evenly—ideal for brittle materials like cast iron.

When you try to mate them, the RF’s raised face sits on the FF’s flat surface. This mismatch leads to uneven gasket compression and potential leakage. The seal isn’t properly centered, and parts of the gasket are either over-tightened or not compressed enough.

Alignment and Pressure Concerns

A correct flange connection requires perfect face-to-face alignment, bolt-hole matching, and gasket compatibility. Mixing RF and FF throws all this off:

• Uneven pressure across the gasket, which compromises sealing
• Mechanical stress on the raised face—risking fatigue or flange distortion
• Bolt torque variation, leading to stress points or gasket blowout

Even if it seems to fit, long-term performance and safety are at risk.

When are exceptions possible? 

In low-pressure, non-critical setups—like certain utility lines—engineers sometimes pair RF and FF flanges with a soft, forgiving gasket. It can hold temporarily, but it’s a workaround, not a best practice. For most B2B industrial applications, consistent face types are critical for meeting compliance and uptime goals.

Best Practice: Stick to Matching Faces

To maintain a safe, efficient, and leak-free system, the best move is to always match flange face types. Using the same type on both sides—either RF-to-RF or FF-to-FF—ensures that the gasket is compressed evenly, the load is distributed properly, and the connection holds up under pressure.

• Raised Face to Raised Face (RF-to-RF):
  This is the most common and recommended pairing in industrial systems. RF flanges concentrate gasket load, making them ideal for high-pressure or high-temperature applications. Using two RF flanges ensures the raised surfaces meet properly and the gasket performs as designed.
• Flat Face to Flat Face (FF-to-FF):
  FF flanges are preferred in systems that use brittle materials like cast iron or PVC, where gasket compression needs to be more uniform to prevent cracking. FF-to-FF also works well in water supply and utility systems with low operating pressure.
• Avoid Mixing RF and FF:
  Mixing these face types means the raised section of the RF flange doesn’t have a matching surface to compress against. This results in an uneven seal, risking leaks, flange damage, or even system failure. It’s not compliant with most industrial standards and may cause inspection failures or maintenance downtime.

Can You Mate RF and FF Flanges: Final Takeaway

Technically, RF and FF flanges can be bolted together, but that doesn’t mean you should. Unless you’re working in a benign environment with a soft gasket—and have engineer sign-off—stick to matching flange-face types. Longan Flange offers a full range of certified stainless steel flanges and ASME‑standard RF/FF flanges, ensuring your system specs are tight and clear. Reach out to us today to know more!