Flat gasket, fibre washer or O-ring —
how to tell the difference
They are all sealing components. They are not interchangeable in normal service. Fit the wrong type back in and the joint may leak even if everything else is done correctly.
Why this matters before you start
Many heating and plumbing leaks get diagnosed on site, in the middle of a job. The joint is already apart. The original seal is on the floor somewhere, or still stuck to the mating face in pieces. Now you need to know what to replace it with.
If you identify it wrong, you buy the wrong part. The joint goes back together. It leaks again. Or it holds for two weeks and then fails when the system goes back under load.
Knowing what you are looking at before the joint comes apart is better. Knowing what you are holding after it comes apart is the minimum. This guide covers both.
This is not a general seal-design comparison — for that, see Flat gasket vs O-ring. It is an identification guide for service situations where a small sealing part has been removed from a plumbing, heating or equipment joint and must be replaced correctly.
The short version: flat gaskets and fibre washers sit between flat mating faces and seal by compression. O-rings and rubber seals sit in grooves or recesses and seal by being squeezed radially or axially. Different geometry, different replacement, different failure mode. Not the same component.
The four types — what each one is
A flat gasket is a ring — or sometimes a sheet cut to shape — that sits between two flat mating faces. It seals by compression: when the joint is tightened, the gasket compresses between the faces and closes any micro-imperfections in the metal surface.
Flat gaskets are made from fibre composite materials — cellulose, aramid, graphite — bonded with rubber compounds. They are rigid enough to hold shape but compressible enough to conform to the surface under bolt load. They behave differently from rubber seals: they are designed for face compression, not groove squeeze.
In heating and plumbing, flat gaskets appear on pump flanges, heat exchanger connections, valve bodies, boiler flanges and any joint where two flat surfaces bolt or screw together with a seating face. This is the type of flat-face sealing application covered by the Kinetics Line flat gasket range, with material choice depending on medium, temperature, pressure and documentation requirements.
Flanged pump connections, heat exchanger headers, valve flanges, boiler primary connections, threaded BSP fittings with flat seating faces
Joints with grooves or recesses designed for O-rings. Compression fittings. Anywhere the seal relies on radial squeeze rather than face compression.
A fibre washer looks like a small flat gasket — and it functions on the same principle. The difference is scale and application. A fibre washer is usually smaller and lighter-duty, used in many threaded flat-face fittings — particularly BSP threaded connections on radiator valves, taps, flexible hoses and similar domestic and light commercial fittings.
A fibre washer is not the same thing as the rubber washer inside a tap. The name causes confusion — "washer" covers both, but the materials and failure modes are different. Fibre washers compress and set. Rubber washers compress and recover. They are not interchangeable even when the size matches.
The failure mode is the same as a flat gasket: compression set over time, degradation under sustained heat, and failure if the wrong material is used for the medium. A fibre washer that has been under load for years will not reseal. In service work it should normally be replaced when the joint is opened, especially if it has taken a compression set or surface damage.
Flexible hose connections, radiator valve unions, tap connectors, small BSP threaded fittings, garden hose fittings, washing machine hose ends
High-pressure industrial fittings. Flanged joints. Anywhere the fitting geometry requires a larger seating area or higher compression load.
An O-ring is a rubber ring with a circular cross-section — it looks like a small tyre in profile. It does not seal by face compression. It seals by being compressed in a groove or recess when the joint is assembled. That compression deforms the rubber slightly, and that deformation creates the seal.
O-rings are designed for specific groove dimensions. The groove depth, width and surface finish are part of the sealing system — an O-ring the wrong size for the groove will either not seal or extrude under pressure. You cannot substitute a different size O-ring and expect it to work correctly.
In heating and plumbing, O-rings appear inside valve bodies, on cartridges, on push-fit fittings, on pump seal housings and on any connection where the fitting has a visible groove or recess. Some fittings include internal O-rings in the body or nut assembly, separate from any washer at the end.
Valve cartridges, push-fit fittings, pump seal housings, mixer tap bodies, pressure relief valves, expansion vessel connections, threaded fittings with internal grooves
Flat face joints without a groove. Flanged connections. Any joint where the seal relies on face-to-face compression rather than groove squeeze.
A flat rubber washer or rubber seal looks similar to a fibre washer but behaves differently. Rubber compresses and recovers more than fibre — it deforms under load and springs back when the load is reduced. This makes it useful for joints that are opened and closed frequently, like tap valves and hose connectors.
Rubber seals are common in domestic tap bodies, garden hose fittings, washing machine valves and similar low-pressure, frequently-operated connections. The traditional "tap washer" that fails and causes a dripping tap is a flat rubber seal — usually EPDM or natural rubber.
Rubber seals are not normally the right choice for high-temperature or high-pressure applications. EPDM handles water, ozone and weathering well when the compound is suitable. It is not suitable for petroleum oils or fuels. Higher-temperature duties still depend on the compound and approval.
Tap bodies, garden hose connectors, washing machine valve seats, low-pressure domestic fittings, ball valve seating washers
Gas lines. Oil or fuel systems. Higher-temperature duties unless the exact rubber compound is specified for that service. Industrial flanges.
How to identify what you are looking at
The joint is apart. You have something in your hand. Here is how to identify it in under thirty seconds.
Colour is only a clue — do not identify material by colour alone. Parts from other suppliers do not follow the Kinetics Line colour system.
A common mistake that causes repeat failures
Fitting a fibre washer where a flat gasket belongs — or a rubber seal where a fibre washer was — because that is what was on the van.
They look similar enough to feel like a reasonable substitute. They are not. The seating geometry, compression load and material properties are different. A rubber seal on a pump flange designed for a fibre gasket will either extrude under pressure or not compress evenly across the face. A fibre washer in a tap body designed for a rubber seat will not recover between operations and can fail quickly.
One specific case worth knowing: mechanical joints on pumps, motors and sump plugs often use a rigid sealing washer — vulcanised fibre rather than soft fibre or rubber. Vulcanised fibre is hard, crush-resistant and holds its shape under high bolt load without deforming. REDSEAL PRO 110 is this material. It looks similar to a standard fibre washer but behaves very differently — it is not compressible in the same way and should not be substituted for soft fibre on a heating flange, or vice versa.
Use the original seal as one clue, not the only authority — confirm it against the fitting geometry, size, medium, temperature and any manufacturer specification. If you are not sure, identify the fitting type and look up the correct seal specification. A five-minute check before the job is worth thirty minutes of callbacks after it.
One rule that covers most situations: flat mating faces need flat seals — gaskets or fibre washers depending on the fitting size. Grooves and recesses need O-rings. Valve seats need rubber washers. The geometry of the fitting tells you what belongs inside it.
When you are not sure — what to check
Look at the fitting geometry. Is there a visible groove or recess on the mating face or inside the fitting body? That is an O-ring joint. Is there a flat, machined face with no groove? That is a flat seal joint — gasket or washer depending on size and pressure.
Check the size. Flat gaskets for heating and plumbing come in standard BSP sizes — 3/8", 1/2", 3/4", 1" and up. If the fitting is a standard BSP thread, the gasket or washer size follows from the thread size. Measure the OD and ID of what came out if you are not sure of the fitting size.
Check the medium and temperature. For domestic water service, both fibre and rubber seals are common — but only in the fitting geometry they were designed for. Gas — the seal material specified and approved for that fitting and jurisdiction; do not substitute based on appearance or material family. Higher-temperature duties require material data and the correct joint geometry; do not substitute a domestic rubber washer where a fibre gasket or another specified material is required. For any gas fitting, stop and consult an appropriately qualified and registered professional before proceeding.
Check what came out. The original seal is an important clue, but confirm it against the fitting geometry, size, medium, temperature and any manufacturer specification. Even if it is in pieces, the material, size and shape tell you what the fitting was designed for.
A note on gas fittings
Gas fittings may use the same basic geometry cues — flat-face seals or groove-based seals — but the approved seal type and material must come from the fitting, appliance and regulatory requirements.
Gas fittings are safety-critical and must not be treated as general plumbing work. The seal type and approved material must follow the appliance, fitting and local regulatory requirements. In the UK, gas pipework and fittings should be handled only by appropriately qualified and registered professionals. This article is for identification context, not a gas repair instruction.
Frequently asked questions
What is the difference between a flat gasket and a fibre washer?
A flat gasket is the broader category: a flat sealing part compressed between two faces. A fibre washer is a smaller washer-format flat gasket, commonly used in threaded flat-face plumbing and heating fittings such as radiator valves, tap connectors and BSP unions. Both seal by compression between flat faces; the size and load characteristics differ.
Can I use an O-ring instead of a fibre washer?
No. A fibre washer sits between two flat faces and seals by face-to-face compression. An O-ring sits in a machined groove and seals by being compressed in that groove (radially or axially depending on the joint design). The fitting geometry decides which one belongs there. Substituting one for the other will usually leak.
Why does my joint leak even with a new washer?
One common cause of repeat leaks is fitting the wrong type of seal — a rubber washer where a fibre washer belongs, or a flat gasket where the fitting was designed for an O-ring. Identify what came out before fitting what goes in, and confirm it against the fitting geometry, size and specification — the original seal is one clue, not the sole authority.
Know what you are looking at before the joint comes apart. Know what to replace it with before the joint goes back together.
Flat mating face — flat gasket or fibre washer. Groove or recess — O-ring. Valve seat — rubber washer. Not interchangeable. Not a close enough substitute. The fitting geometry tells you what belongs inside it.
If the joint needs a flat gasket, the Kinetics Line flat gasket range covers different heating, water, higher-temperature and documented service requirements depending on the selected material grade. If it needs an EPDM rubber seal, check RUBBERSEAL PRO 110 and its documentation against the exact water, outdoor, UV or approval requirement.