Sealing Pipe Threads with Flax Tow and Jointing Paste
The traditional continental-European method of sealing a threaded pipe joint: wind flax tow in the direction of the thread pitch, coat with a jointing paste that binds and preserves the fibre, and tighten. The flax swells with moisture to close the last gap. Handles hot, rough, and oversized threads and stays adjustable after tightening; the costs are speed, skill, and joints that seize over time.
Winding a natural fibre around a male pipe thread and smearing it with a jointing paste is the traditional way to seal a threaded plumbing joint. It remains standard practice across Germany, the Nordics, the Baltics and the post-Soviet countries, while the Anglo-American trade defaults to PTFE tape. The fibre is flax tow — see Plumber's Hemp, Flax Tow, Oakum, Пакля: The Same Material Under Many Names for why it goes by so many names. ## The method 1. **Clean the thread.** Old paste, rust and scale come off with a wire brush. The fibre needs metal to grip. 2. **Wind the tow in the direction of the thread pitch.** For a normal right-hand thread that means clockwise as seen from the open end, so that screwing the fitting on *compresses* the fibre into the thread valleys instead of unwinding it. Winding the wrong way is the single most common beginner error and produces a joint that weeps. 3. **Wind evenly and not too thickly.** The fibre fills the helical gap; it does not act as a wedge. Over-packing generates enormous hoop stress as the fitting tightens and can split a brass or cast fitting. 4. **Coat with paste.** The paste lubricates assembly, binds the fibre into a mass, fills the remaining voids, and — critically — seals the flax from air and water so it cannot rot. 5. **Tighten.** In service the flax absorbs residual moisture and swells, closing the last of the gap. The joint tends to get tighter over its first days rather than looser. ## Why it works where tape does not The fibre is bulk material, so it conforms to threads that are worn, damaged, out of round, or cut to a sloppy tolerance. It scales up to large diameters where tape shreds. It tolerates heat: a typical DVGW-approved paste such as Neo-Fermit is rated to roughly 130 °C at 7 bar on heating systems, and the Danish Unipak White compound to about 140 °C at 8 bar. It also has a property no other method offers: **the joint stays adjustable**. After tightening, an elbow or valve can be rotated a further quarter-turn to aim it without breaking the seal. With PTFE tape, backing off a fitting to align it breaks the seal and means starting over. ## The costs - **Slow and skill-dependent.** Quantity and winding direction are learned by feel. - **Joints seize.** Russian plumbers call it прикипание — a flax-and-paste joint that has been hot for years is miserable to take apart, and the fitting often has to be destroyed. - **It rots without paste.** Bare fibre in a joint will decay and the joint will weep. The paste is not optional. - **Drinking water is a regulatory question, not a technical one** — approved in Germany with a certified paste, prohibited in the UK. See Hemp Thread Sealing and Drinking Water: Approved in Germany, Banned in the UK. For choosing between this and the alternatives, see Thread Sealant Selection: Flax and Paste, PTFE Tape, Anaerobic, and Wound Cord.