The smell hit before the sound did—warm, wet cardboard and something metallic that shouldn’t be breathing. I was halfway up the loft ladder when I noticed the ceiling plaster had domed like a bad soufflé. One gentle poke with my screwdriver and a palm-full of plumber’s putty the previous owner had wedged around a 15 mm compression olive slid out, still grey but now studded with green copper bloom. That plug had held for fourteen years, quietly doing the one job it was never designed for: sealing pressurised pipe. It had failed the way putty always does under constant water load—by turning back into the loose, oily clay it started as.
Why I’m the One Writing This
I’ve been a Gas Safe/Part P registered plumber-heating engineer for twelve years, clocking north of 2,100 leak call-outs across north London. I’ve seen putty mis-used on everything from 28 mm central-flow tees to £450 Grohe shower mixers. Last winter I logged 47 “mystery damp” jobs; eight were down to old putty someone hoped would harden like epoxy. It never does.
Quick Steps:
1. Isolate water and drain the section.
2. Strip the failed putty with a plastic scraper (metal gouges brass).
3. Clean threads with 120-grit then methylated spirit.
4. Re-seal using the right material (see table below).
5. Pressure-test at 1.5× working pressure for 30 min.
What Plumber’s Putty Actually Is—and Isn’t
Putty is linseed oil mixed with powdered clay or limestone. It stays pliable, which makes it brilliant for drain basket seals and tap deck plates where the joint is tightened by hand and sees zero pressure. It is not a pipe-thread sealant; water under 1 bar will slowly push through the oil film. Think of it as flexible wallpaper paste, not glue.
How Leaks Start: Three Real Scenarios I’ve Walked Into
1. Kitchen sink waste: Installer packed 6 mm of Fernox PS20 putty under the strainer flange, forgot the rubber washer. First hot washing-up cycle softened the putty; six months later the cupboard swelled.
2. Basin tap tail: DIYer wound putty over ½-inch threads, then over-tightened. The putty extruded like toothpaste, leaving a 0.2 mm spiral gap. Slow drip, huge damage to veneered MDF.
3. Radiator tail union: Someone “seated” the 15 mm olive with a smear of putty for luck. Heating expanded the pipe, olive bit through the soft filler, pin-hole spray inside stud wall.
When Putty Works—and When It Absolutely Won’t
| Joint Type | Pressure | Right Product | Putty OK? |
|---|---|---|---|
| Sink basket | 0 bar | Plumber’s putty | ✅ |
| Bath clicker waste | 0 bar | Putty + silicone washer | ✅ |
| Tap shank through countertop | 0 bar | Putty for cosmetic seal | ✅ |
| Compression olive | 1–3 bar | PTFE + silicone grease | ❌ |
| Threaded iron pipe | 2–8 bar | Loctite 55 tape or Jet-Lube PTFE paste | ❌ |
My Step-by-Step Putty Leak Repair Protocol
1. Diagnose the Path
Feel around: putty leaks leave a greasy halo. If the water tastes metallic you’re on copper; if it’s chlorinated you’re on the mains side.
2. Choose the Replacement Sealant
- Drain flange: Fresh putty ring, 8 mm Ø, rolled like kids’ plasticine.
- Compression joint: Strip putty, wrap PTFE tape 12 turns clockwise, add silicone grease to olive.
- Threaded steel: Use Loctite 55 cord (£7.49, 50 m) or Hempel hemp yarn + Boss White.
3. Re-assemble & Torque
Hand-tight plus ¼ turn on plastic, ½ turn on brass. Over-tightening is the rookie mistake I see most; it cold-flows putty out of the joint.
4. Test Smarter
Fill sink, let stand 5 min, then empty completely. The sudden weight tests both static and dynamic pressure. For mains, I hook a Kamco 12-bar test pump (£89, 2025 price) and log 30 min with zero drop.
Cost & Time Reality Check
- Putty tub 750 g: £4.79 (Toolstation, Everbuild)
- My labour rate: £65 first hour, £45 thereafter
- Average putty-leak redo: 35 min, total bill £65 inc. VAT
- Ignore it? One client paid £1,800 to re-tank a timber floor after a 9-month drip. Putty is cheap; arrogance is expensive.
Safety Considerations and Legal Requirements
UK Water Regs Schedule 2.7 bans “non-approved sealing materials” on potable pipe. Putty is listed only for non-pressure sanitary joints. Use it on mains and you fail the inspection; water companies can force you to rip it out.
Warning: Pressurised pipe + plumber’s putty → micro-spray inside wall → hidden mould & structural rot → full ceiling replacement. Always match the sealant to the system pressure.
FAQ
Can I just slap more putty over the old stuff?
No. Fresh putty won’t bond to the oily residue; the leak will track sideways and appear 20 cm away, fooling you into thinking the pipe’s fractured.
How long should a putty seal last?
On a drain: 20 years plus. On a compression olive: minutes to months. Putty isn’t designed for radial pressure; the olive deforms it continuously.
PTFE or hemp for threaded iron—what’s quieter?
Hemp plus Boss White absorbs vibration; PTFE can squeak under thermal cycling. I use hemp on 1-inch radiator branches and PTFE on 22 mm zone valves.
Is there a food-safe putty?
Oatey Great White (US import, £12 Amazon) is NSF 61-rated, but still only for zero-pressure joints. Don’t confuse “non-toxic” with “pressure-worthy”.
My putty dried rock-hard—why?
You bought the £1 shop bargain version. Cheap fillers oxidise; oil migrates out. Spend the extra 80 p for Everbuild or Fernox and you’ll stay workable for two years in the pot.
Key Takeaway
Plumber’s putty is a drain-only product. If you find it anywhere upstream of the trap, strip it and swap in the right sealant. Do it once, test it twice, sleep dry.
Alex Thompson