Swap bath wastes and traps without callbacks: survey, choose the right trap, set correct falls, seal properly, test well, and hand over like a pro.
Ditch the paperwork and reclaim your time with Donizo.
Contractors across Europe trust Donizo to handle their quotes and invoices while they focus on what they do best: their craft.
Replacing a bath waste and trap seems simple until you’re back a week later chasing smells, dribbles, or a slow drain. The usual culprits? Wrong trap choice, poor fall on the run, rushed sealing, and no access for future maintenance. Here’s a practical, field-tested guide to do it once and do it right. You’ll see what to check before you start, how to pick the right components, how to set fall and venting to avoid self‑siphonage, the right way to seal and test, and how to hand over cleanly. We’ll also cover a quick, professional way to scope and send a signable proposal using voice capture so you don’t lose the evening to admin.
Many callbacks start with hidden constraints: no access to the trap, warped tubs, cracked overflow gaskets, or low outlet heights that force poor trap geometry. It’s common for the existing fall to be too flat, especially on long runs across joists.
You pull the bath panel and find no access hatch and a flat 40 mm run pinned tight to joists. You propose: new 300 by 300 mm access, solvent weld rerun at about 1:60, and an anti‑siphon P‑trap to protect the 50 mm seal.
Rushing removal leaves old sealant, grit, and deformed fibre washers that prevent a consistent seal. Over‑tightening cracked wastes are another common cause of dribbles.
A steel bath shows a slight dish around the waste. You swap the stock flat washer for a moulded conical seal and add a stainless backing plate to spread the load, avoiding the need to over‑torque the waste nut.
Wrong trap, wrong seal depth, or no protection against self‑siphonage leads to smells and slow drains. Baths need a reliable 50 mm seal under UK guidance (Approved Document H).
| Trap Option | When To Use | Seal Depth | Notes |
|---|---|---|---|
| Standard P‑Trap | Normal space, short branch | 50 mm | Reliable, good flow; simplest choice when it fits |
| Shallow P‑Trap | Low outlet / tight voids | About 38–50 mm | Use with care; protect against self‑siphonage |
| Anti‑Siphon P‑Trap | Long runs / shared branches | 50 mm | Built‑in AAV reduces gurgle and odours |
| Waterless Valve | Intermittent use / smell issues | N/A (membrane) |
1960s bath with a low waste outlet over a deep joist bay. A shallow P‑trap plus an integrated anti‑siphon vent keeps effective seal depth close to 50 mm and stops the gurgle on full‑bore discharge.
Flat or over‑steep runs cause standing water, airlocks, noise, and self‑siphonage. Undersized vents and crowded branches compound the issue.
Existing run crossed three joists with humps and dips. You re‑route in the joist zone with notches compliant to structural rules and clip every 0.6–0.8 m to hold about 1:60 fall. Result: full tub drains clean in one go with no gurgle.
Leaks at the waste flange, overflow gasket, or compression rings show up after you’ve left. Many contractors report that rushing the sealant stage is the number one cause of revisit.
You note a faint weep at the overflow nut only under high flow. A quarter‑turn back, reseat, and a fresh bead cures it. You leave a simple note: “Avoid full‑bore baths for 24 hours while silicone cures.”
Small plumbing jobs often burn more time in admin than in the repair. Many contractors find the back‑and‑forth on scope and price can delay approval by days.
After a 15‑minute survey, you record: “Replace 1 1/2 inch bath waste, install anti‑siphon P‑trap, rerun 40 mm branch at approx. 1:60, add 300 by 300 mm access hatch, sanitary silicone with 24‑hour cure.” You attach two photos of the current trap and the flat branch. Proposal goes out before you leave; the client e‑signs that evening.
For UK residential baths, aim for a 50 mm water seal as set out in Approved Document H. This helps prevent odours and reduces the chance of self‑siphonage. Shallow traps can be used where space is tight, but protect the seal with anti‑siphon features.
Use solvent‑weld on concealed runs for robustness and leak resistance. Use quality compression fittings where future disassembly may be needed or where access is guaranteed. Many contractors use a hybrid: solvent‑weld on the run, compression at the trap for serviceability.
In general, a fall around 1:40 to 1:80 works well. Avoid flatter than about 1:110, which commonly leads to standing water and deposits. Keep the fall consistent and support the pipe to prevent sagging.
Yes—provide reasonable access for inspection and maintenance. Many installers allow a minimum 300 by 300 mm opening where practical. It saves future damage if a gasket or joint ever needs attention.
Most sanitary silicones skin in 10–20 minutes but require roughly 24 hours to fully cure. Advise light use or avoid full‑bore drains until cured. Always check the product’s instructions.
Bath waste and trap replacements are straightforward when you respect the basics: a reliable 50 mm seal, the right trap for the geometry, a consistent fall on 40 mm pipework, careful sealing, and a proper test. Do that, add an access hatch, and callbacks drop away. For the admin, speak your findings, add a couple of photos, and let Donizo turn it into a professional, signable proposal in minutes. Clients e‑sign, and you can convert to an invoice without re‑typing—more time on the tools, less time at a keyboard.
Discover more insights and tips for your construction business
| Follow product guidelines; useful in cold voids |
