How to Fix a Leaking Toilet — Trade Method
Toilet leaks are almost always one of five points: fill valve, flush valve, cistern-to-pan doughnut, pan-to-floor seal, or supply nut. Here is the diagnostic order.
Step-by-step method
1. Continuous fill = fill valve
Water constantly trickling into cistern. Adjust float first; if that doesn't work, replace the fill valve (Fluidmaster PRO 45 = £12).
2. Cistern-to-pan leak (doughnut)
Water under toilet on flush only. The foam doughnut has failed. Isolate water, undo cistern bolts, lift cistern, swap doughnut (£4).
3. Base leak on flush
Water pooling around base after every flush. Pan-to-soil seal failed — pull the toilet forward and re-seat with silicone or a new seal.
4. Supply nut leak
Water at the flexi tail. Tighten quarter turn, or replace the flexi (£4).
5. Flush valve leak
Cistern slowly empties into pan. Replace flush valve washer or full flush valve (£8).
Frequently asked questions
How do I tell if my toilet is leaking silently?+
Add food dye to the cistern. If colour appears in the pan without flushing, the flush valve is leaking.
Can I use PTFE on toilet connections?+
Yes on threaded connections. Never on the doughnut or pan seal — those need silicone or the original rubber component.