With a blocked drain the real damage is usually done not by the blockage but by whatever is poured in to shift it. Caustic soda is the first thing most people reach for and the last thing that should be used. The sequence below runs from lowest risk to highest — and in three cases out of four it stops before any chemistry is needed.
First: work out where the blockage is
This is the one step that cannot be skipped, because it determines everything else.
- One drain slow, the rest normal → the blockage is in that drain or its trap. Solvable without a plumber.
- Every drain in the bathroom slow → the blockage is in the stack serving them. Harder, but often still solvable.
- Water backs up elsewhere when you run this drain → the blockage is in the main run. Call a plumber now.
- Damp on a ceiling or wall below → it is already leaking. Shut off and call.
If you are not in the first or second category, ignore the rest of this article. Chemistry poured into a blocked main run stays there, waiting for whoever opens the pipe.
Step 1 — hot water
A litre or two of hot water, poured slowly, in several goes. It shifts fresh grease and soap scum, which account for a large share of slow bathroom and kitchen drains. On PVC do not use boiling water — hot from the tap is enough.
Step 2 — mechanical
A plunger works on pressure, not chemistry. Fill the sink or shower tray with a few centimetres of water so the plunger has something to push, and work in short sharp strokes.
A drain snake is the most effective tool for hair in a shower drain. On a floor drain, lift the grate and clear the throat by hand — in half of all cases the problem is right there and visible.
Step 3 — mineral granulate
When the mechanical route fails, a sodium bisulphate granulate breaks organic deposit down without the exothermic reaction of caustic soda. It does not heat the pipe, does not attack PVC or rubber seals, and is compatible with septic systems.
The method is the same for all granulates: dry granulate into the drain, then lukewarm water, then leave it to work. Never mix with another product — acid and chlorine together release gas.
| Method | Clears | Risk to the installation |
|---|---|---|
| Hot water | Fresh grease, soap | None |
| Plunger | Soft blockages | None |
| Drain snake | Hair, solid objects | Slight — can scratch glaze |
| Mineral granulate | Organic deposit | Slight |
| Caustic soda | Grease, but aggressively | High — heat, pipe deformation |
| Sulphuric acid | Almost anything | Very high — to pipework and to people |
If the drain runs freely but smells
That is not a blockage. A drain that flows normally and still smells has a dry trap or biofilm on the walls of the throat. Drain cleaner does nothing for either — see why drains smell.
Keeping it from happening again
- A hair trap on the shower drain — catches hair before it reaches the trap
- Cooking fat into the bin, not the sink — it cools and sets in the pipe
- Hot water through the kitchen drain once a week
- In high-traffic buildings, granulate as scheduled prevention rather than occasional rescue