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 more often than not 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 auger is the most effective tool for hair in a shower waste. On a floor drain, lift the grate and clear the throat by hand — often the problem is right there and visible.
Step 3 — mineral granules
When the mechanical route fails, sodium bisulphate granules break the deposit down without the heat that comes off caustic soda. Sodium bisulphate is an acid salt, not a strong mineral acid: it dissolves to about pH 2, the same chemistry used to correct pH in swimming pools. The manufacturer states that it does not attack PVC or rubber seals and that it is compatible with septic systems.
The method is short. Empty the sachet into the drain dry, add a little warm water, leave it 15–30 minutes and then flush with plenty of hot water; overnight for a stubborn blockage. Wear eye protection, because the granules cause serious eye damage. Never mix it with another product — an acid and a chlorine cleaner together give off gas.
| Method | Clears | Risk to the installation |
|---|---|---|
| Hot water | Fresh grease, soap | None |
| Plunger | Soft blockages | None |
| Drain auger | Hair, solid objects | Slight — can scratch glaze |
| Mineral granules | Organic deposit | Slight — wear eye protection |
| Caustic soda | Grease — but violently | 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 waste — 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 busy buildings, a sachet of granules a month as scheduled prevention rather than an occasional rescue
Full guide: drain smell — causes and a permanent fix — why traps dry out, why bleach and vinegar do not help, and what is done in homes, hotels, schools and hospitals.