What is happening
A drain that has always been slow is a different problem from one that stopped suddenly. It is usually not a clog at all.
Drains report problems indirectly. The fixture showing the symptom is often not where the trouble is, which is why the useful first question is not what is blocked but which fixtures are affected and when.
What usually causes it
In this area, four explanations cover most cases:
- Hard water scale in older cast iron
- Grease film in a kitchen branch
- A shallow fall on a long horizontal run
- A partial belly holding water
Which one applies changes the fix entirely, and they are not distinguishable from inside the house. We would want to see it before saying anything for certain.
What you can check yourself
Run the fixtures one at a time and note which ones react to each other. Two drains that respond to one another share a pipe, and that narrows things considerably.
Note whether it is getting worse gradually or started suddenly. Gradual usually means the pipe has been closing up for a long time. Sudden usually means something specific is in the way.
Narrowing it down
Compare it with other drains in the house. If every drain is slightly slow, look at the main. If one is slow and the rest are fine, the problem is in that branch, and the age of the pipe tells you whether to expect scale or grease.
What guessing costs
This one rarely gets acted on because it never quite fails. The cost of ignoring it is that the pipe keeps closing, and descaling a nearly closed line is a bigger job than descaling a half closed one.
When it is worth calling
If it has happened before, it is worth finding the cause rather than clearing it again. A drain that has been opened twice and returned twice will return a third time.
On this particular pattern, descaling is the service that most often resolves it, though we would confirm that by looking rather than by assuming.
