Reading a shutoff valve layout
Applies to: Assumes a house on a metered or mains supply with a main stop, and fixture stops added at various dates.
Last worked on 2015-05-14
Before any plumbing work, and long before any emergency, it is worth knowing exactly what each valve isolates and which of them still turns.
Three levels of isolation
The main stop isolates the whole property and is usually where the supply enters, often in a floor void or under a sink. Floor or branch isolators, where they exist, take out one riser or one floor. Fixture stops isolate a single outlet and are the ones added most recently.
Most houses have all three levels somewhere, but rarely as a designed set: they accumulate. Mapping them once, in daylight, with the house dry, is the difference between a small job and a wet one.
Valves that have not been turned in years
Gate valves seize open and, worse, seize part-closed, which throttles the supply permanently and is often blamed on the main. A gate valve that turns but does not stop the flow has usually lost its gate off the stem.
Quarter-turn ball valves are far more reliable but also fail: the handle can turn without the ball moving if the stem has corroded. In either case the test is not whether the handle moves but whether the water stops.
Making the map
Work fixture by fixture. Close one valve, then check every outlet in the house and record which ones lost pressure. Reopen it fully before moving on, and note valves that are stiff, that leak at the gland when moved, or that do not fully stop the flow.
The result is a short list on paper near the main stop. It is one of the few pieces of house documentation that repays the effort immediately.
Mapping the valves
- Locate the main stop and confirm it fully stops the flow at the lowest outlet in the house.
- Reopen the main stop fully, then back it off a quarter turn so it does not seize in the fully open position.
- Close each intermediate valve in turn and record which outlets are affected.
- Test every fixture stop by closing it and opening the outlet it should serve.
- Record any valve that turns without stopping flow, and treat that valve as absent until it is replaced.
- Write the list, date it, and leave it where the main stop is.
The point of the map is that it is made when nothing is wrong. A valve you have never turned is not an isolation point; it is an assumption.