Discovering a yellowish-brown ring spreading across your ceiling drywall is unsettling. Determining whether the source is a roof penetration or an upstairs pressurized water pipe dictates who you need to call.
Clue 1: Weather Correlation
If the ceiling stain expands exclusively during or immediately following a heavy San Bernardino rainstorm, it is almost certainly a roofing issue (flashing, tile slippage, or vent pipe boots). If the stain grows during hot, dry summer weeks, it is 100% an active plumbing leak.
Clue 2: Upstairs Bathroom Proximity
Look directly above the stain. Is there an upstairs bathtub, shower, toilet, or laundry closet? Even tiny slow leaks from a shower pan or P-trap slip joint can migrate along floor joists and pool in ceiling drywall.