Why does a water heater leak soak the wall behind it?
Water runs down the tank's outside, collects at the base and wicks upward into the drywall and bottom plate behind the heater. The wall holds the water in its cavity, so the damage can be well above the floor and out of sight.
The tank sits close to the wall in most closets and garages, and the gap behind it is too narrow for air to reach. Water that gets in there stays for a long time, and the drywall is the first material to fail.
A bottom plate that stays wet can swell and begin to rot. The crew checks it with a meter and a probe, and any soft wood is reported to the repair trade rather than left to dry in place.
Is it safe to keep a heater on while the leak is fixed?
No. The heater should be off and its cold inlet closed before the crew starts. A gas heater should be off at the gas control, and an electric heater should be off at its breaker, and neither should be relit until a technician confirms it is safe.
A tank that is still heating while water runs under it is a shock hazard and, for a gas unit, a fire and carbon monoxide concern. The crew checks the wiring and the gas connection on arrival, and it asks you to keep the area clear until the heater is cleared.
The heater's pan and drain line should be checked by the technician before the unit is used again. A pan that is full of water or a drain line that is blocked is a common reason for a second leak.
Does the concrete slab have to be dried?
Yes, at least at its edge and wherever the water pooled. Concrete holds water for a long time and releases it slowly, and it can wick moisture into the bottom of the walls above it. The slab is measured and dried, and its surface is cleaned if the water left residue.
Concrete that has been wet is usually dried in place. The surface is cleaned, and the air movers are placed to pull moisture off the surface while the dehumidifier pulls it from the room. The slab is released only when its readings reach the dry goal.
Surface staining from rust or mineral deposit from the tank can remain on the concrete after drying. That is a cosmetic matter for the owner to decide, and it does not affect the drying.
What moves the scope of a heater leak job?
The volume of water, how long it sat, the size of the wet area, the wall construction and the amount of stored material decide the scope. The price is explained and approved on site before work begins, and changes are explained first.
A small leak caught early is usually a drying job with a small area of drywall opened. A heater that leaked for weeks can wet the wall, the bottom plate, the adjoining room and the flooring above, and the work grows with it.
Stored items in the garage or closet add to the scope. Boxes, paint, cardboard and shelving soak up water and need to be moved and cleaned. The crew photographs them before they are moved, so the record is complete.
Who replaces the heater?
A licensed plumber or the heater installer. The replacement is a separate job from the drying, and the crew tells you what the installer will need to see. Keep the heater off until the replacement is confirmed safe.
Replacement decisions often depend on the tank's age, its condition and the cost of the new unit. Those are matters for the plumber and for you, and this site does not recommend one over another.
The drying can run while the replacement is being arranged. The readings from the drying give the installer the information about the wall and the slab, which can change where the new unit is placed or how it is secured.