A wet riser is a system of pipework and valves that is permanently kept with water for the purpose of distributing water within a building for firefighting purposes. This built-in system of water distribution in a building means that there is no need for the firefighters to create another distribution system for fighting fire in case of a fire outbreak in the building.
Unlike wet risers, dry risers are not permanently charged with water, meaning that when they are not in use, the system has no water but, in case of a fire outbreak, the firefighters charge the system with water using the fire service pumping appliances. A dry riser or a wet riser fire main can be used in buildings whose height is less than 165 feet high. However, for buildings whose height is more than 165 feet above the access level of the rescue service vehicle, wet risers are used. This is because the pumping pressure provided by the fire service appliance is not high enough to charge the riser and ensure an adequate supply of water to the riser within a short period of time. The water that is charged to wet riser normally comes from a pressurized supply with landing valves located at specific locations on every floor.