A "wet" standpipe is filled with water and is pressurized at all times. In contrast to dry standpipes, which can be used only by firefighters, wet standpipes can be used by building occupants. Wet standpipes generally already come with hoses so that building occupants may fight fires quickly. This type of standpipe may also be installed horizontally on bridges.
A dry riser is a main vertical pipe intended to distribute water to multiple levels of a building or structure as a component of the fire suppression systems.
The pipe is maintained empty of water. The dry riser is the opposite of a "wet riser" or "wet standpipe" system where the pipes are kept full of water for manual or automatic fire fighting operations. Dry risers have to access fire engine within 18 m of the dry riser inlet box. Dry risers in occupied buildings have to be within a fire resistant shaft, usually one of a building's fire escape staircase enclosures.
Depending on regional nomenclature, the term "dry riser" may refer to a standpipe, intended to provide water to fire hose connections, or a vertical main pipe in an automatic dry pipe fire sprinkler system. A dry standpipe comprises a fire department connection, e.g. Storz, which is an external access point at ground level through which water can be pumped from the fire department's fire engine pump to firefighters' fire hose attachments on each floor, whereas a dry pipe fire sprinkler system is a network of pipes connected to fixed sprinklers inside a building, which are full of air until one of the sprinklers is triggered