What is standpipein construction?

Class I systems provide 2.5 inch hose connections for fire department use and are the standard in most commercial buildings. Class II systems provide 1.5 inch hose stations with hose attached, intended for occupant use, and have become rare in new design. Class III systems provide both outlets on the same riser. The class is set by the building code and the fire marshal, and it changes the valve hardware at every floor, so confirm it before pricing.

Often physically, yes: most mid-rise and high-rise buildings use a combined riser, where the same pipe in the stair serves the floor hose valves and feeds each floor's sprinkler control assembly. Contractually it varies. Some specs bundle standpipe and sprinkler into one fire protection package, others carve the standpipe out, so the estimator's job is to confirm which sub's number carries the riser, the hose valves, and the FDC before bid day.

Whenever the available water supply cannot deliver the required pressure at the most remote hose outlet. Water loses 0.433 psi for every foot of elevation, and NFPA 14 requires 100 psi residual at the topmost 2.5 inch outlet of a Class I system, so a building with 144 ft of riser needs roughly 62 psi for lift plus friction losses on top of the 100. Few municipal supplies can do that, which is why most buildings beyond a few stories carry a fire pump.