Net rentable area is also known as rentable area, net rentable square feet or usable area. The simple definition is that the net rentable area is the area (or square footage) for which rent can be charged. Net rentable area is determined by measuring the overall square footage of a space, floor or building and subtracting all vertical penetrations. Vertical penetrations make portions of the area unusable and include elevator shafts, stairways, heating/cooling venting and mechanical shafts. No deductions are made for necessary columns, load bearing walls or structural projections. Mechanical rooms are also not typically deducted when figuring net rentable area. An example would be a penthouse apartment with elevator service. The physical area of the elevator shaft would be deducted from the overall square footage, yielding a net rentable area.