As soon as a building is constructed or rehabilitated, the natural process of deterioration begins. Preservation has been defined as "the act or process of applying measures necessary to sustain the existing form, integrity, and materials of an historic property. Work, including preliminary measures to protect and stabilize the property, generally focuses upon the on-going maintenance and repair of historic materials and features rather than extensive replacement and new construction."
The most important component of any plan to preserve a historic structure is maintenance. Regular inspection and maintenance of systems will help preserve the integrity of historic building fabric. If that fabric is maintained, deterioration will be minimized or eliminated. Maintenance is the most cost effective method of extending the service life of a building system. By logical extension, maintenance is the key to preservation. While the decay of components of the envelope cannot be avoided, neglect can actually cause this process to increase at an exponential rate. When maintenance has been deferred, and a problem suddenly rears its ugly head, it is not uncommon for the reaction to be swift and inappropriate. The use of the wrong materials and methods will often cause worse damage to irreplaceable historic building fabric.
When considered in the long term, the cost to maintain historic structures is significantly less than the restoration of historic systems and materials, and it creates far less disruption to building occupants. When a property owner or manager creates a maintenance program for their building, it is strongly recommended that they seek the counsel of a preservation consultant, and/or experienced contractor. The maintenance program should clearly identify and describe courses of action that are specific to the building. Every historic structure, no matter how small, should have a written guide that includes:
· Lists and schedules for periodic inspections of each system. These should be set-up in a ‘checklist’ format, to ensure uniformity of procedures over time;
· Blank elevations of the building to be marked up during inspections and after any work takes place;
· A full set of actual photographs that comprehensively document the conditions of the entire structure as well as a digital copy of each. This album will grow over time;
· An emergency list of contractors who can be called upon in an emergency, especially HVAC, electrician, plumber, and roofer;
· Individualized procedures for the historically appropriate handling of the individual systems and materials of the building; and,
· Hard copies of completed reports that document all work and inspections. Include copies of estimates, contracts, warranty cards, paint colors, mortar recipes, materials sources, and any other information that will be needed by future stewards of the structure.
Maintenance is the most important preservation treatment for extending the life of an historic property. It will slow the natural process of deterioration and prolong the natural service lives of the historic fabric of the envelope. A written maintenance plan will help preservation planners organize, schedule inspections, and guide the work necessary to for a historic building. When a property owner or manager creates a maintenance program for their building, it is strongly recommended that they seek the counsel of a preservation consultant, and/or experienced contractor. The maintenance program should clearly identify and describe courses of action that are specific to the building. When the full life cycle of a building is considered, there is no smarter money spent than on maintenance.
 National Park Service, Nationwide Programmatic Agreement Toolkit for Section 106 of the National Historic Preservation Act, glossary of terms