There are many types of decks in common interest communities such as condominiums, cooperatives, apartments, etc. A reserve study provides expected refurbishment and replacement costs and a timeline of the useful life for decks so these communities can adequately budget for the costs associated with them and prepare for large scale projects that will likely impact the community membership.
If a deck has been installed correctly it is typically a long life component that can last for over 25 years when adequately maintained. The most common reason decks need replacement before the 20 year mark is water intrusion/rot issues due to a poor install or lack of maintenance and painting. A community should follow regular paint cycles of at least once ever 5 years in the Pacific Northwest to make sure the wood surfaces are protected from the elements and the decks should be inspected regularly (annually preferred) to ensure their is proper drainage off the decks and away from the building. If the deck has an elastomeric coating this should be top coated every 5 years as well to ensure the deck superstructure is protected. Over time these elastomeric coatings deteriorate and become punctured due to deck traffic and items on the deck like BBQ’s which are heavy and can dig in and through these waterfront coatings causing them to leak.
Depending on the type of deck and how exposed it is to the elements some decks will require full replacement and others can just be refurbished at a lower costs as the deck superstructure being would be left intact (waterproof membrane replaced, boards replaced, railing replaced). It has been our experience though that most communities struggle to adequately maintain their decks over a long period of time and through various Boards so replacement is usually required.
Link: Deck Maintenance