Roughly 20 years ago, a fellow I'd done some residential stair work for asked me to build the stairs for a store he planned to open. In my inexperience and hubris, I built those commercial stairs to residential standards. When they failed the building inspection, I had to replace them. Only my working a series of 16-hour days allowed the grand opening to happen on schedule.
The science fiction author Robert Heinlein opined, "Specialization is for insects." Embracing that philosophy as a carpenter, I always sought new challenges. Occasionally, I'd pull a boner like those stairs, but I worked every day, even through the foul economic times of the late '80s and early '90s.
In better times, however, I never made a pile of money, particularly compared with others who stuck with a niche such as deck building. It's far easier to dial in your costs and procedures in a niche business. Some of the deck builders I've met have developed very profitable businesses.
Specialization succeeds, sometimes spectacularly, but only in a relatively static set of circumstances, and it can make dealing with change hard. In the late '80s and early '90s, when I was still working every day, many of the specialty contractors I'd known were either out of business or finding new income sources. One friend who was a siding contractor got through those times in part by building masonry chimneys.
As in the mid '80s, the steadily booming building economy of the past decade has enabled and encouraged niches such as deck building. A cursory read of any recent Wall Street Journal today suggests that those days are numbered, if not gone. Although the slowdown has affected new construction more than remodeling such as deck building, that's beginning to change. I've talked to deck builders recently, guys who'd been booked for six months out, whose phones have stopped ringing.
If all you build is decks, are you so specialized that you won't be able to pay the mortgage if your niche shrinks? It might be time to consider some de-specializing. You already have a base you can market to, the satisfied customers for whom you've built decks. These people know and like you and would rather hire you than some guy from the Yellow Pages to install doors, windows, or siding. When the money starts to flow again, you can go back to building just decks.
Don't be like me and those stairs, though. If you branch out, take the time to read the directions. Literally. Most construction mistakes I've made, and that I've seen others make, could easily have been avoided by an hour spent reading a manufacturer's on-line instructions or the relevant code book. Being informed about codes and proper techniques will separate you from the competition, and help you prosper in times both lean and fat.