When considering a bookcase, there are always a few things you should consider before engaging a contractor to custom build your bookcase. Lets start off with the following:
1) Booksize - determine the size of your largest book and the average of your existing books collection. Believe it or not but we have seen some customers with a fully installed custom bookcase and yet some books are sitting on the floor beside the bookcase, as the books are unable to fit or they are awkwardly positioned sitting sideways on a shelf
2) Book Weight - it doesnt appear to seem like a big thing, but the weight of dozens of thick books jammed together, it can have a warping effect on thinner shelves that have wide distances between vertical gables
3) Book Accessibility - for those folks who want higher shelving, make sure you do a high reach test. If you can't reach the top shelf easily, maybe consider incorporating a sliding rail ladder into your design. They often come in 3, 5, and 7 step combinations and have a wide range of price points. They are not only functional, but they also add an element of class to your custom bookcase.
Take care
