Building built-in bookshelves can add a unique look to any room in your home. But it is important to know how much weight your shelves will be able to carry before you buy your materials. The weight capacity of your bookshelves depends on the kind of wood you are using. Your local home improvement store can provide you with a chart that shows the weight capacity of various kinds of wood depending on how long the wood plank will be. The narrower your shelf, the more weight it can carry. A three-foot-wide shelf will be more durable than a six-foot-wide shelf regardless of the material. You can also find reference charts on the Internet that will tell you exactly how much weight per foot a wood material can handle when you use it for shelving.