The number of 2x4 timber pieces needed to build a house varies depending on the size of the structure, although it is typically advised that one 2x4 be estimated per foot. One every 16 inches is required by the construction regulations. For example, a house with 4 feet of clear floor space requires 20 2x4s. The decision on how many 2x4s to use is based on cost and availability. If available, straight-grain lumber is less expensive than edge-grain lumber. Also, because straight-grain lumber does not have as much variation in thickness across its width, it can be more efficiently used in building frames. Finally, the more 2x4s that are used, the cheaper the frame will be. A cheap frame is not desirable since it will likely be made from low-quality materials or cut poorly, which could lead to damage or injury if a person walks into the unoccupied frame. Typically, the total cost of lumber used in a house construction project is about equal to the cost of steel, although this depends on what kind of wood is being used. Hardwood is more expensive than pine, for example. Lumber is also expensive compared to other building materials such as drywall or concrete. However, because a house constructed out of lumber will last for many years after it is built, the cost of lumber becomes more affordable over time.