The common nail is called “common” because it is the most practical means for fastening pieces of wood together easily, quickly and inexpensively. Glue is neater, screws are stronger, and either one combined with correct joinery becomes stronger still. But 99 per cent of all the homes of the nation are fastened together with nails because they are quicker and easier to handle.
There are hundreds of different kinds of nails, each kind with its own assortment of sizes. They range from railroad and boat spikes down to brads so fine that a thousand barely make a pound. However, less than a dozen sizes and kinds will serve the needs of the average handyman. Choosing the right nail and driving it right makes the difference between the finished work holding together or falling apart.
Nails hold by friction, that is, the pressure of wood against the shank of the nail. Just how well they hold depends upon three things: the condition of the wood, the shape and texture of the nail, and the size of the nail in relation to the size of the wood.
First, consider the condition of the wood. If it is soft, the nail will drive in easily, but it will also pull out easily. The harder the wood, the more difficult to drive a nail, but the harder it will be held. There is one notable exception – splitting. Hardwoods split more easily, than soft. Starting with softwoods like balsa and pine, through rock maple and oak, and on to ironwood and teak, you come to a point where a nail thick enough to be driven without bending is also thick enough to split the wood. In other words, some woods are so hard that they cannot be nailed without drilling a pilot hole first. When you reach that necessity, it is more practical to use a screw.
Dry wood splits more readily than does wet wood, which often cannot be split at all. When a nail is driven into wet wood, there is a good chance that after the wood dries it will shrink, leaving the nail loose.
The second consideration is the shape and texture of the nail. The more exterior surface of the nail in contact with wood, the greater the holding power. In addition, many types of nails have ridges or spirals along the shank both of which increase the nail’s holding power and also the tendency to split the wood into which it is driven. A long, thin, pointed nail goes in more easily, holds well, but is more apt to split the wood. Blunt-pointed nails, or those blunted by the hammer before driving, have less splitting action but more holding power. The common diamond-pointed nail is the best compromise.
Many types of nails are coated with a special glue. When driven into wood, the heat of passage melts the glue and in a few minutes the nail is glued in fast. If you drive these nails part way, they will freeze in that position and will bend if you try to drive them deeper later on. Also, they will be difficult to pull out. It is best to drive coated nails all the way at once. Nails coated with powdered rosin will react in somewhat the same way.
The final consideration is the size of the nail in relation to the size of the wood. Obviously, a spike will split a thin, dry slat. A good rule is to choose a common nail which will not penetrate all the way through the last piece of wood. If the wood is extremely dry, choose a coated box nail of the same length as the common nail. If you are fastening a thin board to a thick one, use a nail that is long enough to go through the thin piece and two-thirds of the way through the thick one.
This standard can apply to virtually all mixed-size carpentry and all normal household work except the laying of hardwood floors (where special steel-cut nails are used) and the fastening of wood trim. Since nearly all trim is made of softwood, long, slim nails are used. There is little load on the nail, and often the nail must reach through an empty space before reaching the foundation to which the trim is actually fastened.
Locating the right spot in which to drive the nail is also of vital importance. Driving across the grain is usually the correct procedure. It holds better and is less apt to split. Driven along the grain, the nail is easily pulled out. Any shearing stress placed on a nail so driven will split the wood. A nail placed too close to the end grain will split out. Placed too near the edges of the board, it will also produce splits. A single nail will permit a certain amount of swing in the joint, but two nails, not placed in the same grain line, will eliminate swing and more than double the strength.
Although we have discussed nails in relation to their holding power against pulling outward, nails should never be used for that strength alone. The nail should be so placed that strain against it is crosswise, not along its length. A nail can be pulled out, but it is extremely difficult to shear it off. Place all nails in your construction work so that this principle is observed. With the exception of trim, which carries little load, all structural joints can be made so that the weight and live load will drive nails deeper or force the load against their shear angles. If you find a spot where this is not possible, something other than simple nailing strength is needed. Use angle irons, straps, bolts or some additional means of securing. Study the sketches to note the right-and-wrong techniques. They’ll assure safety on your building projects.
Nails are ordered by penny number, or “d”. Once based on price per nail, this number is now related only to length. The table below shows length of nail in relation to this number and the number of nails of that size you get per pound. In deciding what length of nail to use, rule-of-thumb calls for setting the nail two-thirds of its length into the second of two pieces to be joined. Use the thinnest possible nail m very dry wood to avoid splitting. Pre-drill holes for larger nails, particularly in hardwoods. Avoid using rusty nails as they may bend or break.