There is nothing that will destroy a customers confidence in the flooring installation faster than an annoying "squeak". Many times the blame falls on the flooring retailer, when it is really something that is out of their control. The following are a few of the causes of squeaks in new construction.
Wet or green lumber (greater than 19% moisture content) Lumber used in today's structures is usually higher in moisture content than the lumber used in the years past. Once the structure is completed the lumber is subjected to dryer conditions and will dry to somewhere between 6-12%. As lumber dries, shrinkage takes place. For example a Douglas fir or pine will shrink .0026" per inch per percentage of moisture content loss. This shrinkage will result in the nail, which was tight, to loosen from the shrinkage. This in turn allows the underlayment to move up and down on the shank of the nail causing a squeak. Lumber shrinkage is also the major cause of nail pops, which are extremely difficult to repair.
Improper subfloor or underlayment spacing Floor panels will expand slightly when installed over a substrate with a higher moisture content. If no allowances have been made for spacing of the joints there can be enough movement to cause the underlayment to buckle pulling the fastener loose. This is especially true when improper fasteners are used or the correct fastening pattern not followed. Once the moisture content lowers, the wood will shrink resulting in a loose panel free to move up and down on the shank of the fastener.
Improper glued floor construction The construction industry has used the Engineered Wood Association's "Sturd-I-Floor System" for some time. This is the use of a non-staining construction adhesive used to glue and then nail the subfloor to the joist. Sometimes when a carpenter is installing subfloor panels they will either; spread more adhesive than can be covered or fail to have the panel nailed in place before the adhesive skins-over and begins to harden. The result is not only a poor adhesive bond, but built in gaps in the adhesive bead, allowing the panel to move up and down on the shank of the nail resulting in a squeak.
Improper fastening technique Power nailing has become increasingly more popular with "fast track" building techniques. The problem is that when using a pneumatic nailer it is impossible to tell when a nail has barely missed the edge of the joist allowing the shank of the nail to rub the edge of the joist.
The other cause of squeaks is in the spacing and fastening of the underlayment panels. Underlayment must be properly fastened at the edges, usually within 3/8" of the edge and then spaced properly along the edge. Spacing along the edge and keeping the fastener close enough to the edge will prevent the edges from rubbing together. Any rubbing at the edges of the underlayment can cause a squeak.
There are also squeaks that occur between the underlayment and the subfloor. This is caused by improper nailing patterns, improper fasteners or fasteners that are driven too deep into the underlayment. Each of these will allow movement between the underlayment and the subfloor when subjected to loads, causing deflection of the substrate.
Other causes of noises between the underlayment and the subfloor are; installing underlayment over a dirty, gritty surface, installing underlayment over a sticky residue on the subfloor or by attempting to glue the underlayment to the subfloor.
Lack of or improper blocking The blocking of the joists is normally done prior to the installation of the subfloor. The norm is to nail the blocking from the top then after the structure is enclosed, the bottom of the blocking is securely fastened. In more and more cases the blocking is either not done or is not secured on the bottom. This allows the floor joist, when subjected to a bearing load, to rotate or slide rather than transferring the load to the other joist.
Improper fastening of interior wall partitions If there is a squeak near an interior wall partition, the squeak may be a result of the wall plate being anchored to the subfloor rather than to the floor joist. Occasionally, the practice of carpenters is to nail the lower plate into the subfloor which, when walked on, will deflect slightly allowing the subfloor to move, rubbing on the shank of the nail.
Joist hangers When joists are properly set in joist hangers, they should rest firmly on the bottom of the hanger and should be anchored in all of the fastening holes provided. An all to common practice of carpenters is to fasten only the top of the joist hanger. As dimensional lumber dries, it shrinks away from the hanger and can move freely. The squeak occurs when the components rub together.
Improper nails The use of fasteners that are not designed for flooring purposes such as smooth shanked or coated nails are a major cause of squeaks. Coated nails are believed to have superior hold. Initially this is true, however this hold is only temporary and a deterioration occurs between the coating and the nail over time. When lumber dries there is shrinkage, which is not limited to just the outside dimensions of the lumber. There is a direct correlation to the shrinkage and the hold of the nail. As the shrinkage occurs the withdrawal resistance of the nail is compromised leaving a fastener with a poor hold of the panels. With deflection of the substrate the smooth shank of the fastener will start to withdraw. Then the panels will start to rub on the fastener with a squeak resulting.
Unevenness of joists With the use of lower grades of dimensional lumber, high yield growth lumber and higher moisture content in lumber we are seeing much more movement to the floor joists. This movement is a result of the lumber drying to it's in-service moisture content and will result in the lumber shrinking, twisting and bowing. When floor joists are installed they are supposed to be "crowned" upward. If the joists are not "crowned" or are placed randomly, the subfloor will be forced to bridge the high and low joists creating gaps between the subfloor and the low joists, generating a squeak between the movement of subfloor and the low joist.
Ductwork Structures with forced-air heating systems usually have the ductwork running between the joists and come up through the floor where the discharge vent is located. Squeaks occur when the discharge vent is cut too tight to the ductwork, allowing the deflection of the substrate to rub on the metal ductwork.
The above is an attempt to help understand the causes of squeaks and to help prevent them from occurring. All squeaks can be eliminated. It is a matter of determining the source of the squeak and then taking corrective action to eliminate the cause. While this sounds simple, it may not be as sometimes the elimination of an annoying squeak is a major undertaking.