Mechanical Fastening of Wood Floors
October 1, 2008
What is meant by “mechanically fastening wood flooring”? First, some background on the fastener. Before flooring nailers were invented, “8d cut nails” were considered the preferred flooring fastener. The steel nails were about two inches long, had a ¼-inch rectangular cross section at the head, a slightly mushroomed head, and were tapered to a 1/16-inch blunt tip. They were driven using a 20-oz. or heavier claw hammer or flooring hatchet. The hammer handle was often modified so the head actually fit the handle at a slightly drop angle to help with the angle nailing of the flooring. There was basically no mechanical help with nailing the flooring; the installer would simply drive the nail with the hammer using two or three blows. Then he would set the nail head with the head of the next nail to be driven.
The fastening or nailing process
Typically the areas away from the walls are “blind nailed,” fastening at a 45-degree angle directly above the tongue so the fastener does not show on the surface of the finished floor. “Face nailing,” driving the fastener vertically through the surface of the flooring, is done at wall lines where the nailing machine cannot fit or manual angle fastening is not possible. The only opportunity you get to properly blind nail the floor is as the flooring is installed. So, do it right the first time. Adding hidden fasteners later is not an option. The fastening process permanently positions the flooring so that movement between boards is minimized and most creaks, squeaks and noises are eliminated. However, fasteners won’t keep a floor from cupping or gapping that result from moisture issues or seasonal change.
For solid wood flooring the starting run is precisely positioned along the starting line and fastened by both face nailing and blind nailing. The vertical face nailing keeps this run from moving when subsequent runs are nailed against it. In addition, blind nailing this run is absolutely necessary as it holds down the tongue edge. This keeps the flooring from lifting when stressed from normal seasonal expansion. When using trim screws or threaded nails, drilling a pilot hole prevents splitting.
The fastening schedule for the starting run on wood frame construction is to face nail into each joist and at board ends. For joist spacing 19 inches and greater, add an additional nail between joists. Blind nail at the same locations as the face nails. With plywood on slab construction, face nail and blind nail the starting run every 10 to 12 inches and at the board ends. On a slab it is likely necessary to cut the nails to slightly less than 1½ inches long so they won’t go through the vapor retarder under the plywood and can be set below the face of the boards. I often use small bolt cutters to cut the nails and screws short, since lineman’s pliers or diagonal pliers require too much effort and wear on the hands.
The NOFMA recommended schedule for blind nailing the field of ¾-inch thick solid strip flooring using 2-inch fasteners is every 10 – 12 inches. In addition, a fastener should be located within 1 – 3 inches of each end of boards. All boards with factory ends should have a minimum of two fasteners. Where there are smaller cut-off blocks at the ends of runs, 6 inches and shorter, a single fastener is allowed. When nailing plank flooring, 4 inches and wider, or using shorter 1½-inch fasteners, the frequency should be increased to every 8 inches.
Special situations and cautions
First, don’t mix fasteners when nailing in the field. Staples and cleats hold differently, and if mixed can result in irregular seasonal gapping and or movement. It’s OK to use cleats or staples and hand-driven fasteners at wall lines but stick to one type in the field.
Wherever reversals or changes of direction occur, the exposed groove edge should be fitted with a spline or slip tongue and that edge blind nailed as if it were a tongue. This eliminates face nailing and a potentially weakened area where two grooves edges abut without engagement.
When fastening with staples, don’t overdo it. The ram driving the staple is ½-inch wide and if overdriven, it’s like driving a wedge into the wood, splitting it. Also, with a pneumatic tool there is the potential to drive extra fasteners, increasing the splitting risk.
When fastening factory finished flooring, consult the manufacturer’s directions for type of fastener, nailing tool recommendation, and frequency. Some finishes can fracture or bruise when hit or stressed during the nailing process. Using a smaller gauge fastener may be recommended. The recommendation may be the use of the over size base plate in order to distribute the driving force and associated recoil. The act of fitting the boards together may also bruise the edge so a specific hammer may be a recommendation.
Also, the species of wood may tend to split when fastened. Using a smaller fastener or drilling a pilot hole may be required. Again consult the manufacturer.
When face nailing pick areas of the grain that are more porous such as the open grain of oak. When filled, these holes can be color matched or stained to blend better than the tighter closed grain patterns.
When fastening significantly crooked or other out of plane boards, don’t use the fasteners to move the board into place. This extra movement can bend the fastener backwards and split the tongue. Drive the boards together first and then fasten.
You only get one chance to fasten the flooring so do it correctly the first time. Incorrect fastening automatically puts the contractor at risk of responsibility even when the nailing did not cause the problem.