Installation of Wood Flooring
by Charles Peterson

This article refers to the simplest of hardwood flooring to install, _ inch thick solid strip flooring up to 3 _ inches wide strip flooring. Solid strip flooring can only be installed above grade. Solid strip products must be nailed down and not glued. The first step to creating a floor that will last a lifetime is a thorough job-site inspection. A hardwood floor should be one of the last items installed in a home. The hardwood and subfloor should not be exposed to periods of excess humidity or moisture. Wood is hygroscopic, which means it can absorb water and expand in size or loose water and shrink in size. The effects of small changes in the strip flooring dimensions are multiplied over every piece of wood.

It seems that many times the simple facts about wood are forgotten. We can not stop wood from acclimating to its environment. It will gain moisture and swell when the relative humidity increases and shrink when it looses moisture to an environment that is dryer. As we know, wood expands when it gains moisture and shrinks when it looses moisture, every time! This may seem like a simple premise but every day I am called with a problem which is related back to moisture.

Since wood is sensitive to moisture, it makes sense that wood floors should be one of the last items to be installed during the construction of a home. We want the moisture content of the wood to be as close as possible to normal living conditions. Wood flooring that is allowed to gain moisture on the job site prior to installation may have big gaps between the boards during the next dry season of the year. Wood flooring that picks up moisture after installation may experiences cupping or buckling. When the floor buckles it actual detaches from the subfloor and it is not uncommon to see the flooring lift one foot into the air. I have seen construction dampness make many floors expand to the point in which the doors and windows of the building are pushed out of the wall by inches.

Construction dampness must be dissipated prior to the wood flooring being delivered. Construct moisture is accumulated during the building process. Poor building practices may allow the subfloor to become flooded. Framing material may have been stored outside or rained on during construction. Construction material comes from the manufacture kiln dried but this is no longer the case after it has been rained on.

Moisture also comes from the curing and drying of different materials used during construction. Concrete used for the foundation gives off water as it cures. Paint and plastering also liberate large amounts of water as they dry and the wood floors act as a large sponge.

At first, it may seem a little extreme but prior to the wood flooring being delivered, a thorough inspection of the exterior of the building should be accomplished. Water should drain away from the building and not into it. Down spouts from the gutters should also drain away from the building. Windows and doors should be installed and not leak. Moisture from any of these sources has ruined countless wood floors.

The heating and cooling system should be running, if possible, prior to the wood flooring being delivered. This will aid in removing residual construction moisture and help get interior of the home to its normal expected environmental conditions. It may take a week or two to remove the residual moisture out of the building.

Basements and crawl spaces can become areas of high humidity. Moisture may be given off by the foundation’s curing concrete. There may also be moisture vapor permeating through the pores of the concrete from the ground. Basements and crawlspaces can sometimes be cooler than the source of incoming air. As the air cools, the humidity levels rises. The surface of the cool concrete can also condense moisture out of the air. A small relative humidity difference of 10% between the first floor and the basement can cause the wood floors to cup. For these reasons many basements and crawlspaces may require a dehumidifier.

National wood floor guide lines state the moisture content wood flooring should be within four percent of the subfloor moisture content. Although it only takes a 3% difference in moisture content from the top to bottom of the piece of wood flooring to cause cupping. Hopefully the wood flooring and subfloor will equalize before there is a problem. This would correspond to the basement relative humidity and living area relative humidity corresponding to each other.

It is good practice to write the moisture readings and date on the subfloor just prior to laying the hardwood. To estimate the amount of hardwood required accomplishing a job, starting by measuring the square footage of the area. Industry standards allow a tolerance up to five- percent defects of the hardwood square footage. Add five percent for cutting waste. There will not be enough material if the room is irregular or if the hardwood is installed on a diagonal. Order eight to nine percent overage for a floor installed on a diagonal. A few boards should be left with the floor to repair any damage that occurs during the life of the floor.

The subfloor must be structurally sound. Any squeaks should be taken care of prior to the hardwood being installed. Generally the deflection in the center of the room is limited to 1/2'". I measure the difference loaded and unloaded using a string line or a laser. You can generally guess that the floor has too much deflection when it feels a little bouncy.

Wood floors are general installed over wood Subfloors or concrete. We of course want them to be structurally sound, clean and dry. In order for the wood flooring to perform correctly it should be installed over something structurally sound and flat. When we refer to flat we are necessarily referring to level.

The subfloor should be flat within 3/16 of an inch over ten feet. Flatness refers to the highs and lows in a floor and not the levelness of a floor. CDX Plywood, Oriented Strand Board and one by six-inch solid boards are the approved substrates for nailing hardwood strip floor to. Do not install over particleboard! It does not have enough fastener holding power. Minimum thickness for the subfoor should be 5/8 of an inch in order for the fasteners to obtain full holding power. Solid boards installed on a diagonal seem to have the best fasteners holding power. Plywood seems to out perform OSB after moisture cycles in allowing hardwood to return to original position with less permanent gaps between hardwood strips.

The subfloor material should be installed per the manufacture’s recommendations for fastening and also the clearance between sheets. Normally sheets are spaced 1/8 inch apart and nailed every six inches on the joists. Plywood sheets should installed with the face grain at ninety degrees to the joist such that long sides of the sheets are parallel to the joists.

A 15-LB asphalt felt building paper stapled to the subfloor will retard moisture movement from below. Red rosin paper can absorb and retain moisture, which is detrimental to a floor. Red rosin has a tendency to break down over time and the wax coating on rosin paper is not always consistent.

Strip hardwood flooring is normally installed. Perpendicular to the floor joist. This is generally parallel to the longest dimension of the room, which is visually the most ascetically pleasing way. Hardwood flooring may be installed parallel to the joist but the subfloor must be a minimum of 1 1/8 inch thick or the floor joists must be blocked every twenty-four inches with 2" x 6" or larger lumber. Hardwood flooring is generally installed parallel and next to the longest wall of the area being worked. In rooms twenty feet or wider it is good practice to start installation in the center of room. A slip-tongue is used to install the flooring in two directions. Hardwood flooring expands towards the tongue, by installing in opposing directions you lessen the expansion of the floor in one direction.

There does not seem to be any rooms that are square. Many rooms will have walls out of parallel. Compensate for out of parallel walls by splitting the difference between the two walls.

The starting row of hardwood must be a perfectly straight line. When starting the flooring by the wall place marks the width of the flooring plus _ of an inch for expansion and _ of an inch for the tongue. Snap a chalk line between these marks. Use the longest and straightest boards for the first row. Faces nail the first row 1 inch in from the groove every 10 to 12 inches and within 2 to 3 from the ends using 7D or 8D nails. Blind nail the tongue using the same spacing. A clearance of _ of an inch should always be left for expansion between vertical obstructions. Most new trim is not thick enough to allow cover area. A good amount of this clearance can be obtained by trimming the bottom of the sheetrock.

Installations that are started in the center of the room use backer boards screwed along the starting chalk line to keep the first rows of flooring straight. In cases were the strip floor will continue into adjoining areas, it may be necassary to adjust your starting line to benefit the floor in other rooms. After enough rows have been installed the backer boards are removed. A slip-tongue is glued into the groove of the first starting row. The tongue is then blind nailed as normal. A small piece of flooring is slid along the tongue to ensure proper alignment when nailing. Hardwood may then be installed in opposing directions. Slip-tongues are also used to back fill into closets and out cove.

Lay out as many rows of flooring as much as possible. Leave enough clearance between the installed flooring and the unnailed strips to allow access for the nail gun. Blend the board color and grain variations into an aesthetically pleasing pattern. Stagger the butt joints at least six inches apart for aesthetics and strength. Try not to create stair casing or " H " patterns with the butt joints in the floor. Turn the last piece of strip flooring in a row around such that Cut the butt end checks the straightness of your rows as you work and adjust as necessary.

The last few rows of floor will need to hand nailed or top nailed. Many professional use some urethane construction adhesive under the last rows of wood flooring to limit the amount of top nailing on the floor.