Engineered Flooring Installation on Concrete Part 2

Best Practices for Nail-Down

This is the second article in our three-part series covering engineered flooring installations on concrete. Our first article focused on best practices for glue-down installations. In this article, we look at another install type-nail-down. Nail-down involves nailing or stapling the wood flooring material directly to a wood subfloor.

Here are a few key points made in the first article:

• Every on-grade concrete slab has moisture and will always have moisture. As a result, concrete will release moisture from deep within a concrete slab over time. This can jeopardize the stability of solid wood flooring resulting in warping, splitting, or curling.

Nail Down

• To avoid these flooring problems, the NWFA recommends using only engineered hardwood floors when an installation is to be made over below-grade concrete slabs. That’s because, due to its construction, engineered hardwood flooring is more dimensionally stable than solid wood flooring, so there’s less expansion or contraction.

• In addition, engineered flooring can be installed at any grade level, including below-grade.


Just like solid hardwood flooring, engineered flooring needs to be properly aligned with its environment before installation. Consult the specific flooring manufacturer for their requirements.

Do not store engineered wood flooring in garages or where humidity levels are higher.

To allow for proper acclimation, the heating/air conditioning (HVAC) system must be capable of maintaining the requirements set forth by the flooring manufacturer. According to NWFA Installation Guidelines, the HVAC system should be operational for a minimum of five days prior to installation at a temperature between 60°F – 80°F and a relative humidity level should be controlled between 30 – 50 percent at all times prior, during, and after installation.

To ensure the flooring is properly acclimated and has achieved equilibrium moisture content (EMC) with the surrounding ambient conditions, you need to measure the MC in the wood planks. Whether installing on a concrete or wood substrate, always reference the NWFA Installation Guidelines for the appropriate method of moisture testing.

According to the NWFA guidelines, moisture content of the wood subfloor should be no more than 13 percent (specifically in humid geographic regions). And, there should be no more than 2 percent MC to 4 percent MC difference (depending on flooring plank width) between properly acclimated wood flooring and the wood subfloor.


If a contractor plans to install a wood subfloor above a concrete slab in a new or existing home, it’s necessary first to measure the moisture conditions within the slab. Excessive moisture in a slab can damage a wood subfloor and, in turn, an engineered wood floor.

To accurately assess moisture conditions in a concrete slab, one commonly specified test is the ASTM F2170 relative humidity (RH) test. This test uses sensors or probes to measure the RH at a specific depth within the concrete – 40 percent of the slab’s thickness for a slab drying from one side, or 20 percent for a slab drying from two sides.

Scientific research confirms the reliability and accuracy of the RH test at these depths. The readings will most accurately predict the slab’s point of equilibrium, and moisture condition that will exist within the slab after the flooring installation.

Before installing a wood subfloor, a minimum class I vapor retarder meeting NWFA Installation Guidelines should be placed on the slab to protect the subfloor and the flooring from moisture.

How to Install Nail-Down Engineered Wood Flooring

When there’s a wood subfloor in place in the form of plywood or OSB, then a nail-down or staple-down installation is a common way to go.

Required Tools

Nail-down installations require additional tools, including a specialty flooring nail gun, saws, and routers. These are essential to complete the job successfully.

You also will need a chalk line and nail set, power drill, nails and underlay paper, miter saw, table saw, back saw/door jamb cutter, safety glasses, and dust masks.

Jobsite Preparation

Installing wood flooring should always be one of the last jobs on any construction or remodeling project. The contractor, however, must resist pressure to speed up installation for any reason without taking the necessary precautions and steps to ensure a successful installation. Proper job site preparation includes:

• Always following the flooring manufacturer’s instructions to validate warranty requirements.

• Make sure the building is completely enclosed with interior heating and cooling systems running before the flooring material arrives, to bring the building to normal living conditions.

• Acclimate the flooring material as specified by the flooring manufacturer.

• Measure the flooring planks and subfloor with a moisture meter for MC.

• Measure a concrete slab for moisture in the event a subfloor is to be installed above the slab.

Subfloor Preparation

• According to NWFA Installation Guidelines, the concrete subfloors should be clean, dry, stable, and flat to within 1/8” over a 6’ span and 3/16” over a 10’ span.

• Uneven subfloors can result in gaps, squeaks, and poor fitting planks during assembly.

• To achieve floor flatness, you may need to either grind down or build up with a suitable floor leveling material.

• Never apply a vapor retarder over a wood subfloor that has been installed over a slab. The vapor retarder must be installed below the wood subfloor, and over the concrete.


For engineered flooring, follow the manufacturer’s instructions on installation methods. In most cases, the flooring is installed by blind-nailing the material through the tongue of the floorboards into the wooden subfloor.

Nail-down and staple-down wood floors are the most- common and preferred methods if the installation is over a plywood or OSB subfloor. It’s cost-effective and doesn’t require much more than nails or staples.

Since engineered wood flooring comes in various thicknesses, consult the manufacturer for the proper type of nailing gun and length and gauge fasteners. According to NWFA Installation Guidelines each plank or strip should be nailed or stapled at 3”- 4” intervals for staples or 4”- 6” intervals for cleats, and within 1”- 2” of end joints.

Once the final rows are reached, there will be no space to use the floor nailer or stapler on the tongue. It will then be necessary to glue and/ or face-nail. Rip the last boards to fit, leaving the appropriate expansion space from the wall. Fill nail holes with putty. Cut any door jams and fit floor underneath. Install any required reducers at door thresholds. Nail-down installations allow flush-in transitional moldings and vents to be used instead of overlap moldings and drop-in vents – providing the improved look and feel of a flush mount transition.

With the right environment and maintenance, the chances of product failure are minimal. Should a board become damaged, replacement is seamless.

Manufacturer’s Specs

The last piece of advice is the most important: learn, memorize, and follow the manufacturer’s specs for all parts of the install, but most especially regarding cleat, staple, or nail sizes. Whichever you use, make sure you have the right size stipulated by the manufacturer. Follow the specs and advice, and you will be enjoying a beautiful look and feel of having a flush mount transition floor that will last generations.

Tony Morgan is a senior technician for Wagner Meters, where he serves on a team for product testing, development, and customer service and training for moisture measurement products. Along with 19 years of field experience for several electronics companies, Tony holds a BA in Management and has an AAS in Electronics Technology. He can be reached at 800.505.1283 or visit

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