Checking Flatness Before Engineer Hardwood Goes Down (Explained)

Installing engineered hardwood floors can transform the look and feel of any room. However, in order to ensure a smooth and professional result, it is crucial to check for flatness before installation begins. Uneven subfloors can lead to an unsightly end product with gaps, hollow spots, and planks coming loose down the road.

With some careful preparation and the right tools, assessing flatness is not difficult. This comprehensive guide will provide readers with practical tips and solutions for evaluating subfloor flatness prior to engineered hardwood installation.

Introduction: Why Flatness Matters

When installing engineered wood floors, starting with a perfectly flat subfloor is key. Engineered hardwood planks are milled to strict tolerances, with very little play between boards. Any irregularity in the subfloor will telegraph through the finished floor, resulting in an uneven surface and other issues.

Gaps between planks, hollow-sounding areas, squeaking, and even popping out of place can occur if the subfloor is not flat to within 1/8” over 10 feet or 3/16” over 6 feet. This allows for no more than 1/64” variance between adjacent flooring boards. Exceeding these tolerances will likely produce a floor that is visibly uneven and does not perform properly.

Checking flatness applies to concrete and wood subfloors alike. Both must meet the above standards prior to engineered flooring being installed using glue, nails, or staples. Taking the time to correctly evaluate and address flatness will pay off through years of flawless performance.

Common Problem Areas

Subfloor flatness issues generally fall within a few typical categories:

General Unevenness: The subfloor surface simply has too much variation up and down and does not meet the 1/8” over 10’ standard. High and low spots will be readily apparent.

Out of Level: The subfloor slopes significantly in one direction. This occurs if the floor framing rests on uneven footings or the concrete slab was not properly graded.

Low Spots: Isolated depressions are present due to joist/truss spacing issues, surface damage, construction debris, or improper concrete finishing.

Crowns: Localized high points between joists where subflooring or concrete bows upward slightly. Often an issue with fastener spacing.

Cracks and Gaps: Openings have developed in concrete or between subfloor panels allowing movement. These must be addressed and filled.


The good news is there are time-tested solutions for each of these common subfloor problems:

General Unevenness

  • Sand down high spots: For minor high areas in wood subfloors, a coarse #20 grit sandpaper on a floor sander can efficiently level the surface.
  • Fill low spots: Applying a cementitious leveling compound can fill dips and voids in concrete or wood up to 1”. Feather smoothly into surrounding areas.
  • Double up subfloor: A second layer of exterior-grade plywood on top of existing boards can effectively smooth out minor highs and lows.

Out of Level

  • Shim the joists: If unacceptable slope is present, individual joists can be shimmed from below to level the framing within tolerances.
  • Slope the floor: The final engineered flooring can also be installed on a slight angle to match the existing slope without issue. A slope of 1” per 6’ or less will not be noticed visually if consistent.

Low Spots

  • Fill small holes: Use a leveling compound designed for concrete or wood subfloors to fill spots under 1/2″ deep.
  • Cut out and replace: For deeper depressions in a wood subfloor, cut out and replace sections with new exterior-grade plywood.
  • Grind down: A concrete grinder can also lower high areas surrounding depressions to achieve the desired levelness.


  • Refasten Subfloor: Adding additional screws or nails in crow areas can flatten bulges by pulling the flooring back down flush. Use ring or screw shanked nails if needed.
  • Plane or Sand: For minor crowns, a hand plane or belt sander can gradually remove the excess height in the affected area.

Cracks and Gaps

  • Fill cracks: Use a flexible crack filler designed for use under hardwood floors. These stay in place rather than cracking again.
  • Reattach boards: Secure any loose subfloor panels with additional nails/screws spaced 4” apart along the edges.

Evaluating Subfloor Flatness

Now that we’ve covered some of the fixes for uneven spots, let’s go through the process of actually evaluating the subfloor to find them. There are several accurate methods:

Visual Inspection

  • Start by simply looking over the entire subfloor surface and identifying any noticeable high or low spots, slopes, or cracks. Circle or mark areas that are suspect.
  • Glaring issues like large gaps or ridges will be immediately evident. But smaller subtler unevenness is often hard to see with the naked eye.

Straightedge and Level

  • Use a 6’ – 8’ straight edge such as an aluminum carpenter’s level or a straight piece of lumber.
  • Place the edge on the floor and see if light passes underneath when checking flushness. Mark any gaps.
  • A 4’ level is also handy for identifying localized high and low areas by rocking on three corners and marking inconsistencies.

String Lines

  • Stretch mason’s strings or braided fishing line across the floor in a grid pattern, with each line 4’ to 8’ apart.
  • Secure lines by anchoring with nails or taping to the perimeter walls. Look for deviations along the string lines.
  • To check a large area, use florescent spray paint to temporarily mark the string locations on the floor. Remove the strings and a map of any inconsistencies will be revealed.

Flooring Manufacturers Tolerance Gauge

  • Many hardwood flooring companies produce metal tolerance gauges that instantly identify spots exceeding 1/16” over 6’ or 1/8” over 10’.
  • Simply place on the floor and if the gauge teeters or rocks at all, it is out of tolerance in that area. Mark for correction.
  • These gauges are inexpensive and available from flooring retailers. Use the same brand gauge as the engineered flooring for accurate results.

Professional Measurement

  • For full due diligence, hire a certified contractor to digitally measure floor flatness with advanced instruments like a self-leveling rotary laser or digital elevator.
  • They will provide a detailed topographic map of the entire floor surface to identify all low and high areas that must be addressed.
  • While costly, this option ensures a perfect foundation for the flooring using professional diagnostic equipment.

Preparing the Subfloor

Once potential trouble spots have been identified, the subfloor can be prepared using the solutions previously outlined. Be sure to address any issues prior to installation day, as last minute fixes will delay the project.

Here are some final subfloor prep tips before starting engineered hardwood flooring installation:

  • Allow all leveling compounds, replacements, or patches to dry fully according to manufacturer instructions.
  • Recheck for flatness using a method above. Sand or grind down any dried areas that remain high.
  • Vacuum and mop the subfloor to remove dust, dirt, and debris. Screed concrete if needed.
  • Verify moisture testing meets engineered hardwood specifications. Concrete must be cured and dry.
  • Make any necessary structural repairs if the subfloor is damaged or flexes excessively.
  • Snap chalk lines to guide flooring installation and determine if any last minute tapering is required to account for minor slope.


A smooth, flat subfloor is the first step toward enjoying an incredible engineered hardwood floor. Taking the time to properly evaluate flatness, make needed repairs, and prepare the substrate will pay off for years to come. Take it from an expert – the initial investment is well worth it!

With the techniques outlined here, assessing subfloor flatness does not have to be difficult or expensive. A combination of common sense visual inspection, simple tools, and proven fixing methods will get the job done right. Just remember to reference the flooring manufacturer’s recommendations and test completed areas before proceeding with installation. Soon you’ll be walking on a stunning hardwood floor that looks amazing and performs flawlessly.

Similar Posts

Leave a Reply

Your email address will not be published. Required fields are marked *