Record each floor flatness reading with its location, measurement method, reference span, and applicable tolerance. A gap measurement alone is not enough to judge a floor: the reading and the requirement must use the same span and method.
Use the tables below to keep project details, individual readings, and rechecks together. For straightedge measurements, record the maximum gap under each placement. Keep observations about high spots separate from measured gap values unless you have established a reference plane for measuring their height.
Record the Project Requirement First
Before measuring, locate the installation instructions for the selected flooring product and installation method. Copy the flatness requirement exactly, including its reference length and any separate short-span requirement. Do not substitute a tolerance from a different product or installation system.
The NWFA Wood Flooring Installation Guidelines, revised 2025 state that manufacturer recommendations for specific products supersede the association’s general recommendations. That guidance concerns wood flooring; it does not establish one acceptance limit for every floor covering.
| Project detail | Record |
|---|---|
| Project name or address | ______________________________ |
| Room or measurement area | ______________________________ |
| Measurement date and person taking readings | ______________________________ |
| Subfloor material and exposed surface condition | ______________________________ |
| Flooring manufacturer, collection, and product/SKU | ______________________________ |
| Planned installation method | ______________________________ |
| Applicable installation document, revision/date, and page | ______________________________ |
| Required measurement method | ______________________________ |
| Required reference span | ______________________________ |
| Maximum permitted deviation over that span | ______________________________ |
| Additional short-span or other flatness requirement | ______________________________ |
| Measurement equipment and smallest readable increment | ______________________________ |
| Room sketch or photo reference | ______________________________ |
If the flooring product has not been selected, record the measurements but leave the acceptance decision open. An assumed tolerance can support preliminary comparisons, but it cannot establish compliance with an unidentified product’s instructions.
Floor Flatness Measurement Record Table
Use one row for each straightedge placement and orientation. Assign an ID that also appears on a room sketch so another person can find and repeat the measurement.
The table uses feet for reference spans and inches for gaps. If measuring in metric units, relabel the columns and use meters for spans and millimeters for gaps consistently.
| Reading ID | Location and direction | Method / span (ft) | Maximum gap (in) | Permitted gap for this span (in) | Margin (in) | Comparison / observation | Photo ID |
|---|---|---|---|---|---|---|---|
| 01 | __________ | __________ | __________ | __________ | __________ | __________ | __________ |
| 02 | __________ | __________ | __________ | __________ | __________ | __________ | __________ |
| 03 | __________ | __________ | __________ | __________ | __________ | __________ | __________ |
| 04 | __________ | __________ | __________ | __________ | __________ | __________ | __________ |
| 05 | __________ | __________ | __________ | __________ | __________ | __________ | __________ |
| 06 | __________ | __________ | __________ | __________ | __________ | __________ | __________ |
For the comparison column, use specific descriptions:
- Within entered limit.
- At entered limit.
- Exceeds entered limit.
- Not evaluated—applicable tolerance unavailable.
- Not comparable—span or method differs.
- Reading unresolved—straightedge rocks or placement cannot be repeated reliably.
Do not enter zero when a reading was not taken. Write “not measured” and explain why.
Take and Locate Straightedge Readings
For wood flooring over concrete, the NWFA’s 2019 concrete-subfloor guidance, Part XI describes measuring gaps between a straightedge and the substrate, moving the straightedge to multiple locations, and documenting discrepancies with marks and photographs. Use that as supporting measurement guidance, not as a substitute for the selected product’s applicable instructions.
- Identify the measurement line. Mark its reading ID on a room sketch. Include the direction and enough dimensions from fixed features to relocate it.
- Check the contact surface. Remove loose debris that would interfere with the reading. Record any remaining surface condition that affects placement.
- Place the straightedge using the required method. Record its reference length. Do not prop it level or change its support arrangement unless the specified procedure calls for that setup.
- Measure the gaps along the placement. Use a suitable taper gauge, feeler gauge, or other measuring device with adequate resolution. Record the largest observed gap and where it occurs.
- Record instability separately. If the straightedge rocks, note the apparent high area and repeat the placement. Do not assign a high-spot height from a rocking observation alone.
- Repeat across the area. Take separate readings at additional locations and directions, following any specified sampling procedure. Identify inaccessible areas rather than treating them as checked.
- Photograph significant readings. Include the reading ID, placement, and gauge position clearly enough to connect the image to the table.
A reading describes the measured placement, not every part of the room. Do not average away an excessive gap where the applicable requirement limits the maximum deviation.
Keep High-Spot Observations Separate From Gap Measurements
A gap beneath a straightedge is a distance from the underside of that edge to the floor. It is not automatically the depth of a depression relative to the entire room.
Likewise, “high point: 1/8 in” is incomplete unless the reference plane and measurement method are defined. For an apparent high spot, record its location and the straightedge’s behavior. If using a laser or string line to measure elevations, keep a separate record of the datum, setup, measurement points, and sign convention.
Do not combine laser elevations and straightedge gaps in one maximum-gap column as though they were interchangeable measurements. The NWFA concrete-subfloor guidance describes laser, string-line, and straightedge setups separately.
Compare Each Gap With the Matching Tolerance
For a maximum-gap requirement measured over the specified span:
[
M = T – G
]
Where:
- (M) is the remaining tolerance margin, in inches.
- (T) is the permitted maximum gap for that span and method, in inches.
- (G) is the maximum measured gap for that placement, in inches.
Interpret the result only when the measurement and requirement are comparable:
| Margin | Meaning | ||
|---|---|---|---|
| (M > 0) | The recorded gap is below the entered maximum. | ||
| (M = 0) | The recorded gap is exactly at the entered maximum. | ||
| (M < 0) | The recorded gap exceeds the entered maximum by ( | M | ). |
Keep the original measurement precision when making the comparison. Do not round an excessive reading down to the limit. If the difference is smaller than the measuring equipment can reliably resolve, remeasure rather than reporting a definite result.
Example: Completing One Record
Assume, only for this example, that the applicable project document permits a maximum gap of 3/16 in under a 10 ft straightedge. This is an illustrative input, not a recommended tolerance for an unspecified flooring product.
At reading A-01, the maximum observed gap is 1/8 in:
[
M = \frac{3}{16} – \frac{1}{8}
= \frac{1}{16}\text{ in}
]
| Reading ID | Location and direction | Method / span | Maximum gap | Permitted gap | Margin | Comparison |
|---|---|---|---|---|---|---|
| A-01 | Living room, line marked on sketch, east–west | Straightedge / 10 ft | 1/8 in | 3/16 in | 1/16 in | Within entered limit |
This placement is within the assumed maximum-gap limit. It does not establish that other placements, separate short-span requirements, or other installation conditions have been satisfied.
Use the Floor Flatness Deviation Check With Your Record
The Floor Flatness Deviation Check provides fields for the measurement span in feet, maximum measured gap in inches, and selected project tolerance over that span in inches. Its displayed results include measured deviation per foot and remaining tolerance margin. Keep the location, source document, photographs, and observations in your measurement record alongside those numeric inputs.
Treat the per-foot value as a ratio, not permission to convert an installation tolerance to another span. Dividing a gap by its reference length does not describe the shape of the floor between measurement points. A requirement over a longer straightedge cannot be assumed to scale proportionally to a shorter one.
The tool compares entered measurements and limits; it does not identify the correct product requirement or verify how the measurement was taken.
Record Corrections and Rechecks Without Erasing the Original
Keep the initial reading and add a linked recheck entry. Repeat the original location, direction, span, and method where possible so the before-and-after values are comparable. Record nearby additional placements separately.
| Original reading ID | Work completed / date | Recheck date | Method / span | New maximum gap | Applicable limit | New comparison / photo ID |
|---|---|---|---|---|---|---|
| __________ | __________ | __________ | __________ | __________ | __________ | __________ |
| __________ | __________ | __________ | __________ | __________ | __________ | __________ |
| __________ | __________ | __________ | __________ | __________ | __________ | __________ |
A measured gap is not automatically the required grinding depth or patch thickness. The appropriate correction depends on the surrounding surface and the approved preparation system. For concrete receiving wood flooring, the NWFA guidance also calls for assessing moisture before applying patches or self-levelers and following the preparation-product manufacturer’s requirements.
Keep moisture results and substrate-condition findings in their own records. A flatness comparison documents one installation condition; it does not replace those checks.