How is a foundation elevation survey done?

Reviewed by Preston, Oklahoma foundation repair consultant · Updated 2026-10-14

In short: An elevation survey measures relative floor heights across a house from a single reference point and turns them into a contour map of the foundation's shape. Engineering guidelines use it to check deflection (bowing) and tilt against limits such as L/360 and 1 percent, always alongside visible distress. Because slabs are not built perfectly level, comparing surveys taken at different times is the clearest way to measure actual movement.

What does an elevation survey show?

A floor elevation survey records the shape of a foundation at one instant ². Post-Tensioning Institute (PTI) evaluation guidelines describe it as one of the most common tools for judging foundation performance, and note that the only absolute way to measure movement is to compare changes between two surveys ³.

In the ASCE Texas Section guidelines, a "Level B" investigation adds relative foundation elevations, taken with floor finishes considered, in enough detail to show the foundation's shape, plus a drawing of those elevations ².

How are readings taken?

Instruments

The Foundation Performance Association (FPA) guideline calls for a level-measuring device accurate to plus or minus 1/8 inch or better, and says a spirit level should not be used to measure slab slope ¹. The guidelines set an accuracy requirement rather than naming specific tools, and PTI notes that contour intervals should match the instrument's accuracy ¹³.

Reference point

The survey starts from a reference point set to 0.0, ideally a spot that is easy to find again, such as a corner or a point near the center of the house ¹. PTI notes the reference is typically near the center of the foundation and may be an arbitrary point rather than a true benchmark ³. The FPA guideline recommends returning to the reference during the survey to confirm the instrument still reads the same ¹.

Spacing

FPA recommends a reading every 30 to 100 square feet where practical, with more in large rooms ¹. PTI notes that a span needs at least three elevations to describe its shape ³.

Floor coverings

Tile, wood and carpet have different thicknesses, so readings on top of finishes are either corrected for covering thickness or flagged ¹. PTI distinguishes a raw "survey elevation plan" from an "adjusted elevation plan" corrected for coverings and steps, which shows the true distortion ³. At a flooring change, FPA suggests several readings on each side, averaged ¹.

How are the results read?

Tilt versus deflection

• Tilt is the whole foundation sloping one way, like a tabletop with short legs at one end.
• Deflection is bending, where the middle is higher or lower than a straight line between two points, giving a dome or dish shape.

The deflection ratio is the largest departure from a straight line between two points divided by the distance between them ².

Common limits

ASCE Texas treats L/360 as a screening benchmark: below it, material deflection is unlikely unless distress shows otherwise; above it with few symptoms, more information is needed; above it with enough symptoms, an engineer is generally justified in finding excessive deflection ². That guideline also notes that slopes over 1 percent are usually noticeable to occupants ².

Houses are not built flat

FPA cites the ACI 117 concrete tolerance of 1.5 inches overall (plus or minus 3/4 inch) and reports that most residential slabs are built within about 1.0 inch of level ¹. PTI likewise notes that a 1.5-inch spread can be original construction ³. Without an as-built survey, the original shape is unknown, so deflection and tilt from a single survey are approximations ².

Heave or settlement?

Without a fixed benchmark, PTI notes, contours alone do not show whether an area heaved or subsided ³. See heave vs settlement. Crack and distress locations are compared against the contour map using the distress pattern library.

Why repeat a survey?

Time-change elevations are calculated by subtracting the older reading from the newer one at each point, using equivalent plans and the same locations ¹. PTI notes time-change contours are typically drawn at 0.5-inch increments ³. Clay soils move with seasonal moisture ⁵, so survey dates matter; the Oklahoma Mesonet runs 120 weather stations with at least one in every county, a public record of local rainfall around each survey date ⁴. Follow-up surveys after repair are part of a monitoring program.

Related

• Inspection protocol
• Tolerances and standards
• Working with engineers
• Sloping floors
• Mesonet for foundation pros