Manometer / Elevation Survey: How to Read Foundation Levels

A foundation elevation survey — the manometer or zip-level survey — is the measurement that turns "my floors feel off" into a number. A surveyor maps the relative height of points across your floor to reveal exactly how much, and where, the foundation has settled or heaved. That number, the differential settlement across the floor plan, is what separates a house that needs piers from one that needs only monitoring. It quantifies what visual signs like cracks and sloping floors can only suggest. What it does not do is decide the repair — that's the job of the engineer's report and, after it, foundation leveling.

What a Manometer / Zip-Level Survey Is

An elevation survey is a precise floor-elevation map. The surveyor establishes a zero (a reference point) somewhere in the house, then takes readings at points throughout the floor plan and records each one's height relative to that zero. Plotted out, those readings form an elevation or contour map that shows where the slab is high and where it has dropped — the pattern of differential movement.

Three instruments do this work, and the name "manometer survey" survives from the oldest of them:

• Water-level manometer. Uses the simple principle that water equalizes across a connected tube, so the water height at a measuring point reveals its elevation relative to the reference.
• Zip Level (pressurized hydrostatic altimeter). A high-precision instrument that reads elevation to roughly 0.01 inch (0.25 mm) without needing line-of-sight or a tripod, which makes it fast to run through a furnished house.
• Laser and digital levels. Optical instruments used alongside or instead of the above on many jobs.

The defining point: this page is about the measurement. The survey produces elevation data and nothing more. Reading that data — deciding whether the numbers mean active movement, original construction, or something else — is interpretation, and it belongs in the engineer's report.

How the Survey Is Performed

The mechanics are straightforward, which is part of why the data has to be read carefully rather than taken at face value.

1. Set the zero. The surveyor picks a reference point and assigns it zero. Every other reading is relative to it — so a "high" or "low" point is high or low compared to that spot, not to true level.
2. Take readings across the plan. Points are measured room by room across the floor, typically on a grid dense enough to capture the shape of the movement, not just two end points.
3. Record and map. Each reading is logged and plotted into an elevation map, often shown as contours. The total spread between the highest and lowest readings is the differential across the floor plan — the headline number.
4. Locate the affected area. The map shows where the movement is concentrated, which guides where any repair would focus and, in a pier-and-beam home, where to look in the crawl space.

A single survey is a snapshot in time. Its real diagnostic power comes when it's repeated, because a snapshot can't tell you whether a low corner has been low since the house was built or dropped last summer.

Reading the Results: Differential Settlement & Tolerance

This is where homeowners are most often misled. The number that comes off the survey is differential settlement — the difference in elevation across the floor, not an absolute defect score. Here's how to read it honestly.

Perfectly level is not the goal. There is no such thing as a perfectly level lived-in house, and chasing one is a mistake. The working tolerance most engineers reference: a slab home is generally considered out of tolerance once differential settlement exceeds roughly 1 to 1.5 inches across the floor plan. Stay consistent with this number — it's the same threshold used on our foundation leveling page, because it's the line at which the measurement starts pointing toward a repair.

The numbers professionals cite. The two most widely used performance criteria are 1% tilt (about 1 inch over 8 ft 4 in) and L/360 deflection, adopted by the ASCE Texas Section guidelines, the Post-Tensioning Institute, and the Foundation Performance Association (FPA-SC-13). On the construction side, ACI 117 levelness tolerance is about ±3/4 inch — a roughly 1.5-inch range that can exist purely from original construction, per the PTI's 2015 slab-on-ground evaluation guidelines.

Why the reading alone doesn't diagnose. Because homes are routinely built an inch or more out of level, an elevation number by itself does not establish a problem. It has to be read alongside distress evidence — active cracks, sticking doors, separations. Practitioners have produced conflicting conclusions on the same house using zip levels, precisely because two readers can weight the same numbers differently without the distress context. As a rule of thumb, most people don't notice a slope under about 1 inch over 20 feet but clearly feel 1 inch over 10 feet — perception and measurement are not the same thing.

Settlement vs heave. A survey shows where points are high and low; repeated surveys over wet and dry seasons show which way they're moving. That's how engineers tell soil settling down from clay swelling up — by mapping elevations over time rather than eyeballing a slope once.

Baseline & Periodic Surveys (Purchase + Every ~3 Years)

The single most useful way to use this measurement is to take it early and take it again. A baseline survey at purchase, repeated roughly every three years so it spans wet, drought, and normal moisture cycles, turns a one-time snapshot into a movement record.

The payoff is direct: a baseline distinguishes pre-existing movement from new movement. Without it, any out-of-level reading is ambiguous, and a contractor can present a long-standing slope as active settlement that needs piers. With it, you can show whether the house has actually moved since you owned it — and in an expansive-clay region where slabs rise and fall seasonally, that protection routinely saves homeowners from paying for repairs they don't need. If a re-survey shows under about an inch of differential and stable cracks, the answer is often monitoring, not underpinning.

Survey vs a Contractor's Eyeball

A contractor walking your floor and declaring it "off by an inch and a half" is giving you an opinion. A survey gives you a measured elevation map you can hand to an independent engineer, compare against a baseline, and use to bid the job. The difference is incentive: a foundation-repair contractor's free inspection is a sales visit — the company profits only if it sells a repair, which biases toward reading any slope as active settlement. A paid survey from an independent inspector or engineer reports the same number whether or not you ever buy a repair.

What a Survey Costs

An elevation survey is rarely sold entirely on its own — it's usually bundled into a broader inspection or engineering assessment.

Set that against the work it informs. A multi-pier repair can run into the tens of thousands, and the national average foundation project sits near $5,179 (This Old House, 2026), with HomeAdvisor's 2025 range at $2,225–$8,133. Against those numbers, a few hundred dollars for a measurement — and a baseline that can prove you don't need the repair at all — is among the cheapest insurance in the whole process. For where the survey fits in the full workflow, see the diagnosis overview.

FAQ Note

The questions below are what San Antonio homeowners ask most about the elevation survey itself — what it measures, how accurate it is, how to read the differential against tolerance, and why a baseline matters. For the diagnosis that turns the survey into a repair scope, start with an engineer's report; for what the numbers mean for the fix, see foundation leveling.

Get Matched With a Vetted San Antonio Specialist

If a survey has quantified your differential — or a contractor quoted piers and you want the movement measured and read by a PE before you commit — we'll match you with a vetted San Antonio foundation specialist who works from a sealed engineer's design, not a walk-through. The match is free, the quote is no-obligation, and we don't take a fee from you. We screen for an independent elevation survey, a sealed Engineer-of-Record letter, a documented baseline where one exists, and a clean Bexar County permit record. If a quote rests on an eyeballed slope instead of a measurement, we'll tell you. That's the only way an editorial matching service should work.