What a post-tension slab is
A post-tensioned slab is a concrete slab reinforced with steel cables that are tensioned after the concrete cures — that is what "post" means in the name. Plastic-sheathed steel strands, called tendons, are laid in a grid before the pour. Once the concrete reaches a specified strength, each tendon is stressed with a hydraulic jack and anchored at the slab perimeter, putting the whole slab into compression.
The reason that matters is a basic property of concrete: it is strong in compression and weak in tension. When expansive clay lifts the middle of a house, or drops away from its edges, the slab has to resist bending, and bending puts part of the slab in tension. Precompressing the concrete means the tensile stress has to overcome the built-in compression before the concrete sees any net tension at all. The slab therefore resists those stresses without cracking as readily as a conventionally reinforced slab does under the same movement.
How to identify one. The tendon ends anchor at the slab perimeter, and the pockets where they were stressed get patched. Those patches are visible as a regular series of circles around the slab edge, roughly evenly spaced. That is the field identification for a post-tension slab, and it is the thing to look for on a walk-around. Some builders also leave a plaque or slab stamp noting post-tension construction, often in the garage.
One consequence follows immediately and it is the most practically important fact on this page: the slab contains stressed steel under very high load. Cutting, coring or anchoring into it without first locating the tendons is genuinely dangerous and destroys the reinforcement along that line. That applies to plumbers cutting for a repair, contractors setting anchors, and anyone installing a post, a handrail or a car lift.
When post-tension became the North Texas standard
The design lineage runs from empirical methods in the 1950s, through the BRAB method published in 1968, to the Post-Tensioning Institute methods that begin in 1980. PTI developed detailed residential design guidelines in 1980, based on extensive parametric studies. Those guidelines were adopted by the model building codes, and by PTI's own account millions of residential foundations have been constructed using these standards since 1980.
The governing standard for a residential post-tensioned foundation on expansive soil is PTI DC10.5, "Standard Requirements for Design and Analysis of Shallow Post-Tensioned Concrete Foundations on Expansive Soils" — most recently the 2012 and 2019 editions. This is a detail worth getting right, because it is routinely stated incorrectly: residential post-tensioned slabs-on-ground are not governed by ACI 318. If a document about your foundation cites ACI 318 as the controlling standard for the slab design, the person who wrote it is working from the wrong book.
The year nobody can actually give you
Here is the honest position, and it is more useful than a confident wrong answer. There is no primary source that documents the exact year post-tension became the majority foundation type in any specific North Texas county. Not in the city ordinance record, not in the appraisal district data, not in PTI's own publications. Anyone quoting you a precise cutover year — 1983, 1987, 1990 — is repeating a trade impression, not a documented figure.
What can be said and defended: PTI's residential design standard dates to 1980, and post-tension became progressively dominant in North Texas expansive-soil subdivisions through the 1980s. In practical terms, houses from roughly the mid-1980s onward are increasingly likely to be post-tensioned, and houses from the 1970s and earlier are almost always conventionally reinforced. Do not assume from the year alone. Walk the perimeter and look for the patched pockets.
How a post-tension slab behaves in expansive clay
A post-tension slab does not stop the soil moving. Nothing does. What it does is let a stiffer, precompressed slab ride the movement with less cracking than an equivalent conventionally reinforced slab would show.
The two movement shapes are the same as for any slab in the Blackland Prairie. In centre lift, or doming, perimeter soil dries and shrinks while the middle stays comparatively moist, and the slab arches over a high centre; distress concentrates at the perimeter and exterior doors bind. In edge lift, or dishing, perimeter soil takes on water and swells, lifting the edges relative to the centre, and distress concentrates toward the interior. North Texas houses commonly cycle between both across a year, which is why elevation surveys taken in different seasons can legitimately disagree.
Because a post-tension slab tends to move as a stiffer unit, its distress often reads differently. Rather than a scatter of fine cracks distributed across the floor, movement is more likely to show up in the finishes and the veneer — sticking doors, cracking at door and window corners, stair-step separation in brick mortar joints, tile that tents at a line. The absence of visible floor cracking is not evidence that a post-tension slab has not moved. It is partly evidence that the design is doing its job.
Broken and blown tendons. A tendon can fail, usually from corrosion at the anchorage or from having been cut. The characteristic visible sign is a cone-shaped concrete blowout at the slab perimeter where an anchor has released. This is one of the few genuinely post-tension-specific findings, and it is one that belongs in front of an engineer rather than a repair salesperson, because the remedy depends on the tendon layout and the design assumptions.
What an inspector can and cannot determine
Under the Texas Standards of Practice, a real estate inspection is a limited visual survey and basic performance evaluation of the systems and components of a building using normal controls, and it is not intended to be a comprehensive investigation or exploratory probe to determine the cause or effect of deficiencies (22 TAC §535.227). Inspectors are also not required to inspect anything buried, hidden, latent or concealed — a category that includes, in this case, every single tendon.
An inspector can:
- Identify the foundation type from patched stressing pockets at the slab edge, a builder plaque, or a slab stamp.
- Report visible distress at the slab edge, including a cone-shaped blowout consistent with a released anchor.
- Report interior and exterior performance — door reveals, sheetrock and veneer cracking, floor slope, tented tile, separation at porches and patios.
- Report grading, drainage and irrigation conditions at the perimeter, which is where the actual driver of movement usually lives.
- Note where prior repair work is visible, and whether it appears effective and workmanlike.
An inspector cannot:
- Locate tendons. That takes ground-penetrating radar or equivalent scanning, performed as a separate service before any cutting.
- Determine whether a tendon is broken anywhere it is not visible at the perimeter.
- Verify the original design — tendon spacing, beam depth, soil report assumptions, the effective prestress actually achieved. None of that is visible, and none of it is in the inspection scope.
- State whether repair is needed. The Standards of Practice specifically relieve inspectors of the obligation to provide repair cost estimates, recommendations or re-inspection services.
When distress warrants it, the referral is to a Texas-licensed professional engineer, usually alongside a plumbing static test. Some North Texas municipalities reinforce this at the permit counter: Plano's adopted residential code amendments require that every foundation or footing — and any size addition to an existing post-tension foundation — be designed and sealed by a Texas-registered engineer. If a house has had permitted foundation work in a city with that amendment, an engineer's sealed document exists and a buyer should ask for it.
What people get wrong about post-tension slabs
"The house has a post-tension slab" is treated as a finding. It is not. It is a description of the construction type, and in North Texas expansive-soil subdivisions it is the standard construction type for the era. Discovering a post-tension slab in a 1992 house is like discovering it has composition shingles.
"Post-tension means it will not move." No. It means it is designed to resist the tensile stresses that movement generates without cracking as readily. The soil beneath it is the same soil. The maintenance obligations at the perimeter are the same obligations.
The patched pockets get read as prior repairs. Those evenly spaced circles around the slab edge are original construction — they are where the tendons were stressed and the pockets grouted. They are the signature of the system working as designed, and they are periodically written up by inexperienced observers as patched cracks or prior underpinning.
"Post-tension slabs cannot be repaired." Overstated. What is true is that any intervention that penetrates the slab, or that changes how it is supported, has to account for where the tendons run and what the original design assumed — which is why this is engineering work rather than a matter of picking a pier count off a proposal. That is a real constraint, not an impossibility.
The most consequential error is a practical one. Plumbers, remodellers, fence contractors and homeowners cut into post-tension slabs without scanning. A severed tendon releases violently and removes the reinforcement from that line of the slab. PTI publishes a construction and maintenance manual for post-tensioned slab-on-ground foundations, DC10.2, which is the appropriate reference for homeowner-side guidance (Post-Tensioning Institute, slab-on-ground applications).
What to do about it
Confirm the foundation type before you need to know it. Walk the slab perimeter and look for the patched circles. Check the garage for a stamp or plaque. Note it somewhere you will find it again, because the moment it matters is the moment a plumber is standing in your kitchen with a saw in hand.
Scan before anyone cuts. Any slab penetration — a plumbing repair, an anchor, a post base, a floor drain — requires locating the tendons first. This is a routine, inexpensive service and it is not optional. Make it a condition in writing with any contractor working on or through the slab.
Manage the perimeter moisture the same as any other North Texas house. Grade falling away from the foundation, downspouts discharging well clear of the slab, irrigation that reaches all four elevations evenly, no beds ponding at the edge. A post-tension slab benefits from consistent perimeter moisture exactly as a conventional one does.
Take a blowout seriously and take cracking normally. A cone-shaped concrete failure at the slab edge is a specific finding that deserves an engineer. Hairline cracking, a door that sticks in August and frees up in March, and minor veneer mortar separation are the ordinary vocabulary of a house on a Vertisol.
If foundation work has been done, ask for the paperwork. In jurisdictions requiring engineer-sealed foundation design, a permitted repair or addition produced a sealed document. That document — with its elevation data, its date and its scope — is worth considerably more to a buyer than a contractor's warranty certificate, and sellers who have it are usually happy to produce it.
Common questions
How can I tell if my house has a post-tension slab?
Walk the perimeter and look for a regular series of patched circles in the slab edge, roughly evenly spaced. Those are the stressing pockets where each tendon was jacked and anchored, then grouted over. Many builders also cast a plaque or stamp into the slab, often in the garage, noting post-tension construction. Era is a weak indicator on its own — houses from the mid-1980s onward are increasingly likely to be post-tensioned, but you should confirm visually rather than assume from the year built.
Is a post-tension slab better than a conventional slab?
It is better suited to expansive soil, which is why it became the North Texas standard. Precompressing the concrete lets the slab resist the tensile stresses generated when clay lifts the centre or drops away from the edges without cracking as readily as a conventionally reinforced slab. That is a real engineering advantage in this soil. It is not a guarantee against movement, and it does not reduce the need to manage moisture around the perimeter.
Can a plumber cut through a post-tension slab?
Only after the tendons have been located by scanning. The strands inside the slab are under very high tension, and cutting one releases that load violently while removing the reinforcement along that line. Tendon location is a standard, separately performed service using ground-penetrating radar or equivalent equipment, and it should be a written condition of any work that penetrates the slab — plumbing repairs, anchors, post bases, floor drains, saw cuts of any kind.
What standard governs residential post-tension foundations?
PTI DC10.5, "Standard Requirements for Design and Analysis of Shallow Post-Tensioned Concrete Foundations on Expansive Soils," in its 2012 and 2019 editions. Residential post-tensioned slabs-on-ground are not governed by ACI 318, which is a common and revealing error in documents about residential foundations. PTI also publishes DC10.2, a construction and maintenance manual for post-tensioned slab-on-ground foundations, which is the right reference for homeowner maintenance questions.
What does a broken tendon look like?
The characteristic visible sign is a cone-shaped concrete blowout at the slab perimeter where an anchorage has released. Most tendon failures are not visible at all, because everything between the anchors is buried in concrete, and an inspector is not required to report on anything buried, hidden, latent or concealed. A visible blowout is a finding for a Texas-licensed professional engineer, because the remedy depends on the tendon layout and the original design assumptions.