Helical piers for slab homes: how they work and when they fit

How a helical pier works

A helical pier is a steel shaft, round pipe or solid square bar, with one or more spiral plates welded near the tip. A hydraulic motor turns it into the soil like a screw. FPA describes it as an end-bearing anchor "screwed into the soil by hydraulic drive equipment," driven to the torque needed for the required capacity. A steel bracket then moves the weight of the house from the grade beam to the pier^[2].

The key idea is that torque tells you capacity. The harder the pier is to turn at the end of installation, the more load the soil can carry. ICC Evaluation Service, which evaluates building products for code compliance, writes this into its acceptance criteria AC358^[1]:

• Predicted ultimate capacity equals the final installation torque times a factor called Kt^[1].
• Allowable capacity is 0.5 times that predicted ultimate capacity, a factor of safety of 2^[1].
• Kt must be verified by full-scale field installation and load tests, and each shaft size and helix style needs its own correlation^[1].
• For homes under the IRC, if no site-specific soil information is available, the minimum safety factor on torque-based soil capacity rises to 2.5^[1].

So every helical pier comes with a number: the torque it reached. That record is one of the best things about the system. Ask for it.

Helical piers vs push piers on a slab

Both are steel and both go under the grade beam. The difference is what drives them.

	Helical pier	Steel push pier
How it goes in	Screwed in by a torque motor[2]	Pressed in using the weight of the house[2]
What sets capacity	Final torque times Kt[1]	Resistance when the house begins to lift (driven to refusal or a firm layer)[2]
Needs heavy structure?	No	Yes. FPA lists "not enough weight" as a reason push piles fail to reach depth[2]
FPA failure modes	Wrong spacing, wrong angle, not driven to proper torque and depth below the active zone, poor bracket anchorage[2]	Not pushed to refusal, wrong angle, overload, bracket not clamped, drought releasing grip at shallow depth[2]

On a single-story slab house, the house may not weigh enough to push steel piers deep. That is where helical piers fit best: lighter homes, porches, additions and garages. On a heavier house that can drive push piers to a deep firm layer, push piers may reach further. The engineer decides. See steel push piers and pier types compared.

What AC358 covers, and what it does not

AC358 is the main standard behind "code-evaluated" helical products. It has limits worth knowing^[1]:

• Corrosive soils are outside its scope. That includes soil resistivity under 1,000 ohm-cm, pH under 5.5, high organic content, sulfates over 1,000 ppm, landfills and mine waste^[1].
• Corrosion is designed for 50 years. Steel thickness is reduced by a sacrificial amount over 50 years: 0.013 inch for zinc-coated steel and 0.036 inch for bare steel^[1]. Zinc coatings must meet ASTM A123, A153, B633 or B695 as applicable^[1].
• Settlement is not covered by the report. AC358 requires the evaluation report to say that settlement of a helical pile system is outside its scope^[1].
• A registered design professional must size it. The criteria say a registered design professional determines the system, the bracket and the loads^[1].

Myth: "helical piers last forever." The code basis is a 50-year corrosion design period^[1]. Longer numbers are marketing.

Helical piers on expansive clay

Helical piers do not escape the clay problem. FPA lists "not installing the pier to the proper torque and depth below the active zone" as a reason they move with seasonal moisture changes^[2]. The active zone is the depth where soil moisture still swings with the seasons. A pier that stops in it can rise and fall with it.

Heave is the bigger limit. FPA says the repair piers most used on Houston-area homes "are not tied to the grade beams, and thus cannot prevent foundation uplift due to heaving soils." Heave after repair "is not generally considered to be a failure of the piers"^[2]. A helical pile can be built to resist pulling forces, and AC358 covers tension loads^[1]. But a standard lifting bracket only holds the beam up, not down. If heave is the concern, FPA describes lifting the floor above the soil's potential rise, with piers that have enough depth and tension capacity to resist uplift^[2]. That is a custom design, not a standard pier job. See heave vs settlement.

What helical piers cost

Source (date)	Per pier, installed
HomeGuide (Nov 2025)	$2,000 to $4,000[3]
Today's Homeowner (Apr 2025)	$2,500 to $3,500[4]

Most slab repairs need several piers, so the count drives the total. Prices vary by region, depth and access. See foundation pier cost for full ranges. In Houston, the permit plans must show the location, quantity and type of piers, so the engineer's layout fixes the count before bidding^[6].

What to ask before you sign

• The product's ICC-ES evaluation report number. You can look it up in the ICC-ES reports directory^[5].
• Target torque and minimum depth for each pier, set by the engineer.
• A torque log showing the final torque reached at every pier.
• Coating: galvanized, and to which standard^[1].
• Spacing and bracket type, and who designed them.
• What the warranty says about heave and drainage upkeep.

When to call a pro

• A contractor recommends helical piers: have a structural engineer confirm pier type, count and spacing.
• Your soil may be corrosive (fill, landfill, very acidic soil): ask the engineer whether soil testing is needed first.
• Floors are rising, not sinking: ask an engineer to rule out heave before buying piers.
• Piers are already installed and the slab keeps moving: ask for the torque log and an engineer's review.