Piering & underpinning: how many piers does a foundation need?
Underpinning stops a settling foundation by transferring its weight to stable soil or bedrock through piers. The count is simple geometry — affected length divided by spacing — and the count drives the cost.
When part of a foundation sinks because the soil beneath it can no longer carry the load, the fix is usually underpinning: driving or drilling piers down to competent soil or rock and transferring the building's weight onto them. The two common systems are push (resistance) piers, hydraulically driven using the structure's own weight, and helical piers, screwed in like a giant auger and better suited to lighter loads. Both are priced per pier, so the whole estimate comes down to one question: how many piers?
The counting formula
Piers are placed at intervals along the affected length of wall — not the whole perimeter, only the settling section. The count is:
piers = ceil(affected length ÷ spacing)
Spacing is a labeled planning band, typically about 6 to 8 feet, and your engineer sets the real figure based on soil bearing and the load each pier must carry. We round up, because you cannot install a fractional pier and the ends of the run need support. The piering cost tool does the ceiling and the multiplication for you, and the pier & strap spacing table lists the bands.
Worked example
Suppose a structural engineer identifies a 120 ft run of foundation that is settling, and specifies push piers at 6 ft centers. The count is 120 ÷ 6 = 20 piers. If your contractor quotes $1,400 per pier installed, the pier line is 20 × $1,400 = $28,000. Add any engineering and permit line the contractor lists separately. Change the spacing to 8 ft and the count drops to 15 piers ($21,000) — which is exactly why the engineer's spacing decision, not the calculator, governs the bill.
Why piering dominates a foundation budget
At four figures apiece, piers usually swamp every other line item. A single injected crack is a rounding error next to a 20-pier job. That is useful to know when you read a repair quote: if piering is on it, price that first and treat crack sealing, drainage and cosmetic work as the smaller follow-ons. Feed the pier total into the full repair estimator alongside the other lines to see the whole picture, and add a buffer with the contingency planner.
Push piers vs helical piers
Neither is universally "better"; they suit different conditions. Push piers rely on the weight of the structure to reach load-bearing strata and work well under heavier masonry homes. Helical piers generate their own capacity as they are screwed in, so they shine under lighter structures, additions and porches where there is not enough weight to drive a push pier. The engineer picks the system, the depth to competent soil, and the spacing; your job is to get an itemized per-pier price from each contractor and compare on equal footing.
What a pier job includes besides the piers
The per-pier price is the headline, but a complete underpinning job carries other lines worth budgeting for. There is excavation to expose the footing at each pier location, often by hand in tight spots; the engineer's report and design, which some contractors list separately; the permit and inspection; and, once the piers are in, the option to re-level the structure by lifting it back toward its original position rather than simply stabilizing it where it sits. Lifting is not always possible or wise — pushing an old, brittle structure back up can crack finishes — so it is a decision for the engineer, and it affects both risk and price.
Interior finishes add cost too. Piering a corner under a finished basement means removing and later restoring flooring and drywall, while piering an open, unfinished perimeter is far cleaner. When you compare two pier quotes, check that they include the same scope: 20 piers plus engineering, permit and re-leveling is a very different number from 20 piers alone. Feed the whole set of lines into the repair estimator so you are comparing complete jobs, not just pier counts.
What the count does not tell you
The formula gives you a planning count and a planning cost, not an engineering design. Real pier layout accounts for load path, soil bearing at depth, and the exact extent of movement — all of which require a licensed structural or geotechnical engineer. Underpinning is structural excavation and needs a permit and inspection. Use the number to budget and to check that a quote's pier count is sane for the length involved; use the engineer's report to actually build.