How many helical piers you need comes down to one line of arithmetic: the load each pile has to carry (the area it supports times the load per square foot) compared with what one pile can safely hold. On a typical deck the framing usually decides the count, because you need a pile under every post. On an addition or a new home, an engineer sets the spacing along the foundation from the wall loads, and the count falls out of that.
Here’s how to rough it out yourself, with a worked deck example, and where rules of thumb stop being useful.
The load path in one paragraph
Weight goes from the deck boards or floor into the joists, from the joists into beams or walls, from the beams into posts or grade beams, and from there into the piles. Each pile carries the load from its “tributary area”, the patch of floor that drains its weight to that pile. Multiply that area by the design load per square foot and you have the load on the pile. Divide the total load by the allowable capacity of one pile and you know roughly how many you need, then adjust for where the framing actually puts supports.
Worked example: a 12 x 16 attached deck
Take a common deck: 16 feet wide along the house, 12 feet deep, ledger bolted to the house, one beam along the outside edge with no overhangs. The beam needs posts. Put them at 8-foot spacing and you get three posts: two corners and one center.
DCA 6, the American Wood Council’s prescriptive deck guide, gives formulas for tributary area. For a center post it’s half the joist length times the beam span; for a corner post it’s half the joist length times half the beam span (AWC DCA 6, Appendix). DCA 6 also assumes 40 psf live load plus 10 psf dead load, so 50 psf total.
| Post | Tributary area | Load at 50 psf |
|---|---|---|
| Center post | ½ × 12 ft × 8 ft = 48 sq ft | 2,400 lb |
| Each corner post | ½ × 12 ft × 4 ft = 24 sq ft | 1,200 lb |
| Ledger (house side) | Other half of the deck | Carried by the house, not by piles |
Three piles. For comparison, DCA 6 Table 4 calls for a 24-inch round concrete footing, 10 inches thick, under posts on an 8-foot beam span with joists up to 14 feet, assuming 1,500 psf soil. That’s the footing each helical pile replaces.
Two caveats before you take that number to the lumberyard. First, DCA 6 doesn’t apply where decks see snow loads over 40 psf, and Denver’s 2025 residential code lists a ground snow load of 43 psf (Denver Residential Code 2025, Table R301.2), so your plans examiner may want the loads checked by a designer. Second, a free-standing deck has no ledger, so the house side needs its own beam and its own piles. That’s why free-standing decks typically need more piles than attached ones.
Can one pile carry that? The torque check
Helical pile capacity is usually confirmed in the field from installation torque. The relationship is simple: ultimate capacity equals installation torque times an empirical factor, Kt. The Hubbell/Chance design manual lists default Kt values of 10 per foot for 1.5- and 1.75-inch square shafts, 9 for 2-7/8-inch round shafts and 7 for 3.5-inch round shafts (CHANCE Technical Design Manual, Sec. 6). ICC-ES AC358 uses the same approach and applies a factor of safety of 2 or more to get allowable capacity (ICC-ES AC358).
Run the center post from the example. It needs 2,400 lb allowable, so 4,800 lb ultimate with a factor of safety of 2. On a 2-7/8-inch round shaft with Kt = 9, that’s a final torque of about 535 ft-lb. That’s a light load for a helical pile, which is the point: on decks, pile capacity is rarely the limit. The framing is.
This is also why you’ll hear installers quote “8 to 10 feet between piles” for decks, a figure Techno Metal Post publishes as its rule of thumb. It’s really a beam-span number. A bigger beam spans farther and needs fewer posts; a smaller beam needs more.
What changes the count on a deck
- Beam size and span. Longer spans mean fewer posts but bigger beams.
- Attached vs free-standing. Free-standing adds a beam line along the house.
- Concentrated loads. Hot tubs are outside DCA 6’s scope entirely and need their own design. Roofs and pergolas add posts and snow load.
- Stairs. Stair stringers need their own landing support.
- Height. Tall posts need bracing; the pile count doesn’t change but the connection detail does.
- Basement proximity. Footings within 5 feet of the house foundation must bear at the house footing elevation under DCA 6, which affects depth more than count.
Helical pile spacing rules
Piles that are too close start sharing the same soil, and capacity drops. The Hubbell/Chance manual recommends center-to-center spacing of at least five times the diameter of the largest helix, with a three-foot absolute minimum that should be used only when there’s no other way to do the job (CHANCE Technical Design Manual, Sec. 5.2.5.2). For a 12-inch helix, five diameters is 5 feet.
The same section notes that only the helix plates need the spacing; shafts can be battered (installed at an angle) to spread the helixes apart under a tight cluster. On decks this rarely comes up. On a column with a heavy point load, where two piles share one cap, it does.
Additions and new homes: the engineer sets the count
For a building, the pile count isn’t something to estimate off a rule of thumb. Under the IRC, the ICC-ES report for the Chance system requires a registered design professional to design the helical pile foundation, including the bracket (ESR-2794). The engineer works from:
- Line loads along each wall (roof, snow, floors, walls), and point loads at columns and girder bearings.
- The grade beam or footing that spans between piles, since the concrete has to carry load between supports.
- The soils report: the bearing layer, the depth of the active zone on swelling soil, frost depth and groundwater.
- Uplift. On expansive clay the soil can grab the shaft and push up; tension capacity and embedment matter as much as compression.
The output is a pile layout drawing and a schedule: pile locations, model, minimum depth and minimum installation torque. The installer’s job is to hit those numbers and log them. If you’re comparing quotes for an addition or a house, compare them against the same pile schedule. A bid with fewer piles isn’t cheaper if it doesn’t match the engineer’s layout.
Quick checklist before you ask for a count
- A framing plan showing beams and post locations (decks) or foundation plan (buildings).
- Deck or building dimensions and height above grade.
- Any heavy items: hot tub, roof, masonry, kitchen island, safe.
- Soils report, if one exists.
- Access width to the work area.
Get a pile count for your project
Site Safe Helical Piers installs helical piles for new decks, additions and homes across Denver and the Colorado Front Range, built to the engineer’s layout with a torque log for every pile. Call 970-673-5615 or send us your plans. Related reading: helical piles for decks, helical piers for home additions, and our services.
