| Scenario | Deck (L × W) | Loads (dead/live/addl) | Pier capacity | Max spacing | Output (piers, grid) |
|---|---|---|---|---|---|
| Residential deck | 20 ft × 12 ft | 10 / 40 / 0 psf | 5,000 lb | 6 ft | 12 piers, 4 × 3 grid |
| Light commercial | 6 m × 4 m | 0.6 / 2.0 / 0.5 kPa | 35 kN | 2 m | 12 piers, 4 × 3 grid |
- Deck area: A = L × W
- Uniform load rate: q = dead + live + additional
- Uniform load total: Wu = A × q
- Total service load: W = Wu + point_load
- Design load: Wd = W × load_factor
- Allowable per pier: Pa = (pier_capacity × reduction) ÷ safety_factor
- Required piers by capacity: Nreq = ceil(Wd ÷ Pa)
- Auto grid: rows = ceil(L ÷ max_spacing) + 1; cols = ceil(W ÷ max_spacing) + 1
- Final piers: Nfinal = rows × cols × cluster_per_point
- Select your unit system and enter deck length and width.
- Enter dead, live, and any additional load rates.
- Add optional point loads for localized heavy items.
- Provide pier capacity, safety factor, and reduction factor.
- Choose auto grid for spacing-based layout sizing.
- Or choose manual grid and enter rows and columns.
- Click calculate to view results and utilization.
- Download CSV or PDF for documentation and sharing.
Load inputs and tributary area
This tool estimates total gravity demand from deck area loads plus optional point loads. Enter dead load for framing and finishes, live load for use and furniture, and additional load for items like planters or snow. Total area load = (DL + LL + Add) × Area. Point loads are added directly to reflect localized heavy features.
Effective pier capacity and safety
Helical pier ratings are often stated as ultimate capacity. To plan conservatively, the calculator converts ultimate to effective capacity using: Effective = Ultimate × Reduction ÷ Safety Factor. The reduction factor covers installation variability and correlation limits. The safety factor provides design margin so early counts do not understate support needs.
Minimum piers versus layout piers
Minimum required piers are based on capacity: Nmin = ceil(Total Load ÷ Effective). Layout may require more piers to meet a maximum spacing target or to align with framing. Auto mode computes rows and columns from spacing and deck dimensions. Manual mode lets you test an existing grid and see utilization.
Practical interpretation on site
Use results for preliminary planning and budgeting. Final design should verify beam reactions, cantilevers, uplift, lateral resistance, and settlement. Soil conditions can change capacity substantially, so confirm with site-specific geotechnical data and manufacturer installation criteria. When heavy edge loads exist, consider tighter perimeter spacing.
Example data set
The values below show how a spacing-based grid can govern the final pier count even when capacity suggests fewer supports.
| Item | Value | Derived |
|---|---|---|
| Deck | 16 ft × 12 ft | Area 192 ft² |
| Loads | DL 12, LL 40, Add 5 psf | Total 57 psf |
| Point load | 400 lb | Included in total |
| Pier | 4500 lb ultimate | Effective 1440 lb |
| Factors | Reduction 0.80, SF 2.50 | Nmin 8 piers |
| Spacing | 7 ft max | Auto grid 4×3 = 12 |
1) Does this replace an engineered foundation plan?
No. It provides a preliminary pier count from loads, capacity, and spacing. Final design should confirm soil parameters, structural reactions, lateral resistance, and code-required details through a qualified professional.
2) What safety factor should I use?
Use the factor required by your engineer, jurisdiction, or manufacturer guidance. Higher safety factors increase pier count. If unsure, select a conservative value and validate against site-specific recommendations.
3) Why include a reduction factor?
Reduction accounts for uncertainties like installation variability, torque-to-capacity correlations, and group effects. It lowers the effective capacity per pier so the output remains conservative for early planning.
4) When should I use manual grid mode?
Use manual mode when you already have a framing layout, pier line constraints, or obstructions. It lets you test utilization for a fixed number of grid points without changing spacing logic.
5) How do I handle irregular deck shapes?
Approximate with an equivalent rectangle using the effective supported area, then refine with manual rows and columns. For large cutouts, split the deck into zones and evaluate each zone separately.
6) What does “cluster per grid point” mean?
It multiplies the grid points to represent multiple piers installed at one location, such as twin piers under a girder. Set it to 1 for typical layouts and increase only when required.
7) What’s included in the CSV and PDF exports?
Exports capture your inputs, calculated loads, effective capacity per pier, required minimum piers, spacing-based grid size, and final recommended pier count. They’re useful for records and quick sharing.