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Calculate the number of deck posts, footing depths, and concrete requirements for your deck project. Ensure structural safety with proper post sizing and spacing.
Check local frost line requirements
Post is adequate
Standard spacings: 6 ft, 8 ft, 10 ft, 12 ft
Maximum spacing: 12 ft for residential decks
Typical posts needed: 1 post every 64 sq ft
Post size selection: 4x4 for heights up to 6 ft, 6x6 for taller decks
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Proper post spacing is critical for deck safety and structural integrity. The spacing of posts affects the load distribution, the ability of the beams to span, and the overall stability of the deck.
Common Residential Post Spacings:
Formula:
Number of Posts = (Deck Length / Spacing) × (Deck Width / Spacing)
Example:
20 ft × 15 ft deck, 8 ft spacing:
(20/8) × (15/8) = 2.5 × 1.9 ≈ 5 posts
Frost Depth Check: Local building codes specify minimum frost depths for your region
Concrete Grade: Use 4000 PSI concrete minimum for structural footings
Post Setting: Posts should be set on concrete pads, not directly in soil
Drainage: Ensure adequate drainage around footings to prevent water accumulation
| Post Size | Max Height (ft) | Load Capacity | Best Use |
|---|---|---|---|
| 4x4 | 12 | 3,500 lbs | Standard residential decks |
| 6x6 | 14+ | 8,500 lbs | Multi-story, heavy loads |
Note: Post sizing also depends on lateral wind loads, snow loads, and your local building code requirements. Always check with your local building department for specific requirements.
| Region | Frost Depth |
|---|---|
| Northern US, Canada | 36-48 inches |
| Midwest US | 24-36 inches |
| Southern US | 12-24 inches |
| Southwest, West Coast | 12 inches or less |
| Warm climates | Check local code |
Pressure-Treated: Best choice for outdoor decks - chemically treated for rot/insect resistance
Southern Pine: Strong, readily available, good for structural posts
Douglas Fir: High strength-to-weight ratio, excellent for beams and posts
Footings must extend below your local frost line to prevent heaving during winter freeze-thaw cycles. Frost depth varies from 12 inches in warm climates to 48 inches in northern regions. Check local building codes for your area.
Use 4x4 posts for most residential decks up to 12 feet high. Use 6x6 posts for taller decks or when supporting heavier loads. The post size depends on height, spacing, load, and wood species. Check your local building code for specific requirements.
Standard residential decks use 8-foot post spacing. Spacing can range from 6 feet (most conservative) to 12 feet (maximum for residential). Tighter spacing provides more support and accommodates longer beam spans. Local codes may have specific spacing requirements.
Concrete volume depends on footing diameter and depth. A typical 12-inch diameter footing at 36-inch depth requires about 3.5 cubic feet of concrete. Use this calculator to determine exact requirements based on your footing specifications.
Use concrete with a minimum 4,000 PSI compressive strength for structural deck footings. Pre-mixed concrete bags, ready-mix concrete trucks, or continuous concrete are all suitable options. Ensure proper mixing and curing time (typically 7 days).
Yes, pressure-treated posts (rated for ground contact) are designed for this use and are the most common choice. However, they should still sit on concrete pads rather than directly in soil for better longevity and to avoid rot at the base.
A post capacity check compares the actual load on a post against its maximum bearing capacity. If the actual load exceeds capacity (over 100%), the post is undersized and can fail. Use a larger post size or increase post spacing to reduce load per post.
Frost heave occurs when water in the soil freezes and expands, pushing footings upward. Prevent it by: 1) Digging below the frost line, 2) Using proper drainage around footings, 3) Setting posts on concrete pads in the footing, 4) Using a concrete pedestal above grade.
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