Deck Footing Spacing Guide

Estimate Workflow

1. Sketch deck size and ledger condition.
2. Choose joist direction and beam lines.
3. Use code tables or engineered drawings for beam/post spacing.
4. Calculate pier concrete with the sonotube calculator.
5. Confirm permit requirements before digging.

Laying out footings from framing specs down to holes

1. Start with the deck load calculation. Multiply deck area by 50 psf (40 live + 10 dead). A 16x20 deck = 320 ft² x 50 = 16,000 lb total load to distribute.
2. Divide by footing bearing capacity. A 16 in diameter footing on 1,500 psf soil has 1,400 lb/ft² x 1.4 ft² = 1,960 lb capacity per footing. Need 16,000 / 1,960 = 8.2, so 9 footings minimum. Layout becomes 3 rows of 3 footings.
3. Check AWC table for beam size against your joist span. AWC DCA6 is the standard reference. ICC/IRC 2024 references the same limits.
4. Confirm footing spacing equals beam span. Each footing falls below a beam support point or a post location.
5. Mark footing centers on the ground. Use stakes and string lines. Squared corners verified with a 3-4-5 triangle or laser.
6. Dig to required depth: frost depth + 6 in, with local code overlay. In Delaware I dig 36 in. In Minnesota I dig 60 in.
7. Pour concrete footings with anchor bolts or post bases set before concrete cures.
8. Inspect before backfill. Local code usually requires a footing inspection before the holes are filled in. Don't skip this; a failed later inspection means digging out.

For authoritative spacing and beam references, I rely on the AWC DCA 6 Prescriptive Deck Guide, the IRC 2024 section R507 for decks, and AWC DCA 7 for residential guard and handrail.

On a deck I rebuilt in Maryland in 2022, the original construction had used 4x4 posts on 10 ft beam spans with 2x10 beams. Every support condition was non-compliant with DCA 6. The deck was legal when built in 1998 under an older code, but it had never been structurally sound. When we pulled the cedar decking, the beam had sagged 7/8 in at mid-span and the 4x4 posts showed compression cracks at the bases. Rebuild specified 6x6 posts on 8 ft beam spans with 2-ply 2x12 beams. The new deck has zero visible deflection under a dozen people.

Code Cross-Checks Before Footing Layout

When I review deck beams, joists, 6x6 posts, concrete piers, and post bases on a job, I treat the published rule as the starting point, not the finished answer. The missing layer is the field condition: moisture, compaction, soil behavior, delivery tolerance, and the specific code table that applies in that county. In a Maryland deck rebuild where the old 4x4 posts were spaced 10 ft apart under a beam that AWC DCA 6 limited to 8 ft, the calculator math was not the problem. The problem was that nobody translated the calculator output into a field-controlled specification.

The checks below are the ones I use before I approve an order or a layout. They are deliberately numeric because vague wording such as "good gravel," "deep enough," or "standard slope" is where residential projects lose money. If the number is written down, a supplier, inspector, or crew lead can challenge it before material is placed. If the number is only assumed, the mistake usually shows up after the truck has left.

• 40 psf live load
• 10 psf dead load
• R507 deck framing
• DCA 6 beam table
• 1,500 psf soil

The recurring risk is starting hole layout before confirming the beam span table. My field correction is simple: mark every post center only after the IRC R507 and AWC DCA 6 spans agree. This is a small step, but it creates a paper trail and a repeatable decision. It also gives the homeowner a fair way to compare bids. A bid that includes density, compaction, depth, or code reference is usually more reliable than a cheaper bid with only a lump sum.

I also price the cost of being wrong. On one recent job, $220 in extra concrete and two additional posts prevented a $4,100 permit redesign. That is the kind of practical difference a guide page should help you catch before you call the supplier. The calculator gives the quantity; the field check protects the quantity from becoming the wrong purchase.

Sarah's pre-order verification notes

1. Write down the assumed density, depth, spacing, or slope. I do not let a number remain implied. If it drives cost, it belongs on the order sheet.
2. Confirm the unit with the supplier or inspector. Feet, inches, cubic yards, tons, percent slope, and ratios are all easy to mix when a quote moves from phone call to invoice.
3. Check the tolerance. I allow 5% on simple rectangular material orders, 10% on irregular shapes, and 15% when curved edges, wet material, or compacted volume are involved.
4. Photograph the condition before covering it. A photo of a tape measure in a footing, a delivery ticket next to a stone pile, or a laser reading on a slope has settled more disputes for me than any email thread.
5. Do one reverse calculation. Convert the final order back into area, depth, or load. If the reverse answer does not match the site sketch, the order is not ready.

That five-step habit is not glamorous, but it is how I keep small residential jobs from developing commercial-sized change orders. We have measured the same pattern across driveways, patios, decks, grading work, and concrete pours: the expensive mistake is usually visible in the numbers before it is visible in the finished work.