Introduction
Most deck structural failures trace back to a short list of post-and-footing errors — and nearly all of them are made during estimating and layout, long before the first board goes down. This guide covers the mistakes that distort deck post results, why each one matters structurally, and exactly how to fix or avoid it. Use it alongside the Deck Post Calculator: correct inputs in, correct deck out.
Undersized Footings
The mistake: pouring footings too small in diameter — or too shallow — for the post and the load. A 6×6 post (5.5 inches / 140 mm actual) on an 8-inch pad leaves almost no bearing margin, and soft soils need more area, not less. Undersized footings settle differentially, and a deck that settles unevenly racks, gaps, and eventually fails inspection or worse.
The fix: size footings to the post and the soil. Minimum 12-inch (30.5 cm) diameter for 4×4 posts, larger for 6×6 posts and heavy or elevated decks, and always below the local frost line. When in doubt, go one size up — concrete is the cheapest structural insurance in the project, as the cost guide shows. If footings are already poured too small, the only real fix is supplemental footings or an engineer’s retrofit — there is no cosmetic repair for inadequate bearing.
Posts Not Bearing on Footings
The mistake: posts that miss their footings — set beside the pad, half-on, or floating on backfilled soil because the layout drifted during digging. A post that does not bear fully and concentrically on its footing is not supported at all; the load path is broken at the most critical point in the frame.
The fix: prevention beats repair. Use batter boards and string lines (see the pro tips guide) and keep them up until every base is set. Verify each post lands centered on its footing before the concrete cures. If you discover a miss after the pour, stop and get a structural assessment — shifting a post off-center or bridging the gap with shims is not acceptable, and a new properly-placed footing is usually the answer.
Wrong Post Spacing
The mistake: spacing posts to suit the lumber on hand — “8 feet is close enough” stretched to 9 or 10 feet (2.7-3.0 m) — instead of the beam span tables. Over-spanned beams deflect, bounce, and in the worst case fail; undersized spacing just wastes money on extra footings.
The fix: spacing comes from IRC Section R507 span tables for your beam size, species, and load — typically 6 to 8 feet (1.83 to 2.44 m) — never from convenience. Estimate with (beam length / spacing) + 1 and round up, as derived in the formula guide. If an as-built deck has over-spanned beams, the remedies are adding intermediate posts and footings or sistering/laminating a larger beam, both worth an engineer’s review.
Burying Posts Without Protection
The mistake: setting wood posts directly in soil or in concrete with the end grain buried — sometimes even backfilling with the excavated dirt. End grain wicks moisture relentlessly, and a buried post base rots from the inside out, often invisibly, until the deck leans or a railing gives way.
The fix: posts belong on concrete, connected with a standoff post base that holds wood at least 1 inch (25 mm) above the concrete. Use ground-contact-rated pressure-treated lumber (UC4A or better) anywhere near grade, slope footing tops away from posts, and keep soil and mulch pulled back. Already-buried posts should be inspected by probing the base with an awl — soft wood means replacement, and replacement means doing it right this time. The material guide details treatment ratings and base hardware.
Notching Posts Incorrectly
The mistake: cutting deep notches to seat beams without understanding the structural cost. A notch removes cross-section exactly where the post carries maximum load, and notching on the tension side or over-notching can reduce a 6×6 to the strength of a 4×4 — or less. Many jurisdictions restrict or prohibit notching for this reason.
The fix: prefer through-bolting the beam to the face of the post or using an engineered beam-to-post cap connector — both keep the full post section intact and both satisfy inspectors. If your local code permits notching, follow its exact depth and location limits and never notch both sides at the same elevation. When in doubt, defer to the connector manufacturer’s specs and your local building department, per the code compliance guide.
Skipping the Frost Depth
The mistake: pouring footings at a convenient digging depth — 24 inches because the auger gave up — in a region where the frost line is 36 or 48 inches (91 or 122 cm). Seasonal freeze-thaw then jacks the footings upward unevenly, racking the frame and popping fasteners.
The fix: look up your local frost depth from the building department and treat it as a hard minimum for the bottom of every footing, no exceptions. Bell out footing bottoms where soil allows for extra heave resistance. Heaved footings cannot be pushed back down — repair means excavating and re-pouring below the frost line, the most expensive lesson on this list.
Forgetting Guardrail Posts in the Count
The mistake: estimating only the structural posts under the beams and discovering on build day that the railing needs a dozen more posts — a separate lumber run, mismatched stock, and blown schedule.
The fix: count guardrail posts in the same pass: exposed railing perimeter divided by 6 ft (1.83 m) maximum spacing, rounded up, plus corners. The estimating guide walks through a full takeoff that includes them, and the calculator counts both categories together.
Related Guides
- Deck Post Complete Guide — the flagship overview with correct estimating procedure from the start.
- Deck Post Pro Tips — the field techniques that prevent these mistakes on build day.
- Deck Post Code Compliance — permits, inspections, and what IRC R507 requires.
- Deck Post Build Checklist — a step-by-step checklist that bakes the fixes into your workflow.
- Deck Post FAQ — quick answers when you need to double-check a detail fast.
Get Your Number
Every mistake above starts as a wrong number on paper — a spacing stretched too far, a footing sized too small, a count that forgot the railing. Run your layout through the Deck Post Calculator with honest inputs, check the results against this guide’s fixes, and you will catch the errors while they are still free to correct: on the plan, not in the ground.
Disclaimer: Results are planning estimates only. Verify quantities and prices with your supplier and consult a licensed professional or your local building authority for structural or code-related decisions.
