Freestanding deck over an old concrete patio

The brief

The owners were putting in a new slider and wanted a low composite deck outside it, with storage underneath. What was there: an old concrete patio that heaved every winter and flooded every spring. Tearing it out would have been expensive and messy. Attaching the deck to the house would have meant cutting into new siding for a ledger.

So the plan does neither. The deck stands on its own two beam lines, with its footings cut right through the old slab.

What the plan decides

1. No step out of the house. The deck surface sits 1 in. below the slider threshold. The door goes in before the framing so the height is set from the real sill.
2. Footings that ignore the slab. 12 piers, 24 in. wide by 48 in. deep. The inner six are cut through the old slab with an isolation joint around each one.
3. Framing for composite. 2-ply 2×10 beams on 6×6 posts, and 2×8 joists at 12 in. on center, the spacing composite decking needs for a stiff surface.
4. Hardware and protection. A hurricane tie on every joist and joist tape on top, so the framing lasts as long as the decking.
5. Stairs, rail and storage. Two 4 ft stairs with equal risers, 36 in. aluminum rail on the open sides, and a lattice skirt with a 4 ft storage door.

The framing sheet is a starting point for the builder and the building department, not a stamped drawing. It still gets everyone pricing the same structure. More detail on spans, bracing and footings in freestanding deck plans.

Reading the framing sheet: what the code guides say

A framing sheet is a short list of structural decisions. Below are the four that matter most on a freestanding deck, with what the published guides actually say about each. Your building department has the final word, and its rules can differ from these.

1. Freestanding vs. ledger-attached

The American Wood Council’s Prescriptive Residential Wood Deck Construction Guide (DCA 6) is a prescriptive deck guide based on the 2015 IRC. Its scope is narrow: it applies to single-level decks “attached to the house to resist lateral forces.” Its commentary separates two other cases. A non-ledger deck carries its own weight on posts but still leans on the house for sideways loads. A free-standing deck is “both vertically and laterally independent.” This plan is the second kind. So DCA 6 is a good reference for footings, spans and hardware, but the deck’s sideways bracing has to come from its own posts and beams. The IRC rule, as quoted in Penn State’s PHRC deck design handout (R507.9.2), is simple: lateral loads go to the ground or to a structure that can carry them there.

2. Footing depth below frost

DCA 6 says footings bear on undisturbed soil at least 12 in. below the ground surface or below the frost line, whichever is deeper. It tells you to get the frost line from the authority having jurisdiction. Pennsylvania leaves that number to each municipality; see our frost line depth by state guide for how it’s set and local examples.

The IRC does let some freestanding decks skip frost depth. Per Penn State’s PHRC deck design handout, the exception needs all three: joists resting directly on precast pier blocks at grade with no beams or posts, a deck no bigger than 200 sq ft, and a walking surface no more than 20 in. above grade. This deck is 340 sq ft, about 32 in. high, on posts and beams. It doesn’t qualify, so its footings have to go below the local frost line. The sheet takes every pier to 48 in.

3. Joist spans and spacing

DCA 6 Table 2 lists maximum joist spans by species, size and spacing. It assumes a 40 psf live load, 10 psf dead load, No. 2 lumber and wet service. For Southern Pine 2x8s it allows 13 ft 1 in. at 12 in. on center and 11 ft 10 in. at 16 in. A cantilever can’t be more than a quarter of the main span. On a deck 10 ft deep, even a joist running the full depth stays under the 12 in. number. Other species have different numbers, so read the row for the lumber you’re buying.

Span isn’t the only limit. The PHRC handout notes that maximum joist spacing is also limited by the decking. That’s why this plan runs joists at 12 in.: check your composite brand’s install guide for its spacing.

4. Lateral bracing

DCA 6 calls for diagonal bracing parallel to the beam at each corner post taller than 2 ft, and doesn’t allow it on center posts. On a freestanding deck with no house to lean on, bracing is how the frame resists racking. Our freestanding deck plans guide covers bracing options, including knee braces.

Permits usually apply

Pennsylvania’s Uniform Construction Code exempts from permits an uncovered deck whose floor is no more than 30 in. above grade, as long as the work doesn’t violate a law or ordinance (34 Pa. Code § 403.62(c)(8)). At about 32 in., this deck is over that line. DCA 6 also says a deck shouldn’t be used until final inspection and approval. Outside Pennsylvania, the threshold is whatever your state and town adopted.

1. Ask for the local frost depth before you size piers.
2. Measure finished height from grade to the deck surface; it decides the permit question.
3. Match the span table to your actual species and grade.
4. Check decking spacing in the manufacturer’s install guide.
5. Show bracing at every corner post on the drawing you submit.
6. Call 811 before augering any pier.

Sources: AWC DCA 6 · PHRC deck design handout · 34 Pa. Code § 403.62 · 811

Questions