The load path, from the ground up
A deck is five parts stacked on each other. Each one carries the part above it, so the weight travels down: decking to joists, joists to the beam, the beam to posts, posts to footings, footings to the ground. One weak link and the whole frame moves.
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1Footing
Concrete footings
Concrete under each post.
- What the code asks for
- Depth below undisturbed ground, size for the load it carries, firm soil under it.CRC R507.3
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2Post
Posts
Carries the beam down to the footing.
- What the code asks for
- A metal post base or embedment, height for its size and wood, bracing on loose or expansive soil.CRC R507.4
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3Beam
The beam
Carries the joists to the posts.
- What the code asks for
- The plies fastened together, how far the joists overhang it.CRC R507.5
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4Joists
Joists
Carry the decking.
- What the code asks for
- Spacing the decking allows, span for the wood and size, hangers where a joist ends at a ledger.CRC R507.6–R507.7
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5House
Ledger and lateral connection
The ledger and the lateral connection to the house.
- What the code asks for
- The ledger on the band joist, never on veneer, flashing above it, hold-down devices.CRC R507.9
Ground
Why footing size depends on your yard
Each footing carries the deck area around its post, half the distance to the next post each way. Softer soil needs a bigger footing; on a hillside it also has to reach firm soil. That is why the same deck costs more on a slope: about 22% more in our calculator, and a raised deck, 5 ft and up, $5 to $10 more per sq ft. Sizing footings is a job for whoever builds the frame and answers for it, not a chart read at the store.
Deck blocks or poured footings
The code answers this one precisely. A free-standing deck can sit on precast concrete pier blocks, with no footings at all, only when all three are true:
- the joists bear directly on the blocks, with no beams and no posts;
- the deck is 200 sq ft or less;
- the walking surface is no more than 20 inches above grade anywhere within 36 inches of the edge.
Attach it to the house, stand it on posts, or make it bigger, and it needs footings.
Why posts have a height limit
A post may stand only so tall for the deck area it carries: the more it holds up, the shorter it has to be, and a thin post or a softer wood like redwood runs out first. That is one reason the frames we build are pressure-treated lumber, with redwood or composite on top.
The beam: what changed in the 2024 code
The 2024 IRC, which California adopted for permits from January 1, 2026, sizes a beam for the joist overhang it actually carries. In practice, a cantilevered deck needs more posts under the same beam than one where the joists stop at the beam. CRC Table R507.5(1).
Joists: span is not the length of the deck
A joist's span is the clear distance between supports. Wood, size and spacing set how far it may go, and a joist at the very edge of what the code allows is legal but flexes underfoot more than most people like. CRC Table R507.6.
Not sure what your yard needs? The calculator prices new construction by size and ground.
Attaching the deck to the house
The part of the frame the code spells out in the most detail, and the one most decks fail on. The ledger bolts into the house's band joist, not into siding, stucco or stone veneer, with flashing above it so water does not rot the wall behind it. Hold-down devices keep the deck from pulling away from the house when people move on it or the ground moves (R507.9.2). If there is no sound band joist to bolt to, the deck has to stand on its own posts instead.
Why the frame is contractor work
Once the decking goes on, nobody sees the frame again. A frame that is a size short, a ledger on the wrong layer, or a footing in soft fill shows up years later as a bouncing floor or a deck pulling off the wall, and fixing it means taking the deck apart. That is why the frame is part of our job, from the footings up, and why it is its own line on our estimate. And if your frame is sound, we'll tell you, and you only pay for new boards (see deck remodeling).
What we put into a frame
Pressure-treated lumber, concrete, and the metal that holds it together: joist hangers, post bases, structural connectors and anchors. On most of our estimates it is its own line, priced per sq ft, apart from the decking you walk on. Redwood and composite go on top; the frame underneath is treated wood.
On a hillside or raised deck, framing takes a bigger share of the cost. See the Los Gatos hillside decks for what that looks like built, or the frame below, finished.
Before you build: questions
Can a deck sit on concrete deck blocks?
Sometimes, and the code says exactly when. CRC R507.3 lets a free-standing deck skip footings if its joists bear directly on precast pier blocks with no beams or posts, the deck is 200 sq ft or less, and the walking surface is no more than 20 inches above grade within 36 inches of the edge. Attach it to the house, add posts, or build it higher and you need real footings.
How deep do deck footings go in the Bay Area?
Deeper than most people expect. The code minimum is 12 inches below undisturbed ground (CRC R507.3.2), your building department can require more, and on a slope or in soft fill the footing goes down to firm soil, which is why a hillside deck costs more.
What joist spacing do I need?
The decking decides first: wood boards and composite each have their own limit, and composite follows the maker's install guide. Then the span decides. We set spacing for the boards you pick. See our composite guide.
Do I need joist hangers?
Wherever a joist ends against a ledger or a rim instead of sitting on top of a beam, yes. The hanger is what carries it; nails alone pull out. Most of our estimates list joist hangers, post bases and structural connectors as their own line.
Can the ledger go on stucco or stone?
Not on stone or masonry veneer: CRC R507.9.1.1 says so in plain words. Through stucco the ledger bolts into the house's band joist, with flashing above it. If the band joist is not sound to bolt to, the deck has to stand on its own posts (R507.8).
Do I need a permit for this?
A deck bolted to the house through a ledger needs one at any size. Our permit guide has the four conditions from the state code and the rule in each of the 33 towns we work in.
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