Why Build Over Concrete Instead of Starting Fresh? Concrete demo in Seattle runs $3–$8 per sq ft — a 400 sq ft patio costs $1,200–$3,200 just to remove. If the concrete is structurally sound and relatively level (within 1.5"), a sleeper system lets you lay a deck surface over it without removal. Lower total cost, faster install, no hauling fees. The right call when: concrete is in good shape, no significant grade change needed, drainage is adequate.
The Sleeper System: How It Works Sleepers are pressure-treated 2x4 or 2x6 lumber laid flat on the concrete surface, typically 16" on center, fastened with concrete screws (Tapcons) or construction adhesive. The deck boards span across the sleepers exactly like a conventional joist system. Key requirements: - Concrete must be clean, dry, and flat within 1.5" across the span - Sleepers must be pressure-treated (ground-contact rated, UC4A or UC4B) — they sit directly on concrete which holds moisture - A moisture barrier (6-mil poly or dimple mat) between concrete and sleepers significantly extends sleeper life - Minimum 1/8" per foot slope away from house must be achievable — if the concrete slopes toward the house, correct with tapered sleepers before installing decking
Material Choice: Composite and PVC Are Strongly Preferred Cedar on sleepers over concrete: not recommended. Cedar needs air circulation below the boards to dry out — the low-clearance sleeper system creates a moist, stagnant environment that accelerates cedar rot significantly. You'll lose 30–40% of cedar's expected lifespan in this application.
Composite (capped): Good choice. The polymer cap protects the wood-fiber core even in a low-airflow environment. Use 4-sided-capped boards only — Trex Transcend, TimberTech Legacy, Fiberon Paramount.
PVC (AZEK, TimberTech Pro): Best choice for over-concrete. Zero wood fiber means zero rot risk regardless of moisture conditions below. Worth the premium in this specific application.
Hidden Fasteners Work Fine on Sleeper Systems Standard hidden fastener clips (Trex Hideaway, TimberTech Secret Fix) clip into the board grooves and fasten to the sleeper top, same as a conventional deck. The installation process is identical — the only difference is that your "joists" are resting on concrete rather than hanging in a frame.
Drainage: The Critical Variable If water pools on your concrete, it will pool under your deck. Ensure: - Concrete has at least 1% slope away from the house (about 1/8" per foot) - Perimeter has a clear path for water to exit — don't box in the edges - If using a dimple mat moisture barrier, the dimples face down to create drainage channels
Cost Breakdown — Seattle - Sleeper system materials (PT lumber, hardware, moisture barrier): $800–$1,800 for 300–400 sq ft - Composite decking materials (Trex Select): $2,800–$4,000 for 300–400 sq ft - Installation labor: $3,000–$5,000 - **Total over-concrete deck:** $7,000–$11,000 for 300–400 sq ft
vs. full demo + new conventional deck: $18,000–$35,000 for the same footprint.
The savings are real if the concrete is in good shape. Get a written structural assessment of the concrete before committing — significant cracking, heaving, or soft spots mean demo is the better call.
Permit Requirements In most King County cities, a deck surface over concrete that doesn't add railing or raise the height above 30" from grade may be permit-exempt. Adding railing triggers permit requirements in all King County jurisdictions. Confirm with your city's building department before starting.
For more on material options, see our guides on [composite decking](/composite-decking) and [PVC decking](/pvc-decking). For full project cost context, visit our [deck cost Seattle guide](/deck-cost-seattle). And if you're weighing structural approaches, the [freestanding vs attached deck guide](/blog/freestanding-deck-vs-attached-deck-seattle) covers the tradeoffs in depth.