How To Attach Deck Posts And Beams To An Existing Concrete Patio Without The Correct Footings
This video addresses a common scenario faced by homeowners and contractors: how to securely attach deck support posts and beams to an existing concrete patio slab, particularly when the slab lacks pre-existing, correctly placed footings for a new deck structure. It explores two distinct approaches: one considered structurally sound and code-compliant, and an alternative that carries significant caveats.
The Fundamental Importance of Proper Footings (The "Correct Method")
The video rightly highlights the "correct method," which involves cutting through the existing concrete slab to excavate and pour new, appropriately sized concrete footings. This is the industry standard and generally required by building codes for several critical reasons:
Load Distribution: Decks impose significant loads, including the weight of the structure itself (dead load), people and furniture (live load), and potentially snow or wind loads. Footings are designed to distribute these concentrated loads from the posts over a wider area of undisturbed soil, preventing the posts from sinking or shifting.
Frost Heave Prevention: In regions with freezing temperatures, footings are typically placed below the frost line. This prevents frost heave (the upward movement of soil during freezing conditions) from lifting and damaging the deck structure. While the video mentions a 12x12x12 inch footing, local codes will dictate the precise depth based on frost line and soil conditions.
Stability and Uplift Resistance: Proper footings, often incorporating embedded anchors or post bases, provide stability against lateral forces (swaying) and uplift forces (from wind).
The Alternative Method: Standoff Base Plates on Existing Slabs (with Major Considerations)
Purpose of Standoff Bases: These metal plates serve a primary function of elevating the wooden post slightly off the concrete. This prevents direct contact, reducing the risk of moisture wicking up into the post and causing rot.
Critical Limitations and Risks
Slab Integrity is Paramount: This method relies entirely on the existing slab's ability to bear the new deck loads. If the slab is too thin (typically less than 4 inches for any load-bearing), unreinforced, cracked, or poured on poorly compacted soil, it is likely unsuitable and could crack or fail under the new concentrated loads from the posts.
Not a True Footing: A slab itself, unless specifically designed as a structural slab with thickened edges or integrated footings, is not a footing. It's primarily designed for surface loads like foot traffic or patio furniture, not concentrated point loads from a deck structure.
Code Compliance Issues: As the video stresses, this method often does not meet local building codes for decks of significant size, height, or those attached to a house. Building departments and structural engineers typically require dedicated footings for safety and long-term stability.
Anchor Reliability: The security of the anchors depends on the quality of the concrete and proper installation. Over-tightening can crack the concrete, and under-tightening can lead to loose posts.
Additional Information and Professional Advice
Load Path Understanding: For any structure to be safe, there must be a continuous "load path" transferring all forces from the highest point of the structure down to the supporting ground. With the alternative method, the slab becomes a critical, and potentially weak, link in this path.
Type of Deck Matters
Small, Freestanding, Ground-Level Decks: The base plate method might be considered for very small, low-level, freestanding decks (often called "floating decks") that are not attached to the house and carry minimal loads. Even then, the slab's condition is key.
Elevated or Attached Decks: For decks attached to a house with a ledger board, or for any elevated deck, proper footings are almost always non-negotiable. The forces involved are much greater.
Consult a Structural Engineer: If you are considering building a deck on an existing slab without adding new footings, it is highly advisable to consult a qualified structural engineer. They can assess the existing slab's condition, calculate the anticipated loads from your proposed deck, and determine if such an approach is feasible and safe, or if modifications are necessary.
Permitting: Most deck construction projects require a building permit. The permit application process will typically require detailed plans, including how the deck will be supported. A plan relying solely on surface-mounted base plates on an existing patio slab for a substantial deck is likely to be rejected without an engineer's stamp and justification.
Long-Term Performance: Decks built on inadequate foundations are prone to settlement, instability, and premature failure, potentially causing damage to the deck, the existing slab, or even the adjacent house.
In summary, while the video presents an alternative for attaching posts to an existing slab, it's crucial for homeowners and contractors to understand the significant structural implications and potential risks involved. Prioritizing safety, code compliance, and long-term durability by installing proper footings is generally the recommended and most reliable approach. When in doubt, always seek professional engineering advice.
