A deck is a structure, not just a surface. The decisions that determine whether it performs safely and durably over 20 years are structural — footing depth, beam sizing, joist spacing, ledger connection — not aesthetic. And the aesthetic decision that most homeowners focus on — the decking surface material — is actually the component most easily changed later. Getting the structural questions right first is what matters.
“The decking material is the last decision on a deck, not the first. Pick the wrong footing depth and you have a problem you can never fix without tearing the whole thing out. Pick composite instead of cedar for the decking surface and you can change that in 15 years. People spend 80% of the design conversation on the decking and 20% on the structure. It should be the reverse.”
— Brendan Tse, licensed carpenter and deck construction specialist with 17 years of experience in residential deck and outdoor structure building in the local area
Table of Contents
Structural Questions
How will this deck be designed — is there an engineer or building code span tables being used?
Deck structural design in many regions must conform to the applicable building code or be designed by a licensed professional engineer. For most residential decks, the OBC prescriptive requirements (using span tables for beam, joist, and post sizing) are adequate. For complex decks — elevated decks, decks supporting hot tubs, or decks with unusual loading conditions — engineering may be required.
Ask whether the deck is being designed to OBC prescriptive requirements or to an engineer’s design, and confirm the structural member sizes are based on one or the other — not guesswork.
What a good answer looks like: A specific reference to OBC span tables or an engineer’s drawing, with structural member sizes derivable from one of those sources.
What size are the beams, joists, and posts?
Ask specifically:
Posts: What diameter or cross-section? What species and grade? For elevated decks, post sizing is critical to the structural integrity of the entire assembly.
Beams: What cross-section? For a given span and joist load, the OBC specifies minimum beam sizes. Ask the contractor to confirm the beam sizing against the span tables.
Joists: What depth and spacing? Joist size and spacing determine the deck’s live load capacity and how the decking surface will perform over time. 16″ on-center spacing is typical; some composite decking products require tighter spacing.
What a good answer looks like: Specific dimensional lumber sizes — “doubled 2×10 beam, 2×10 joists at 16″ OC, 6×6 posts” — with a reference to how those sizes were determined.
What will the post-to-beam connection look like?
Posts that are simply nailed or screwed to a beam without a proper connector can fail under lateral loads. Ask whether post caps or other structural connectors are being used at the beam-to-post connection.
What a good answer looks like: Engineered metal post caps (Simpson Strong-Tie or equivalent) specified at all post-to-beam connections.
What post bases will be used?
Where posts sit on top of footings (rather than being embedded in concrete), post bases must be used to create a positive connection and to keep the end grain of the post out of standing water. Ask whether adjustable post bases are being used and what product is specified.
Decking Material Questions
What specific decking product is being recommended?
Ask for the specific brand and product line — not just the category. “Composite decking” covers a wide range of products from entry-level to premium, with significant differences in warranty, heat retention, fade resistance, and stain resistance.
Pressure-treated (PT) lumber: ACQ (alkaline copper quaternary) treated lumber is the current standard, free of the arsenic used in older CCA-treated lumber. Ground-contact rated PT (UC4B) is required for structural members in contact with or near the ground.
Cedar: Western red cedar is naturally rot-resistant and beautiful. Requires staining or sealing every 2–3 years to maintain appearance and durability.
Composite decking: The dominant market choice. Key specifications to ask about:
– Capped vs. uncapped: Capped composite has a hard polymer shell on all four sides, dramatically improving stain and fade resistance. Uncapped composite absorbs stains more easily and fades faster.
– Core type: Solid core vs. hollow core affects weight, heat retention, and structural spans.
– Brand: Major brands include Trex, Fiberon, Azek/TimberTech, and Deckorators. Each has different performance profiles and warranty terms.
PVC decking: All-plastic, no wood content. Maximum durability and moisture resistance but higher thermal expansion/contraction than composite — gaps must be sized appropriately.
Ask why the recommended product is appropriate for your climate, usage, and maintenance preferences.
What is the warranty on the decking product?
Premium composite and PVC decking products carry 25–50 year warranties on fade, stain, and structural integrity. Ask for the specific warranty terms and what conditions maintain coverage — particularly, whether installation by a certified contractor is required for the full warranty.
What is the substructure spacing requirement for this decking product?
Composite and PVC decking products have specific requirements for joist spacing that differ from wood products. Some require 12″ on-center spacing rather than the standard 16″. Ask whether the substructure is being designed to the decking manufacturer’s installation specifications.
Railing Questions
What railing system is being proposed, and is it code-compliant?
Deck railings in many regions must meet specific requirements under the applicable building code:
– Required for decks more than 600mm (approximately 24″) above grade
– Minimum height of 900mm (approximately 36″) for decks less than 1.8m above grade
– Minimum height of 1,070mm (approximately 42″) for decks higher than 1.8m above grade
– Baluster spacing not to exceed 100mm (approximately 4″) — to prevent a 4″ sphere from passing through
Ask specifically what railing system is being proposed and confirm it meets these OBC requirements. Ask whether the post attachment method (surface-mounted vs. through-bolted) has been verified to be code-compliant.
Related Questions
- Questions to Ask a Deck or Fence Contractor Before You Hire One
- Questions to Ask a Deck or Fence Contractor About Permits and Inspections
- Questions to Ask When Getting a Deck or Fence Quote
- How Much Does a Deck Cost ?
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