The frame beneath your deck does the unseen work. Beams carry the load down to the posts, and joists span between beams to support the decking boards above. Get this structural layer right, and everything you build on top of it stays level, solid, and safe for years. Get it wrong, and no amount of nice decking will fix a bouncy or sagging deck.
A deck frame has three main parts:
Each piece depends on the one below it, so the frame needs to be planned as a whole system, not built piece by piece without a plan.
Beam size depends on the span between posts, the joist span the beam supports, and the load the deck needs to carry. A longer span or wider deck generally calls for a larger beam, or closer posts to shorten the span. Doubled or tripled dimensional lumber, engineered lumber, and solid timber beams are common choices, each with its own span capacity. Ontario’s Building Code requires a built-up wood beam to be nailed together with a double row of nails at least 89 mm long, spaced no more than 450 mm apart, with end nails 100 to 150 mm from each piece’s end, or bolted together as an alternative. Instead of guessing, check the span tables for your specific lumber species and grade, or your local building code, since these numbers vary by region and material. When in doubt, oversize rather than undersize. It is far cheaper to use a slightly bigger beam than to repair a sagging deck later.
Joist spacing is governed by the decking material and thickness:
Always check the decking manufacturer’s span rating and your local building code before finalizing your layout, since the exact limits can vary. Mark your layout on the beams with a tape measure and square before cutting or setting a single joist, so you catch any math errors on paper instead of mid-install.
There are two common ways to attach joists to beams. The first sets joists directly on top of the beam, which is simple and gives good bearing, but raises the overall deck height. The second uses joist hangers to attach joists to the side of the beam at the same height, keeping the deck lower to the ground but relying entirely on the hanger hardware to carry the load. Whichever method you use, every joist should sit square and plumb, with full, even contact at its bearing point. A twisted or partially seated joist will concentrate stress in one spot rather than spreading the load as the frame is designed to.
Deck framing hardware is constantly exposed to moisture, so it needs to be corrosion-resistant: look for hot-dip galvanized or stainless-steel joist hangers, hanger nails, and structural screws rated for exterior and ground-contact use. It’s generally best to avoid mixing dissimilar metals in your hardware and fasteners, for example, standard galvanized hangers with stainless screws, since manufacturers often recommend matching the metal type to reduce the risk of accelerated corrosion over time. Joist hanger manufacturers typically specify filling every fastener hole for the hanger to perform as rated, and skipping holes to save time is a common shortcut that’s worth avoiding.
The most frequent deck-framing mistakes are:
Any of these can become a safety issue well before it’s visually obvious. If you’re not confident sizing and spanning the frame yourself, it’s worth having the layout checked against your local building code, or having a professional install the framing, before the decking goes down and the structure underneath is hidden from view.
Once your frame is square, level, and properly fastened, the next decision is what finishes it off, including the railing along any open side. We build custom metal railings for decks, porches, and stairs across Toronto and the GTA, designed to attach directly to a finished frame or ledger. If a railing is part of the plan, it’s worth confirming post and blocking locations before the decking goes down, since that’s far easier to adjust while the frame is still open.
