✓ The Decision
- Attached to the house → below-frost foundations, period. Concrete footings or engineered screw piles. An attached deck that heaves tears at the ledger — the most dangerous connection on a house.
- Freestanding and low (commonly under ~60 cm) → floating on deck blocks is often legal and fine. It moves a little with the seasons; a freestanding deck can tolerate that.
- Tight access, late season, or you hate digging → screw piles. Installed in hours, loaded same day, no concrete cure, and certified installers provide torque-verified capacity.
- You're already mixing concrete and the holes are easy → sonotube footings remain the budget classic. Dig below local frost depth — not "deep enough-ish."
- High deck, hot tub, or roof above → engineered design, whatever the foundation type. Loads change everything.
Why Frost Rules Deck Foundations
The same physics as garage foundations: water in soil expands when it freezes and lifts whatever sits on it. A deck post on a footing that stops above frost depth gets jacked upward — unevenly, post by post — then settles somewhere new each spring. Boards twist, railings rack, stairs drift off level, and the bolts pull at whatever the deck is attached to. Frost depth runs from about a metre in the mildest regions to 2.5 m+ in the Prairies and the North, and your building department publishes the local number. Every deck foundation strategy is one of two answers: get below the frost, or float on top of it and never attach to anything that doesn't.
Option 1: Concrete Footings (Sonotubes) — The Budget Classic
A cardboard form in a hole augered below frost depth, filled with concrete, with a post saddle on top. It's the traditional below-frost footing and usually carries the lowest material cost per point — the real price is the digging and the labour.
Soil is the deciding factor. Tube footings shine in firm, cohesive ground — clay and loam — where an augered hole holds its walls long enough to set the tube and pour. In loose, sandy, or gravelly soil the hole tends to cave or collapse as you dig, which makes tube footings frustrating and sometimes impractical; that's the situation where screw piles earn their keep (below). Rock and tree roots are the other digging-killers. If you don't know your soil, dig a quick test hole — what happens to its walls tells you most of what you need.
- Pros: lowest material cost, code-familiar everywhere, generous bearing when sized right, fully DIY-able in soil that holds a hole.
- Cons: serious digging (a 4-ft hole per post is real work × 8 posts), concrete cure time, inspection usually wants to see the open hole, and miserable-to-impossible in sandy/collapsing, rocky, or rooty ground.
- Watch: bell or proper diameter at the base for bearing; saddle elevated above grade; backfill that drains.
Option 2: Helical Screw Piles — The Modern Default
Steel screw anchors driven below frost by a certified installer (or hand-installed light-duty versions for small projects). The installation torque correlates with load capacity, so a good installer hands you documented capacity per pile — engineering comfort that a hand-dug footing can't match.
This is the soil answer to tube footings. Screw piles go into the exact ground that defeats Sonotubes — loose, sandy, or gravelly soil where an open hole won't stay open — because nothing has to hold a hole open: the pile is the hole. They need no digging spoil hauled away, no concrete, and no cure time, so they suit tight-access yards and let you build the same day. The trade-off is a higher cost per point, and heavier or taller decks generally need the professionally installed, load-rated versions rather than DIY-grade piles.
- Pros: work in sandy/loose soil that collapses on an auger, no digging or spoil, installed in hours, loadable immediately, consistent depth even past 2 m, torque-verified capacity documentation that inspectors like.
- Cons: higher cost per point than a poured footing, needs a reputable installer for load-rated piles (the product is only as good as the torque numbers), and rock can refuse them too.
- Watch: insist on the installation report (pile size, depth, torque). "We screwed them in pretty deep" is not documentation.
Option 3: Floating on Deck Blocks — Legal More Often Than You'd Think
Precast blocks (or pads) sitting on compacted granular at grade. The deck rides the frost up and down a little — and that's acceptable when the structure is freestanding and low, which is exactly how most codes scope the exemption (commonly: not attached to the building, under ~60 cm, sometimes area limits). It's the cheapest, fastest deck foundation there is, and for a garden-level platform it's often the *right* answer, not the cheap one.
- Pros: almost free per point, zero digging, build the same afternoon, easy to re-level (that's the maintenance model: shim, don't fight).
- Cons: seasonal movement is part of the deal; not for attached decks, tall decks, or heavy point loads; needs decent drainage and a compacted base under each block — blocks on lawn sink.
Quick Comparison
| Concrete footings | Screw piles | Floating blocks | |
|---|---|---|---|
| Relative cost | $$ — low material, high labour | $$$ — installed | $ — lowest |
| Best soil | Clay & firm loam (hole holds) | Sand & loose ground (holes collapse) | Any — but low decks only |
| Frost immunity | Yes (if below frost) | Yes (torque-verified) | No — rides it |
| Attached decks | Yes | Yes | No |
| Speed | Days (dig, pour, cure) | Hours | Hours |
| Best for | Budget builds in soil that holds a hole | Attached/tall decks, sandy soil, bad access, fast schedule | Low freestanding platforms |
What It Ballparks At
Foundation cost scales with two things: how many support points your deck needs — a function of its size and beam layout — and how deep your frost line is. As a rough orientation, cheapest to priciest: a small freestanding deck on blocks costs very little; DIY tube footings and DIY screw piles sit in the middle; and a larger attached deck on professionally installed, load-rated piles is the most expensive foundation by a wide margin. The bigger the deck and the deeper the frost, the more the gap between methods matters. For an actual dollar figure — point counts, concrete volume, and local prices — put your real dimensions into the Deck Foundation Cost Calculator rather than guessing from a per-unit number.
Deck Foundation Cost Calculator Wall Framing Calculator
✗ Deal-Breakers — Stop Before You Build
- Attaching a floating deck to the house. The house doesn't move; the deck does. The ledger connection becomes the failure point — this is the cardinal deck sin.
- Footings that stop above frost depth. "Three feet felt deep" in a 4-ft frost zone buys you an annual re-levelling ritual, or worse.
- Mixing foundation types under one attached deck (some posts below frost, some floating) — differential movement guaranteed.
- Deck screws in structural connections. Joist hangers, post bases, and ledgers take structural screws or through-bolts with rated hardware — deck screws hold boards, not buildings.
- Skipping the permit on an attached or tall deck. Ledger detail, guard heights, and footing depth are inspected items for good reason — see the permits guide.
The Safest Path Forward
- Ask your building department two numbers: local frost depth, and the floating-deck exemption limits (height, attachment, area). That defines your legal options.
- Decide attached vs freestanding first — it forces the foundation answer more than budget does.
- Match the foundation to the site: easy digging → sonotubes; bad access, deep frost, or speed → screw piles; low freestanding platform → blocks on compacted granular.
- Use rated hardware throughout — saddles, hangers, structural fasteners, hot-dip galvanized against treated lumber (see lumber choices).
- Permit and inspect what needs it — and keep the screw-pile torque report or footing inspection record with your house documents.
Frequently Asked Questions
How deep do deck footings need to be in Canada?
Below local frost depth — commonly 1.2 m in milder regions to 2.5 m+ in the Prairies and the North. Your building department publishes the exact local requirement; it's not a guess.
Are screw piles better than concrete footings for a deck?
Functionally they solve the same problem. Screw piles win on speed, access, no digging, and documented capacity; concrete wins on material cost where digging is easy. Attached and tall decks are well served by either — done to depth.
Can I build a deck on deck blocks?
Often yes, if it's freestanding and low — commonly under ~60 cm and not attached to the building, though limits vary by municipality. Put blocks on compacted granular, not lawn, and expect minor seasonal movement.
Do I need a permit for a deck?
Low freestanding platforms often don't; attached, raised, or guarded decks usually do — and zoning setbacks apply either way. One call to your municipality settles it for your address.