Most deck failures start at a connection rather than in the boards: a ledger board poorly fastened or unflashed against the house, posts on shallow footings that heave or sink, rotted joist ends, and guards that loosen. A deck that sways or bounces, pulls away from the house, shows rust streaks at the ledger or soft wood at post bases should be kept clear and assessed.
- The ledger connection to the house carries one end of every joist; when it fails, the deck can drop without warning.
- Water behind an unflashed ledger rots the deck and the house rim joist, turning a deck problem into a house repair.
- Footings that stop above the frost line heave in winter; Calgary requires deck foundations at least 1.2 m deep unless an engineer reviews a shallower design.
- Sistering a deck joist helps only if both ends and their supports are sound; rotten ledgers and post bases come first.
01 /Why do decks fail?
A deck is a floor built outdoors. It carries people, furniture, barbecues, planters and snow, like any floor, but it does so in rain, sun and freeze-thaw cycles, often with its framing hidden under boards and skirting where nobody looks for years. Wood that stays wet rots, steel fasteners in treated lumber corrode, and footings in frost-susceptible soil move with the seasons.
When decks fail badly, the boards are rarely the cause. The weak points are connections: the ledger to the house, joists to the ledger and beam, the beam to the posts, the posts to their footings, and the guards to the frame. A crowded deck puts its highest load on exactly those connections, which is why an old deck can feel fine for years and then let go at a party.
02 /What is a deck ledger, and why does it fail?
The ledger is the board fastened to the side of the house that supports the house end of the deck joists. It transfers roughly half the deck's load into the house's rim joist, so it must be fastened into solid structure and kept dry. Most dramatic deck collapses start here.
- Fastened with nails alone. Nails can pull or shear out under a crowd or a heavy snow load; ledgers are normally bolted or attached with structural fasteners into the house rim joist.
- Hung on the wrong thing. Brick veneer, stucco, siding over a gap, or a cantilevered floor that overhangs the foundation are not a sound base for a ledger. Edmonton's deck guide says not to plan a deck where the house floor is cantilevered without an engineer-stamped design, unless the builder installed a ledger for it.
- No flashing. Water running down the wall collects behind the ledger, rots it and the house rim joist behind it, and corrodes the fasteners. Edmonton's guide stresses flashing to deflect water away from the ledger.
- Corroded fasteners. Treated lumber is hard on ordinary steel; fasteners and hangers need to be rated for it.
- Joist hangers missing or half-nailed. Joists that simply sit on a ledger strip or are toe-nailed rely on fasteners that were never meant to carry them.
Codes say less about ledgers than you might expect. Edmonton's Uncovered Deck Design Guide notes that the Alberta edition of the National Building Code gives no specific direction on ledger board fastening and points builders to other recognised guidance for best practice. That is one reason a municipal deck guide, a designer or an engineer should set the ledger detail, not habit.
03 /How deep do deck footings need to be?
Deep enough to sit on firm soil below the frost line, or designed by an engineer to cope with frost another way. Frost heave happens when water in frost-susceptible soil freezes into ice lenses that push upward; a footing above the frost line rides up and down with the seasons, racking the deck and loosening every connection. Settling footings on soft or disturbed soil do the opposite.
Local rules set the depth. Calgary's uncovered deck checklist says that to prevent frost heave, deck foundations must extend at least 1.2 m (4 ft) below ground, and that a shallower foundation needs a design review stamped by a registered professional engineer; screw piles also need an engineer's pile design. Edmonton's guide describes footings on undisturbed soil below frost level, with at least 1.2 m typical there but dependent on soil, and notes that very low decks on pads can work if the owner accepts seasonal movement and re-levelling. Frost depth varies widely across Canada, so confirm the requirement with your municipality.
Post bases matter as much as depth. A post buried in soil or sitting in a pocket that holds water rots at grade, and a post notched to hold a beam can split along the notch. Posts that are out of plumb, or a deck that is higher on one side every spring, point to footing movement.
04 /What are the warning signs of a failing deck?
| What you see | What it suggests | How soon |
|---|---|---|
| Gap opening between the ledger and the house wall | Ledger fasteners failing or rim joist rotting | Keep clear; assess now |
| Rust streaks below ledger bolts or hangers | Corroding fasteners, water behind the ledger | Assess soon |
| Soft or spongy wood at the ledger, joist ends or post bases | Rot at the connections that carry the load | Assess soon; now if widespread |
| Posts leaning or out of plumb | Footing movement, rot at the base, missing bracing | Assess soon |
| Deck higher on one side in winter or spring | Frost heave of shallow footings | Plan a repair |
| Noticeable bounce or sway when walking | Overspanned joists, loose connections, missing bracing | Assess |
| Guard or handrail moves when pushed | Loose posts or rotted guard connections | Fix before use |
| Split beam at bolts, or a beam sitting in a notched post | Overloaded or poorly detailed connection | Assess soon |
Probe suspect wood with an awl or screwdriver: sound treated lumber resists; rotted wood lets the tip sink in with little effort and breaks out in short, cross-grain pieces. Check from underneath with a flashlight, paying most attention to where wood meets wood or wood meets the house. The signs of structural damage guide covers the house side of the same checks.
05 /Can deck joists be sistered?
Yes, and it follows the same logic as indoor floors: a new full-length member fastened beside the old one, bearing on the same supports, so the pair carries the load together. Deck versions add three conditions. The new lumber must be rated for exterior ground-contact or above-ground use as appropriate, the fasteners and connectors must suit treated wood, and the joist's ends must be sound. A sister fastened to a joist whose end has rotted off the ledger adds little.
Sistering from below without removing decking is often possible when there is room to work under the deck; it is harder on low decks close to grade. How far a sister must overlap, how it is fastened and when a joist should be replaced outright are decided by span tables or an engineer, not by a rule of thumb; Calgary directs deck builders to the Canadian Wood Council's Residential Prescriptive Exterior Wood Deck Span Guide for beam and joist spans. The indoor version, with the reasoning behind overlap and fastening, is in sistering floor joists.
If several joists, the ledger and the posts all need work, rebuilding the frame is often cheaper than piecemeal repair, and it gives a chance to fix the ledger flashing and footing depth properly.
06 /When does deck repair need a permit or an engineer?
Permit triggers are local. The two Alberta examples below show the kind of thresholds to ask about; check your own municipality before starting, including for repairs.
| Trigger | Calgary | Edmonton |
|---|---|---|
| Building permit | Uncovered deck higher than 0.6 m above grade at any point, including rebuilding, extending or adding stairs to one; any covered deck | Deck higher than 600 mm above adjacent ground |
| Foundations | At least 1.2 m below ground, or an engineer-stamped design review; engineer's design for screw piles | Below frost level on undisturbed soil; engineer-stamped foundation and post connection for decks over 1.8 m up to 3.6 m |
| Whole-deck engineering | Cantilever over the beam beyond 600 mm, retaining walls over 1.2 m, frameless glass guards, spans beyond the span guide | Decks over 3.6 m high, and decks carrying a hot tub, fireplace, roof or similar heavy load |
| Guards | 0.9 m high where the deck is under 1.8 m above ground; openings no wider than 100 mm | Required where the deck is more than 600 mm high or beside a steep drop within 1.2 m |
Notice how often a heavy or unusual load triggers engineering. A hot tub full of water and people is one of the largest loads a house structure ever carries, and it does not belong on an existing deck without a design. The general permit picture for structural work is in do structural repairs need a permit.
07 /What do deck structural repairs cost?
Deck repairs are priced per job rather than by the item, so the most useful planning numbers are the ones that fall on the house side of the ledger. The ranges below are from the site's cost guide.
| Scope | Typical planning range |
|---|---|
| Engineer's site visit and letter | $500–$1,500 |
| Engineer's field review during construction | $250–$750 per visit |
| Replace a section of rotted sill plate and rim joist behind a leaking ledger | $1,500–$6,000 |
| Temporary shoring while the house rim is repaired | $1,000–$5,000 |
Rot behind a ledger is the expensive surprise, because it sits inside the house wall and may reach the floor framing. For a quick read on the house-side signs, use the Structural Warning Sign Triage, and send a request with photos of the ledger, the underside and the post bases. StructuralRepair.ca is part of HAUSE Home, an independent information and service-request network, not a deck builder or engineering firm.
FAQQuestions people ask
Can a deck ledger be attached to brick?
Brick veneer is a cladding layer tied to the wall behind it, not a structural support, so it is generally not a sound base for a ledger. The usual alternatives are a freestanding deck on its own posts or a detail designed by an engineer.
Is a bouncy deck dangerous?
Bounce alone often means joists are overspanned or connections are loose, which is a performance problem. Bounce combined with sway, a moving ledger, rot or leaning posts is a safety problem; keep people off until it is assessed.
Do I need a permit to repair or rebuild a deck?
Often, above a set height. Calgary requires a building permit to rebuild, extend or add stairs to an uncovered deck higher than 0.6 m above grade. Rules vary, so ask your municipality before replacing framing.
Why is my deck higher in winter than in summer?
Frost heave. Footings that stop above the frost line are lifted as ice lenses form in the soil, then settle back in spring. The fix is footings or piles to the depth your municipality requires, or an engineered alternative.
Can I put a hot tub on my existing deck?
Not without a design. A filled hot tub with people in it is a very heavy concentrated load, and Edmonton, for example, requires an engineer-stamped design for any deck carrying one. Its guide notes that hot tubs are most often set on the ground with the deck built around them.
How do I check deck posts for rot?
Look where the post meets the ground or its footing and where the beam sits on it. Push an awl into the wood: if it sinks easily or the wood crumbles in short pieces, the post needs replacing, with temporary support first.