Foundation cracks: which ones are normal, and which ones are the house talking

The field rules of thumb, labelled as exactly that

These are the working heuristics the trade uses. They are not a code standard, and the moment a crack matters, the opinion that counts is a professional engineer's:

• Hairline vertical cracks (under roughly 3 mm) in poured concrete are usually shrinkage: the concrete curing, not the building moving. Seal against water, monitor, move on.
• Wider cracks (roughly 6 mm and up), cracks that grow over weeks, or cracks wider at one end are worth professional eyes.
• Any horizontal crack is the serious category. Horizontal cracking suggests lateral soil pressure pushing the wall inward, the classic southern Ontario pattern where wet, heavy clay and poor drainage load a block wall.
• Horizontal cracking plus bowing, wall rotation, or separation at floors: stop guessing, engage a structural engineer. We will say so on a site visit, and we mean it.

"The crack itself is a symptom. The disease is nearly always water: soil soaked by a downspout dumping at the wall, clay swelling against block, a footing sitting above frost. Fix the water and the movement usually stops. Skip the water and the prettiest crack injection just becomes a dated photo of where the next one starts."

Mitch Fraser, owner, Ontario Contracting Co.

Why frost is usually the author

Ontario's Building Code prescribes minimum foundation depths precisely because of this: for heated basements, 1.2 metres in clay; for foundations not enclosing heated space, bearing below the depth of frost penetration (and in clay, 1.2 m but never less than frost depth). Structures built shallower, the porch on blocks, the addition on skimpy footings, the deck on 2-foot sonotubes, heave every winter and settle every spring, cracking themselves and whatever they are attached to. It is the same physics behind heaved deck posts, which is why our decks bear below frost without being asked.

The repair menu, honestly ordered

1. Water management first: grading, downspout discharge distance, and drainage, the same order of operations as our waterproofing practice. Cheapest, and it addresses the cause.
2. Crack sealing (injection from inside, membrane from outside) for stable, non-structural cracks that leak.
3. Structural repair, engineered: reinforcement, bracing or rebuild of bowing walls, underpinning where bearing has failed. This tier always starts with an engineer's design, not a contractor's opinion, ours included.

In century homes with stone and rubble foundations, the whole assessment changes character; the honest approach is the one in our pre-Confederation foundations material. And if a seller or contractor waves off a horizontal crack as cosmetic, the vetting chapter you want is our contractor-vetting library.