Which foundation cracks should I actually worry about?
Direction tells you most of it. Fine vertical cracks in drywall and hairlines in concrete are usually shrinkage and mean nothing. Cracks running at a diagonal from the corner of a door or window, stair-step cracks following the mortar line in brick, and anything wide enough to slide a coin into are worth having looked at, particularly if your doors have started sticking at the same time.
Diagonal beats vertical as a warning sign
A wall is weakest at its openings, so when a structure racks slightly out of square the stress finds the corner of a door or a window and travels away from it at roughly forty-five degrees. That diagonal is the signature of movement rather than of ordinary drying and shrinking.
Outside, the brick equivalent is a stair-step: a crack that climbs the wall by following the mortar joints. Same cause, different material.
Corroborate the crack with something that is not a crack
A single crack on its own proves very little. What makes it meaningful is a second symptom agreeing with it: an interior door that has started catching on the jamb, a gap opening where the ceiling meets the wall, a window that has gone stiff, or a floor with a noticeable slope.
On pier and beam houses the floor tends to speak first and the walls later. On slab houses it is usually the other way round.
Watch it before you panic, but do not watch it forever
Marking the end of a crack with a pencil and a date is a genuinely useful thing to do. If it has not moved in six months, that is real information. If it has grown, that is real information too, and cheaper to have collected than to guess about.
What is not worth doing is watching for years while the doors get worse. Movement is easier and cheaper to address before finishes have been damaged.
Related questions
Is a crack in my garage floor a foundation problem?
Usually not on its own. Garage slabs are often poured separately and crack as they cure. What matters is whether the two sides have shifted relative to each other.
My house is new. Should new cracks worry me?
Some cracking in the first year or two is normal as materials settle and dry. Diagonal cracking plus sticking doors is worth checking at any age.
Do cracks close up after repair?
Some close partly. A lift brings the structure back toward level, and touching up drywall and paint afterward is normal rather than a sign something went wrong.
