Tips to help determine the cause of diagonal foundation cracks in basements:
In building corners in cold climates - frost heave, shallow footings, water problem, or poor backfill. In a raised ranch with a garage located in part of the basement, often you will find step cracks in the front and rear foundation walls on the garage-end of the home. These cracks may correspond to some related observations:
(1) If a downspout or gutter spills rain water against the home, these forces will often combine to make more severe frost cracks appear on the garage side of the home.
(2) There may be less backfill against the front and rear foundation walls at the front of the garage.
(3) The decrease in backfill combined with an un-heated garage may expose these building corners to more frost damage
In the foundation wall anywhere, wider at bottom than top – often due to foundation settlement.
From corner towards adjacent opening, wider at top than bottom - often due to foundation settlement, expansive clay soil, frost damage, or damage from a shrub/tree close to the foundation wall.
Under a ground floor window, from sill to ground, sill bowed up - often due to foundation heave, clay soil, frost, shallow or missing footings.
Over a window or door, straight or diagonal - may appear as horizontal along top or bottom of header, vertical at ends of header (possibly due to differences in thermal expansion of different materials of header vs. wall) or vertical/diagonal at center of header (loading failure) or at corners (possible point-load failure)
Cracks in a poured concrete basement walls which are diagonal or vertical and which are generally uniform in width, or which taper to an irregular hairline form, usually in fact a discontinuous crack in the hairline area, are usually shrinkage cracks and should not be ongoing nor of structural problem, though they may invite water entry through the wall.