Hairline in plaster or drywall
Fine, even cracks in a finish, often at a corner or a board joint.
Guidance BRE groups cracks up to about 1 mm with damage treated in normal decoration.
Read it, note it, watch for growth.
Field guide
Crack patterns, drawn and explained
Which way a crack runs, where it sits and what is around it can say more than its width. Here are seven patterns buyers meet at viewings, with what the published guidance says about each. Diagrams, not photos of real houses.
Diagonal from a door or window corner
Starts at the corner of an opening and runs away on a slant.
Guidance InterNACHI lists cracking over openings where a lintel sags, deflects or rusts; settlement cracks are most likely at corners and next to openings.
Ask what is above the opening and whether it has been repaired.
Stair-step in brick or block
Follows the mortar joints up and across, like a staircase.
Guidance InterNACHI: settlement cracks usually follow a rough diagonal path along mortar joints.
Measure the widest step; ask an inspector to judge the cause.
Horizontal in a foundation wall
Runs side to side along a block or brick foundation wall, sometimes with the wall bowing in.
Guidance InterNACHI calls horizontal or sweeping cracking of brick and block foundation walls potentially serious.
Worth a structural engineer before you offer.
Tapered: wide at one end
Wide at one end and narrowing to a hairline at the other.
Guidance InterNACHI: more rapid settlement produces cracks that taper; drying-shrinkage cracks in block can look similar.
Note which end is wider; in masonry, get a professional opinion.
Edges that do not line up
One side of the crack sticks out or sits lower than the other, often where an addition, porch or bay meets the house.
Guidance InterNACHI links cracks whose edges do not line up at bay windows, porches and additions with differential settlement.
Worth a structural engineer before you offer.
Map cracking in a floor slab
A web of fine, branching cracks across a basement or garage floor.
Guidance Fine cracks from drying shrinkage are common in concrete floors. The sources this site uses are written for walls, so the width scale is only a rough guide here.
Note them; ask about water and any heaving or sinking.
Seen one of these? Measure the widest point and read it in the Crack Reader; it combines the width, the pattern and the signs around it.
Sources: InterNACHI, "Inspecting Foundation Walls and Piers" (Gromicko and Gromicko, rev. 2013); BRE Group, "Assessing cracks in houses" (2014); BRE Digest 251 (1995). A reading of published guidance, not an inspection. Buying Crack Book cannot see the house and never tells you a house is fine. Hire a qualified home inspector (licensed where your state requires it), and a structural engineer when the reading says so.