Hairline Cracks
These are the most common and least concerning. Thin lines, usually less than 1/16 inch wide, that appear randomly on wall or ceiling surfaces. They don't follow any pattern and often appear in clusters.
Cause: normal expansion and contraction of building materials with temperature and humidity changes. Every house does this. It's not settling, it's not structural, it's just physics.
My 1978 house has dozens of hairline cracks. I don't fix most of them because they'll just come back. If they really bother you, a skim coat of joint compound will hide them temporarily. Paint alone won't fill them since they'll show through.
When to worry: if hairline cracks are multiplying rapidly or appearing after some event like nearby construction, heavy traffic vibration, or an earthquake. Might be worth a closer look.
Seam Cracks
These follow straight lines along drywall joints. They're usually visible as a single crack running the length of a seam, sometimes with the paper tape visible or peeling.
Cause: tape failure. Either the original taping job was poor, with insufficient mud under the tape, or the house has moved enough to stress the joint past what the tape can handle. Sometimes both.
Seam cracks look alarming because they're often long and very visible. But they're not structural. The drywall itself is fine, just the tape over the seam has given up.
The fix is to remove the failed tape, re-tape with proper technique, and refinish. I've fixed probably ten of these in my house over the years. They're annoying but not scary.
When to worry: if the same seam keeps cracking after proper repair, or if the crack is accompanied by a noticeable ridge or gap. That suggests ongoing movement that might have a root cause worth investigating.
Corner Cracks
Cracks where walls meet each other or where walls meet ceilings are extremely common. They're stress concentration points where two surfaces move independently.
Cause: differential movement between the wall and ceiling, or between two walls. This is especially common with roof trusses that lift seasonally, pulling the ceiling away from the walls. Also common in older houses that have settled unevenly.
Corner cracks don't usually indicate structural problems unless they're severe. They're an aesthetic issue. The corner bead or tape has failed at the stress point.
The fix depends on the underlying cause. If it's truss uplift, you need floating corner details that allow movement without cracking. If it's just old tape failure, standard retaping usually works.
My bedroom has corner cracks that I've fixed three times. They keep coming back because of seasonal truss movement. Eventually I'll install proper floating corners, but for now I just patch them every few years when they get bad enough to bother me.
Diagonal and Stair-Step Cracks
These are the ones that should get your attention. Cracks running at angles, especially 45 degrees, or following a stair-step pattern through joint compound, can indicate foundation or structural movement.
Cause: uneven settling causes shear stress in the wall. The drywall cracks along the path of least resistance, which is often diagonal. Stair-step patterns happen when the crack follows the weakest points in the compound.
Location matters. Diagonal cracks radiating from window or door corners are extremely common and usually not serious. They happen because openings are stress concentration points. Diagonal cracks in the middle of a wall with no obvious stress point are more concerning.
Look for other signs. Is the crack getting wider over time? Is one side higher than the other? Are there similar cracks elsewhere in the house? Are doors or windows sticking? If yes to any of these, get a professional opinion.
My neighbor Linda's basement crack started as a diagonal line near a window. Over two years it widened to almost half an inch and developed obvious displacement. The foundation was rotating. Classic warning signs that she, and I, didn't recognize until too late.
Horizontal Cracks
Horizontal cracks running along a wall are uncommon and should be taken seriously if they appear on foundation walls or in conjunction with bowing.
Cause: lateral pressure, usually from soil against basement walls. The wall is being pushed inward and cracking at the stress point.
On above-grade walls, horizontal cracks are less alarming. They're often just seam cracks where horizontal drywall joints were taped, which is unusual but not problematic.
The test is whether the wall is bowing. Sight down the wall from one end. If it curves inward at the crack, you have a structural problem. If it's flat, you probably have a tape failure.
I've only seen one actual horizontal pressure crack in person, in a neighbor's basement. You could see the bow from across the room. That repair involved carbon fiber straps and cost about $4,500. Caught it before it failed catastrophically.
Cracks with Displacement
This is the big red flag. When one side of a crack is higher or further out than the other, you're looking at structural movement, not cosmetic cracking.
Run your finger across the crack. If you can feel a step or ridge, measure it. Displacement over 1/8 inch needs professional evaluation. Displacement that's increasing needs urgent professional evaluation.
Cause: differential settlement, foundation failure, or structural damage. Something is moving that shouldn't be moving.
Do not repair displaced cracks without addressing the cause. The repair will fail immediately as the movement continues. You're spending money and effort on something that can't possibly work.
Dave's living room crack, the one he called the engineer for? It was a seam crack with zero displacement. Linda's basement crack? Obvious displacement from the day it appeared. Learning to recognize that difference is the most important crack-reading skill you can develop.
When to Call a Professional
Based on my experience and everything I've learned from contractors and engineers:
Call someone if you have cracks with displacement, rapidly widening cracks, cracks accompanied by other symptoms like sticking doors or sloping floors, or horizontal cracks in basement walls.
Don't worry about hairline cracks, seam cracks, corner cracks, or diagonal cracks near door and window openings. These are almost always cosmetic.
If you're unsure, monitor the crack over time. Mark the ends with pencil and date them. Measure the width and write it down. Check again in a month, three months, six months. Cracks that stay stable are not urgent. Cracks that grow need attention.
A structural engineer consultation costs $300-500 typically. It's worth the peace of mind if you're losing sleep over a crack. Just don't be Dave and call them for obvious tape failure.