Wall and ceiling cracks: which are cosmetic, which need an engineer

You notice it while hanging a picture: a thin line climbing from the top corner of the bedroom door toward the ceiling. In a house built before the Second World War, a line like that is usually old news. The job is telling old news from new movement, and that takes a few weeks, a ruler and a pencil, not a tub of spackle.

#Most old-house cracks are in the plaster, not the frame

Walls in a Victorian, an Edwardian flat or a 1920s bungalow were usually built the same way: thin strips of wood lath nailed across the studs, then wet plaster troweled on in coats. The plaster squeezes through the gaps and curls over the back of each strip, forming the keys that hold the sheet up, as the National Park Service's guide to historic plaster explains. Once set, that sheet is hard and rigid, and the wooden house behind it never quite stops moving.

Wood lath swells and shrinks as the air turns damp and dry. Floors deflect under a century of pianos and bookcases, and an earlier owner may have cut in a doorway without a proper header. The Park Service names all of these as reasons old plaster cracks. A rigid skin breaks where stress gathers, which is why so many cracks start at a door or window corner, and why one horizontal line can return along the same lath joint year after year.

By the late 1930s most builders had switched to rock lath, paper-faced gypsum panels that took two coats instead of three, and later houses moved to drywall. HUD's Residential Rehabilitation Inspection Guide puts most wall and ceiling cracks down to settlement, sagging or warping wood, and small seasonal movements, and says settling cracks can be repaired once the movement stops, as it often has. Hanging a mirror on these walls? Start with the plaster walls guide.

#What the crack in front of you probably means

Find your crack on the drawing and tap its number. Each note says what that crack usually means and who should look at it first. Cracks that look alike can mean different things, though, and change over time is what separates them.

#Is it still moving? A month of watching

A crack that has stopped is a patching job. A crack still moving is evidence, and filling it destroys the evidence. Give it four weeks, through at least one change in the weather, and keep the record in one place.

1. Photograph it with a scale

   Hold a ruler against the crack and shoot straight on, from the same spot each time, with the date on a strip of painter's tape beside it.

2. Tick the ends

   Draw a short pencil line across each end and date it. If the crack runs past a tick, you'll know without guessing.

3. Measure the widest point

   Note the width at one marked spot with a ruler showing sixteenths of an inch, once a week.

4. Try the doors and windows

   A latch that used to catch and now misses, or a door rubbing at its top corner, belongs to the same story. HUD's guide reads binding doors and out-of-square frames as possible signs of settlement.

5. Look again after the weather turns

   Check after the first hard rain and in the next dry spell. A crack that opens and closes with the seasons is the seasonal kind HUD describes: it needs a flexible filler, not an engineer.

6. Decide

   Unchanged after a month: patch it. Wider, longer, joined by new cracks, or matched by doors that changed: stop patching and have an engineer look.

A crack that has stopped moving needs a patch; one that hasn't needs an engineer.

• Heard this?A diagonal crack over a door means the foundation is failing.Tap for the factsWhat's trueThe Park Service describes diagonal stress cracks that start at door and window frames, and shows one over a kitchen door repaired with mesh tape and joint compound. Worry about diagonals running opposite ways, or any crack that keeps growing.
• Heard this?Fill a crack well enough and it stays filled.Tap for the factsWhat's trueA crack that opens and closes with the seasons will split any rigid patch. HUD's guide says those can be filled with a flexible, paintable sealant and otherwise can't be effectively repaired.

#Ceiling cracks and plaster that can fall

A single straight crack across a plaster ceiling is common, and on its own it usually traces small movements in the joists above. Shape changes the picture. When parts of a ceiling come loose, the Park Service says, the keys holding the plaster up have probably broken, and left alone, gravity brings chunks of it down. A sag you can see from the doorway, a bulge, or a crack with one edge hanging lower than the other belongs in that group.

Keep people, beds and cribs out from under a sagging section until someone has looked at it. Where the surrounding plaster is sound, the Park Service describes screwing the loose area back to the lath with plaster washers; where it has separated over a wide area or a long leak has rotted the lath, HUD's guide suggests replastering or covering the ceiling with board. A brown ring means water, so the leak gets found first, and a long-running one is worth checking for rot in the framing.

#Filled ground, old creeks and cracks that start below

Since 1845, fill has turned more than 40 square kilometers of the bay's tidal edge into land, says the USGS report on liquefaction in the 1989 earthquake. Ground like that settles, and in strong shaking it can liquefy. The Marina took the worst of it in 1989; a USGS fact sheet says it was built on uncompacted sandy ground over a shallow water table. The report also covers Treasure Island, and found 1989 damage in the Mission and South of Market in the same general places that failed in 1906.

In the Mission between Folsom and Harrison, roughly 15th to 22nd Street, houses sit over what was once Mission Creek, a tidal estuary USGS researchers traced out through South of Market to Mission Bay. The thickness of fill lying below the water table best explained where the ground sank and spread. HUD's guide adds that settling usually happens early in a building's life or after a big change underground, so a new crack in an old house on fill deserves a second look.

Settlement leaves a pattern, not a single line. HUD lists doors that bind, frames knocked out of square, sloping floors and cracks that taper from wide to hairline, and notes that one settlement crack can run through the foundation, the wall above and the plaster inside. The same crack outside and in, or in the garage wall under a cracked room, is one problem with one cause.

#After an earthquake: check before you patch

Most large earthquakes are followed by aftershocks, and the USGS aftershock forecast page is plain that while most are smaller than the main shock, they can still do damage, and a small fraction of quakes are followed by a bigger one. Plaster that cracked in the first jolt can keep cracking for days. Record everything now and patch later.

Some houses deserve a closer look whatever the cracks say. USGS's Bay Area handbook, Putting Down Roots in Earthquake Country, gives 5 points for a house built before 1960 and 5 for living space over a garage, adds at least 5 for the ground, and suggests an evaluation by an engineer, architect or contractor at 13 points or more unless the house was strengthened recently. The earthquake-proofing guide covers what a retrofit involves.

Weak ground floors matter in flats too. San Francisco's Safety and Resilience Element says soft-story failure, where a ground floor of garage doors and open space gives way, was behind nearly half of all homes made uninhabitable by the 1989 Loma Prieta earthquake. New cracks around a garage after shaking deserve an engineer's visit, even if upstairs looks fine.

#Engineer, plasterer or handyman: who looks first

When a crack points at the structure, the first visit should come from an engineer, not a contractor who will also sell the repair. California's engineering board says a civil or structural engineer should judge whether a quake-damaged building is safe to occupy and can be repaired, sometimes with a geotechnical engineer's soils report. A structural engineer is a civil engineer who passed an additional exam. The board can't recommend anyone, but its license lookup shows whether a license is current.

The minor-work limit also bars splitting a bigger job into small contracts, so most crack patching fits under it and foundation repair rarely does. Any repair that needs a building permit goes to a licensed contractor.