Most drywall is installed over wood studs, but wood moves when the humidity and temperature in a house changes and drywall doesn’t. So something’s gotta give, and it’s always the drywall. This is the reason for many of the annoying small cracks in a house. 

    "If someone invented wood today it would never be approved as a building material,” according to Joe Lstiburek, an engineer and building scientist. "It burns, it rots, it has different strength properties depending on its orientation, no two pieces are alike, and most cruelly of all, it expands and contracts based on relative humidity."

    The tree trunk that is the source of wood studs served the tree in two ways: structural strength to support the branches and leaves, and as a membrane to transport water and nutrients up to them. When we cut down a tree and use the wood for its strength we also have to contend with its inherent moisture absorption ability.

    Wood fiber expands and shrinks more perpendicular to the grain than along the grain. In other words, a wood stud or beam gets thicker when it absorbs moisture, but only negligibly longer. Also, wood usually has higher moisture content when it is installed than after a few months in place inside an air conditioned house and, as it dries, the wood gets thinner—but not much shorter—and a few studs will twist as they dry.

   Savvy builders try to let corners and intersections of walls and ceiling “float” whenever possible to accomodate a little movement between them. And we always tell homebuyers to wait to fix any small, cosmetic cracks until they have occupied the house for a few months, and the interior humidity has stabilized based on their heating/cooling, cooking, clothes washing, and bathing patterns. Cracks can also open and close seasonally, based on changes in relative humidity.

    So, even though homeowners tend to envision looming structural problems whenever a drywall crack appears, simple wood movement is often the underlying cause. Most cracks that are caused by wood movement occur along drywall seam lines and where the wall meets the ceiling, especially at the intersection with complicated ceiling framing. Cracks that shear across the drywall are more likely to be related to a structural problem. See our blog post How can I tell if a diagonal crack in drywall at the corner of a window or door indicates a structural problem? if the crack is more than hairline and appears to be growing.

    Garage ceiling drywall has its own unique problems, and can sag and crack over time due to the increased heat and humidity exposure (especially if the garage door is left open for extended periods), corrosion of fasteners, and flexing of the bottom chord of trusses. To read about it, go to our blog post Why is my garage ceiling sagging? 

    Another cause of ceiling cracks is an accumulation of heavy stored items in the attic above the cracks, so this problem is more likely to occur near an attic hatch opening. It can be further aggravated by the removal of the center chords of the roof trusses to get more clear area of attic storage. Don’t do it. See our blog post What are the mistakes to avoid when doing attic improvements? for more on this.

    And there is another type of crack that forms along the line between the ceiling and walls that is caused by phenomenon called “truss uplift.” See What causes cracks between the ceiling and walls? for more on this.

    The worst drywall cracks of all are ones that run in a long line up the wall and across the ceiling. This is an indication an indication of a foundation problem.Want to learn more about cracks? Check out Where are the places to look to find structural cracks in a house? for a short course in “crackology."

      And, by the way, even with its annoying quirks, Joe Lstiburek still thinks wood is a wonderful building material.

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Here’s links to a collection of our other blog posts about STRUCTURE AND ROOMS:

• What causes dark or light "ghost" lines on ceilings and walls?

• Why is the grout cracking and coming loose at my floor tile?

• What are the building code requirements for notching and boring holes in a wall stud? 

• Can you access or exit a bedroom through another bedroom?

• What is the difference between a carport and a garage? 

• What are simple ways to find the cause of a ceiling stain?

• What is the minimum size of habitable rooms in a house according to the building code?  

• How can I identify what kind of wood flooring I am looking at?

• Why does my concrete floor slab sweat and get slippery?

• What is the minimum ceiling height for rooms in a house? 

• Why are there score line grooves in the concrete floor of the garage?

• How much can I cut out of a floor joist? 

• How can I tell if my floors are sloping?

• Why do the floors slope in this old house? 

• What are the common problems when a homeowner converts a garage to conditioned living space, such as a family room?

• How can I tell if a wall is load-bearing? Which walls can I take out? 

   Visit our STRUCTURE AND ROOMS page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.