Wood Shrinks Across the Grain
Wood shrinks in all directions, but most shrinkage is across the grain or across the width of the wood. Our home construction practices have adapted to this movement to limit its effect on building components.
Wood Doors Shrink
Let’s look at a wood panel door (Photo 1). In this photo, I’m pointing to white lines along the panel. The door was stained and finished while the wood was still damp. As the door dried and shrank, unfinished wood was exposed as a white gap.
Why is the gap on the side of the wood panel and not below it? Because wood shrinks across the grain or across its width. The horizontal frame across the width of the door shrank much less than the panel, so the side of the panel was exposed. Normally, this problem is avoided by allowing doors to dry before a finishing coat of paint or stain is applied.
Wood Framing Shrinks
Illustration I075 shows how framing shrinks across the width of the wood. In this case, a significant gap has developed at the top framing of the wall. A gap behind the vapor barrier creates a direct path that allows moist, heated air into an attic.
To see this problem identified during an inspection, look at Photo 2, which shows the attic of a house in a cold climate. The home, about five years old, had an attic moisture problem. Lifting the insulation revealed black stains on the fiberglass where air flows into the attic. The dark areas align with gaps, while the clean areas align with solid wood framing.
How much does framing shrink?
Shrinkage varies according to the type of wood and its moisture content. Under typical conditions, a two-story home will shrink about ¾ inch in height during the first year of occupancy (Illustration S138). Most shrinkage occurs across the width of the dimensional lumber. Builders adapt for this movement by leaving slight gaps around windows and doors. Many of the drywall cracks that occur during the first year of occupancy are due to shrinkage of the framing, not structural defects and movement.
Wood shrinkage is normal and natural. Drying lumber before use limits the shrinkage as the framing adapts to ambient conditions. Good carpenters leave gaps for shrinkage and movement.
Have you ever seen a stone sill on top of a brick veneer wall crushed by the siding and tipping into the wall structure? Maybe the gap did not allow for the wall shrinking.