What to Do With the Butt Joints on Siding


All siding, regardless of whether it's wood, vinyl or a composite, needs to be butted up against other pieces. It is a common misconception that the material should be stacked tight against other pieces. Actually, butt joints need space for several reasons.

Seasonal Movement

  • All materials, ranging from vinyl to cement board, have seasonal movement. Known as expansion and contraction, this is the movement of a material as the molecules within expand during the summer months, thus causing swelling to occur. They shrink during the cooler months. Gaps need to be left between butt joints or the swelling can cause buckling. The exact gap depends on manufacturer guidelines.

Moisture Swelling

  • While vinyl and composite sidings will only swell with the temperature, wood sidings are affected by water. When wood is soaked with water, it naturally swells. Joints are needed where the material butts together not simply for seasonal movement, but also for cases when the wood swells because of condensation, humidity or rain. If you don't have any room for the wood to swell, the pieces of siding will pop off the wall.


  • Most siding manufacturers request that you put caulking between the vertical joints of siding. The exact spacing between the siding panels and sheets will vary depending on the manufacturer. For example, wood siding usually needs a quarter-inch of space and composites require about an eighth of an inch. However, some type of caulking is needed to keep water from entering the gap between the siding pieces.

Flashing Strips

  • For the horizontal butt joints, flashing is the preferred method for most siding installations. While caulking can be used in some circumstances, flashing is preferred because it does more than merely fill the gap between the two pieces of siding. Instead, flashing works by forcing the water out and away from the joint. The water is forced to the outside of the siding where it belongs, draining down the face of the home.

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