Blog

Fiber cement planks are renowned for their exceptional dimensional stability, meaning they are far less likely to crack or warp compared to wood or vinyl. However, like all building materials, they do undergo slight physical changes.

The key to their durability is that fiber cement is “Engineered for Climate.” Here is a detailed breakdown of how it handles thermal expansion:

1. Minimal Thermal Expansion (vs. Other Materials)

Fiber cement has a very low Coefficient of Thermal Expansion (COTE). Because it is a masonry-based product, it doesn’t “grow” or “shrink” significantly when the temperature swings from freezing to blistering heat.

• Vinyl Siding: Can expand up to 12 mm over a 3.6-meter length. This often leads to “oil canning” (buckling or waving) if installed too tightly.
• Fiber Cement: Typically moves only about 1.5 mm to 2 mm over the same length. This movement is so minimal that it is almost invisible to the naked eye.

2. The Role of Humidity (The Real Driver)

Interestingly, fiber cement is more sensitive to moisture (relative humidity) than to temperature.

• When it is very humid, the cellulose fibers inside can absorb microscopic amounts of moisture, causing the plank to expand slightly.
• In dry, hot weather, it may shrink.
• The Counterbalance: Often, high temperatures occur when humidity is lower, and vice versa. These two forces (thermal expansion and moisture contraction) tend to counterbalance each other, keeping the plank stable.

3. Why Cracking or Warping Might Occur

If you see a fiber cement plank cracking or warping, it is almost always due to installation errors rather than a material defect:

• No Expansion Gaps: While movement is minimal, installers must still leave a 3 mm (1/8 inch) gap where planks meet trim or at “butt joints.” If the planks are “slammed” together with no room to breathe, the internal stress can cause the board to bow (warp) or the edges to chip.
• Improper Fastening: If the planks are “over-driven” (screws/nails driven too deep) or “pinned” too tightly, the material cannot slide slightly to accommodate natural movement, leading to stress fractures.
• Unsealed Edges: If “field-cut” edges (edges cut on-site) are not sealed with paint or primer, they can absorb water, leading to localized swelling or “mushrooming” at the joints.

Summary Table: Thermal & Moisture Stability