We do not have a definite answer to this question. But there are several likely culprits listed below, and cracking could also be caused by any combination of them: 

• Over-nailing with a pneumatic nailer - Can cause a hairline crack that will expand over time.

 • Under-nailing - The raised nail head can start a fracture line at the bottom of the board above it when it is installed.

 •  Nailing too high or close at an end - Creates a small crescent-shaped crack high on the board next to an end.

 • Settlement of the structure - Movement of the foundation would cause cracks in the siding, but it would not be a likely cause unless there was also corresponding cracks on the interior wall surface in the same area.

• Improper handling - The boards—especially long ones—should be supported at both ends when moved. Fiber cement planks are floppy until secured on the wall and holding only in the middle or transporting over a shoulder will cause a dramatic sag at both ends. The stress can cause a crack at the center of a board. That is, coincidentally, where we tend to see more of them. 

• Different rates of expansion/contraction between wood and cement products - The thermal expansion coefficient of cement products is approximately twice that of the wood structure underneath it. But, conversely, wood responds more dramatically to changes in moisture level.  We tend to see more cracks on the side a house that gets full afternoon sun; but that evidence is only anecdotal, and we know of no research studies to back it up.

• Defective material - Although the two major manufacturers (James Hardie and CertainTeed) regularly respond to any claims for damages due to failure of their building products by stating that the problem was caused by incorrect installation, both have been entangled in class action lawsuits over the adequacy of both their products and warranties. CertainTeed settled a few years ago and, as far as we know, the James Hardie battle is still ongoing.

Impact Cracking Not Likely

    Cracking due to an impact on the surface of a fiber cement board is less likely because the material tends to dent rather than fracture when well supported. An example would be the pock-marked siding of homes facing a golf course caused by wayward golf balls—something we see pretty regularly.

Manufacturer’s Butt Joint Specs

    For the manufacturer’s specs for butt joints, see our blog post What are the vertical butt joint requirements for fiber-cement (Hardiplank) lap siding?

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To learn more about exterior walls and structures, see these other blog posts:

• What is the average lifespan of a house foundation?

• What causes vertical cracks in fiber cement siding planks?

• What causes raised white lines of residue on a block wall that are crusty and crumbling? 

• What is the difference between soil subsidence, heave, creep, and settlement? 

• How much ventilation is required for the under-floor crawl space of a home? 

• What causes stair-step cracks in a block or brick wall?

• What causes a horizontal crack in a block or brick wall? 

• Should I buy a house with a crawl space? 

• Why is my stucco cracking?

• There's cracks running along the home's concrete tie beam. What's wrong? 

• What would cause long horizontal lines of brick mortar to fall out?

• How do I recognize serious structural problems in a house?

• What is engineered wood siding?

• Should I buy a house that has had foundation repair? 

• What are those powdery white areas on my brick walls?

• What causes cracks in the walls and floors of a house?

• What are the warning signs of a dangerous deck?

• How can I tell whether my house foundation problems are caused by a sinkhole or expansive clay soil?

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