How To Look At A House

McGarry and Madsen's home inspection blog for buyers of  

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What are the vertical butt joint requirements for fiber-cement (Hardiplank) lap siding?

Friday, June 15, 2018

They aren’t what they used to be. A gap of up to 1/8”, and caulked at the joint, was an option in 2001 edition of the James Hardie installation manual, as shown below.

    The current edition of the James Hardie manual, along with the International Residential Code (IRC) and Residential Edition of the Florida Building Code (FBC), offers three options for butt joints:

  1. Strip of flashing (water-resistive barrier) under the joint.
  2. Caulking at joint.
  3. “H” joint cover (like the one shown at top of this page)

    A gap between exposed ends is no longer on the list and only “moderate contact” at exposed butt joints is specified.

Here’s a view of an “H” joint cover from below, which covers a small gap between board ends.

    “Moderate contact” is a term that seems to be intentionally vague, but we suppose it means “touching, but not jammed together.” Occasionally we see homes with moderate contact butt ends that were either not contacting moderately enough, or the movement of the underlying structure caused a compression problem, like the example shown below. In this case it was a siding plank near a corner, which normally sustains more movement.

    Also see Is Hardie fiber-cement siding waterproof? and What causes vertical cracks in fiber cement siding planks? 

        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.

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