The answer to this question depends on whether you want to know if the fiber-cement siding material itself is waterproof, or if the wall is waterproof after installation of the siding.

MATERIAL - Hardie and other brands of fiber-cement siding receive a factory coat of primer, or primer and paint, that waterproofs the surface. But some water can be absorbed into the cut ends over time if they are not sealed with paint before installation or caulked. Because fiber-cement is inorganic, it will not rot. But dust accumulation on the satura areas may allow minor mold growth or staining. This is most likely to happen when an unsealed cut end is at the intersection of the side wall of a roof dormer and the roof slope, and too close to the roof surface. 

WALL - No horizontal lapped siding wall is waterproof, whether it is wood, engineered wood, vinyl, or fiber-cement. Lapped siding is very water resistant, but it depends on gravity to create a downward flow of water over the unsealed lap joints after it hits the wall. The driving rain and wind in a severe thunderstorm or hurricane will push some water up into the laps between boards. Because a waterproof housewrap is normally installed under the siding, the water will run down the house wrap and exit harmlessly at the bottom of the wall without damaging the structure. 

    Also, see our articles What causes vertical cracks in fiber cement siding planks? and What are the vertical butt joint requirements for fiber-cement (Hardiplank) lap siding? and What is the average life expectancy of fiber-cement siding? 

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Here’s links to some of our other articles about EXTERIOR WALLS AND SIDING:

• What are the pros and cons of vinyl siding?

• How long does exterior siding last? 

• Can termites eat through siding? 

• Do stucco walls mean a house is concrete block?

• What is board and batten siding?

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

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

• How can I tell if a diagonal crack in drywall at the corner of a window or door indicates a structural problem?

• Why is my stucco cracking?

• Can vinyl siding be painted? 

• What does freeze damaged brick look like?

• What causes paint to peel prematurely on the exterior of a house? 

• What are the pros and cons of aluminum siding?

• What is engineered wood siding?

• Should I buy a house with asbestos siding?   

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

• What is the difference between "composite" and regular wood siding? 

• What causes a vertical crack in an exterior concrete block or brick wall? 

• Can vinyl lap siding be installed diagonally?  

• Does stucco need expansion joints? 

• What is a vinyl siding zipper tool? 

     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.