Can You Pressure Wash Fiber Cement (Hardie) Siding?
James Hardie, the biggest name in fiber cement siding, says a pressure washer can damage it. What the maker recommends instead, and how fiber cement gets washed.
The short answer
James Hardie says using a pressure washer can damage the surface of its siding. It recommends a low pressure spray from a garden hose and a soft, medium bristle siding brush, once or twice a year. A low pressure soft wash fits that advice; a high pressure wand does not.
What fiber cement is
Fiber cement siding is made from cement, sand and wood fiber, pressed into boards that look like wood lap siding, shingles or panels. James Hardie is the best known brand. It is tough, but its painted surface is what you see, and that surface is what cleaning can hurt.
What James Hardie says
From the maker’s own maintenance page (James Hardie fiber cement siding maintenance):
- Using a pressure washer can damage the surface of Hardie siding.
- Clean with a low pressure spray from a garden hose and a soft, medium bristle siding cleaning brush.
- Clean once or twice a year.
The page does not list cleaning chemicals or address mildew directly.
Why that matters for hiring someone
On fiber cement, a narrow, high pressure stream is the exact thing the maker warns against. Ask anyone you hire what pressure they plan to use on your siding, and why.
How we wash fiber cement
We do not blast siding. Our house wash is a soft wash: low pressure and a professional grade sodium hypochlorite (SH) cleaning solution, given time to work, then rinsed. The solution does the work pressure would otherwise do, which is the same idea James Hardie describes: gentle water, not force. Fiber cement is on the list of sidings we wash, along with vinyl, aluminum, brick, stucco and wood.
How often
James Hardie suggests once or twice a year. Shaded walls and walls near trees usually need it more than a wall in full sun, for the same reasons covered in green film on siding.
What cleaning will not fix
Cleaning will not fix chipped or faded paint, cracked boards, or failed caulking at the joints. If the finish is worn, it needs paint, not more washing. James Hardie's page does not tie cleaning methods to its warranty terms, so read your own warranty before any work.
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