Hardie board—or as it’s referred to by its generic name, fiber cement—is one of the most popular materials for use as external siding material on homes across the United States.

Hardie board is especially sought after for its ability to mimic the texture of wood siding while also being more durable, longer-lasting, and with a wider range of colors. 

However, over time you might notice your Hardie board siding losing its color, with the outer surface of your Hardie board siding becoming chalky to the touch.

These are signs that your Hardie board has oxidized. 

Why Is My Hardie Board Oxidized?

Before you start thinking that choosing Hardie board for your home’s siding was a bad investment, let’s clear the air on what oxidation actually is.

Setting aside all of the complicated chemistry, oxidation is a chemical reaction between two different substances. In the case of your home, the reaction happens between the siding material and the oxygen and dirt particles in the air. 

What the oxidation does to your Hardie board siding is as we touched on at the beginning of this article—faded color, a chalky dust that collects on the surface, and the potential for further damage over time. As such, you’re going to want to fix any oxidation problems when they arise. 

Today’s Homeowner Tips

An important thing to note here is that oxidation on your siding is not exclusive to Hardie boards. If you’ve seen faded paint on cars or outdoor furniture, this is precisely what is happening.

In fact, oxidation will happen to just about any material given enough time and exposure to the sun, since heat and UV function as catalysts that make the process go faster. 

Fixing Hardie Board Oxidation

Now that we understand what the problem is, our next step would be to fix it. Thankfully, the process is no more complex than a regular cleaning job. We’ll give you the cleaning process below:

1. Determine the extent of the oxidation.

The first thing you need to do is observe where and how much oxidation is forming on your siding. If your siding is of a darker color, then you’ll notice the color becoming more faded. The aforementioned chalk is another sign of oxidation, which might not be easily spotted on lighter-colored siding unless you run a wet finger over it.

In any case, finding out how much there is to clean is important as it’ll help you determine how much cleaner you’ll need to use and weigh your options of whether or not to have a professional do the cleaning for you.

2. Prepare your cleaning equipment.

Once you know what you will be working on, the next step is to prepare your cleaning solution. Since Hardie board siding generally holds up to chemicals and weather pretty well, you have several options when it comes to things you can use for cleaning.

Outdoor cleaning chemicals are a pretty reliable option for this sort of cleaning job, but you can also get away with genetic alternatives and still get similarly satisfactory results.

Today’s Homeowner Tips

Dish soap works quite well if you have extra lying around, and if you really want to go DIY, diluted white vinegar is very effective.

3. Protect yourself.

Now, fiber cement is not exactly the safest substance in the world, barring mention the chalky powder that is the byproduct of the oxidation process. For this reason, you would want to minimize exposure to these substances as much as possible when you are cleaning. 

The oxidized Hardie board is going to be composed of very fine particles that can easily get into your eyes, nose, and mouth and cause irritation. Sensitive skin may also react to the powder upon coming into contact with the stuff. 

A pair of goggles, a face mask, and sleeves would generally be nice to have to maximize your protection. 

4. Apply the cleaner.

When applying your cleaning formula to your Hardie board siding, it’s typically recommended to use a relatively mild cleaning tool to work the cleaner into the siding and remove the chalky oxides. 

Using a rough scrubber could scratch the layer of the intact fiber cement underneath, which would only accelerate oxidation after you’ve finished cleaning. Something like a sponge or a soft brush works pretty well for this purpose.

If you feel hand-cleaning would be a bit too tedious (which it is), a pressure washer is a great alternative to both applying your cleaner and rinsing it off in the next step. But as with the scrubber, though, you would want to lower the pressure of your washer to avoid damaging the board, as the force of the water could crack the Hardie board if you’re not careful.

5. Rinse.

After scrubbing, you will have already noticed some oxidation already coming off of your Hardie board. From here, removing it is just a matter of rinsing with water. If you still see chalky powder, simply go over it with a scrubber and your cleaning fluid and repeat the process. 

Preventing Hardie Board Oxidation

Once the oxidation has been cleaned out of your Hardie board siding, the next step would be to prevent it from forming again—or at least, keep it at bay for some time. 

The fact of the matter is that oxidation is a continuous process so long as all of its ingredients are in place. And in the case of your Hardie board siding, its constant exposure to the sun’s UV radiation practically guaranteed continued oxidation over time.

For this reason, painting or refinishing the Hardie board is the most effective known way to prevent further oxidation for a long time. The paint acts as a protective layer that seals the boards and any of their imperfections, blocking them from oxidizing under the sun. 

With such a measure, one can expect the lifespan of their Hardie boards to extend at least another 5 to 10 years—not to mention the extra visual appeal of freshly-painted siding.

Editorial Contributors
avatar for Matt Greenfield

Matt Greenfield

Matt Greenfield is an experienced writer specializing in home improvement topics. He has a passion for educating and empowering homeowners to make informed decisions about their properties. Matt's writing focuses on a range of topics, including windows, flooring, HVAC, and construction materials. With a background in construction and home renovation, Matt is well-versed in the latest trends and techniques in the industry. His articles offer practical advice and expert insights that help readers tackle their home improvement projects with confidence. Whether you're a DIY enthusiast or a seasoned professional, Matt's writing is sure to provide valuable guidance and inspiration.

Learn More