What Is This Gunk Staining My Driveway?

Is it just me, or did this last season cause way more staining than normal on my hardscape? My front porch, driveway, and back patio are all gunked up with black stains - and they were all cleaned in the last 18 months. I feel like in previous years the staining hasn’t returned so fast. So what is causing it?

There are actually several things that cause exterior surfaces to become discolored, and they all impact certain areas more than others. The big causes of hardscape staining are: mold/fungus, tannins, pollution, moss, and plain ol’ dirt. Let’s take a look at each of these factors so that you can get an idea for what might be causing the discoloration on your own surfaces.

First up: mold and other fungi. I don’t know about you, but I don’t usually think of mold or fungus as things that affect concrete or siding. Mold is that fuzzy white gunk growing on those leftovers that I forgot about in the back of my fridge, and fungus is those red and white mushrooms that grow on fallen logs in fairy tale illustrations. But the reality is that mold and other fungi can take many forms and look a lot of different ways. Lichen is a form of fungus that often grows on rocky surfaces, forming white or green patches. Mold and mildew are related fungi that often form gunky white, black, or greenish areas on hard surfaces. This type of discoloration is especially prevalent in damp areas, so you may see your exterior surfaces being especially discolored in shaded areas, in places where water pools, or in spots with overgrowing plants. On concrete this will look like irregular patchy areas, and on siding or wood this will look greenish or grayish and streaky.

Next is tannin. Tannins are pigments that are found naturally in many plants. These pigments are what gives wine its color, and what causes your tea to darken as it steeps. But tannins are also present in plants that grow around your property. As water flows over fallen or decaying plant parts, the tannins in those plants are released and can stain hard surfaces. You may have seen tannins at work if you have ever let fallen leaves lay on your patio or sidewalk for a few months. When those leaves are cleared away, the concrete may have had orangish or brownish discoloration shaped just like the leaves. Over time, tannins can cause hardscape to discolor even when you are diligent about removing plant debris. This will look like swaths of brownish stains, especially in places that are downhill from a lot of plants.

Pollution surprises me as a component of hardscape staining, but it makes sense if you think about how interconnected all of our environmental systems are! Anything that gets into the air will also get into the rain as it falls through the air, and then anything that is in the rain will get on all the surfaces that the rain falls on, including your roof, gutters, siding, and hardscape. This pollution could be from vehicle exhaust, wildfire smoke, factories, agriculture, and even dust. All of that gunk gets washed down onto your exterior surfaces and builds up over time, causing an even coating of black or dark gray discoloration. This happens especially on rough and porous surfaces like concrete. Personally, I think this is the main culprit of how stained my driveway looks this year: all the ash from those wildfires last year has settled into all the nooks and crannies of my concrete!

Finally we have arrived at the staining caused by regular old dirt. Dirt settles into the nooks and crannies of exterior surfaces and just kind of hangs out there. This kind of staining can be found under places where grass or plants have overgrown onto hard surfaces, and in places that are downhill from dirt beds with loose soil that is getting washed out by rain or sprinklers. These stains generally have a more defined shape than the discoloration cause by pollution.

Here’s the good news: you don’t need to diagnose the cause of the staining on your exterior surfaces because our skilled technicians are experts in identifying the right tools and methods to remove gunk from all kinds of surfaces, no matter where that gunk is coming from. Often these stains are deeply ingrained in your surfaces and cannot just be hosed away with a garden sprayer. You need the pros to come get the job done right! Regardless of the cause of the stains, it is a good idea to have your exterior surfaces regularly cleaned to avoid moisture retention that can increase the rate of erosion and cracking, and to keep your property looking well cared for with maximum curb appeal. Are you curious about how we can help keep your property looking its best? Fill out our free quick quote form, or give us a call at (253) 414-5304 to have your property assessed!