A backyard stone patio can be a homeowner’s dream. But when surrounded by trees and shade, it can also be a dream for mold and mildew. When a homeowner faced the problem of mold and mildew taking over his patio, he called the Ask This Old House team for help. Mason Mark McCullough brought several solutions (of the cleaning variety!) to help restore that bluestone beauty.
What Causes Patio Discoloration?
Patios are exposed to the elements all year long, and in shady, tree-covered backyards, they can become discolored with mold and mildew. This usually produces green, brown, or black streaks and a slippery film. Luckily, there are solutions.
Understanding the difference between mold and mildew can help you identify what’s growing on your patio. As This Old House has covered, mildew is always flat — if you rub your hand against it, you’ll see a powdery residue. Mold, on the other hand, has a noticeable texture and can appear in many colors, including red, brown, purple, and black. The good news: the treatment for both is the same.
Solution Choices
There are several options for cleaning a backyard patio. Muriatic acid is the strongest option, cutting through mold and mildew quickly, but it’s not great for plants or pets. White vinegar is also an option, but it only works for efflorescence. Bleach is another product that will work on mold, mildew, and algae. All-purpose outdoor cleaners can handle everything the other products can, and more.
These products do need to be cut with water:
- Muriatic acid: 5 parts water to 1 part acid
- Vinegar: 3 parts water to 1 part vinegar
- Bleach: Varies, but around 3 parts water to 1 part bleach
- Outdoor cleaner: Can be used straight
How To Clean a Patio
The exact process might adjust slightly depending on the solution used, the wait time, and the stains. However, this is a helpful overview of the patio cleaning process.
- Remove all of the furniture and plants from the patio area and wet the surface of the patio with the garden hose.
- Mix a small amount of the cleaning solution in the bucket following the manufacturer’s instructions. Use the scrub brush to apply the solution to a small section of the patio. Let it sit for the appropriate amount of time (according to the manufacturer) before scrubbing and rinsing the solution off. Check for discoloration and proceed accordingly.
- Fill the spray bottle with the appropriate amount of water and cleaning solution. Spray the patio surface evenly and avoid spraying the solution onto other surfaces, plants, grass, and anywhere else you don’t plan to scrub.
- Leave the solution on the surface for the appropriate amount of time according to the manufacturer.
- Connect the scrubbing attachment to the pressure washer. After waiting for the solution to dwell, begin scrubbing the patio with the pressure washer. Move the scrubber back and forth and be sure to cover the entire surface.
- Use the scrub brush to reach the edges, corners, and anywhere else the scrubbing attachment can’t reach.
- Rinse the surface with a garden hose. Take note of any areas that might require a follow-up cleaning.
Choosing the right cleaner matters. As Mark explained on Ask This Old House, muriatic acid is often too strong for bluestone patios. Vinegar is “really specific to one stain that is common with masonry” called efflorescence. Bleach is “the stuff that is most commonly used on the mold, the mildew, and the algaes.” And an all-purpose masonry cleaner can handle a range of issues. Be sure to wet surrounding landscape before applying any solution to dilute runoff and protect nearby vegetation.
Pro Tip: Mark, a masonry expert featured on Ask This Old House, emphasizes the importance of testing first: “We’re always going to want to do a test area. That way we can make sure the cleaner doesn’t discolor the bluestone, ruining the entire patio.” He recommends spreading a small amount of solution onto an inconspicuous stone and observing it during the dwell time — which can be as quick as 30 seconds to a minute depending on the product — before committing to the full patio.
When using a pressure washer on bluestone, keep the pressure regulated to avoid damaging the joints between stones. As Mark demonstrated on Ask This Old House, polymeric sand joints “can be blown out” if the pressure is too high. He recommends using a double-wand attachment that helps regulate water flow — one hole acts as the jet applying pressure while the outer ring tempers the stream — so you get effective cleaning without dislodging joint material.
Resources
Using a pump sprayer, Mark applies a multipurpose outdoor cleaner to the bluestone and allows it to sit for a few minutes. Once the solution is set, Mark sprays down the bluestone using a power washer with a surface cleaning attachment.
