Why Black Spots Keep Coming Back on Block Paving
Black spots on block paving are colonies of algae, lichen and moss that feed on damp, shaded surfaces. They return because the root structure survives surface cleaning, and because the conditions that let them grow, mainly shade and trapped moisture, stay the same.
If you have scrubbed a driveway, watched it come back within a season, and wondered what you did wrong, this guide from Cotswold Exterior Surface Cleaning is for you. We clean block paving across the region for homeowners, holiday lets and commercial sites, and we see the same pattern constantly: the spots are treated as a stain when they are actually a living growth.
That distinction changes everything about how to remove black spots from block paving properly.
What Causes the Black Discolouration
Three things cause the black patches on block paving:
Algae - a green-black film that spreads across the surface in damp, shaded spots
Lichen - a crusty growth that clings to the texture of the blocks and resists light cleaning
Moss - a root-forming plant that settles in the joints and holds water against the paving
All three thrive in the same conditions. North-facing drives, areas under trees, and paving that never fully dries are the worst affected. Poor drainage makes it worse again.
Watch Out Never treat black spots as a simple stain. If you scrub the surface without killing the growth, the roots stay in the joints and the spots return within months. That is the single most common reason DIY cleaning fails.
What You'll Need Before You Start
Getting the right kit together first saves a second trip and protects the paving. Here is what we would gather:
A dedicated black spot remover for paving (not household bleach)
A pressure washer with a rotary surface cleaner attachment
A stiff brush and a soft broom
A watering can or low-pressure sprayer for applying treatment
Kiln-dried sand for re-sanding joints afterwards
Protective gloves, goggles and old clothes
Check the weather before you start. Dry, overcast days work best. Avoid cleaning in strong sun, as treatment dries too fast to work, and avoid frost.
Step 1: Choose the Right Black Spot Remover for Paving
A black spot remover for paving is a cleaning agent formulated to kill algae, lichen and moss at the root rather than just bleaching the surface. Most are applied diluted and left to dwell, then washed off.
Here is how we would approach it:
Read the label and follow the dilution rate exactly. Stronger is not better; it can stain or damage the blocks.
Test on a small hidden patch first, especially on coloured paving.
Apply evenly with a watering can or sprayer, working in sections.
Let it dwell for the time stated on the product, usually long enough to see the growth soften.
Keep pets and children off the area until it is fully rinsed and dry.
How to Apply It Safely
Safety matters more than speed here. Wear gloves and goggles, and never mix products together. Keep treatment away from drains, ponds and planted borders, because most black spot removers will harm plants and aquatic life. Cover or wet down nearby borders first, and rinse any overspray straight away.
Step 2: Pressure Washing Block Paving Without Damaging It
Pressure washing block paving removes the loosened growth, but the wrong technique strips the jointing sand and scours the block faces. Use a rotary surface cleaner rather than a bare lance.
A few rules keep the paving intact:
Keep the nozzle moving; never hold it in one spot.
Work at a moderate pressure, not maximum.
Wash in the direction of the joints, not across them.
Rinse the whole area afterwards to clear residue.
Pro Tip The mistake most DIY cleaners make is blasting the joints. That washes out the kiln-dried sand that locks the blocks together, and once the sand is gone, water sits in the gaps and the black spots come back faster than before.
Step 3: Lichen Removal from Block Paving
Lichen removal from block paving needs a different approach to algae. Lichen bonds tightly to the block surface and often needs a second treatment. Apply your black spot remover, allow a longer dwell time, then agitate with a stiff brush before rinsing.
On stubborn patches, a repeat application a week later usually finishes the job. Do not scrape hard at the surface, as this can chip the block edges.
Step 4: Re-Sanding Block Paving Joints After Cleaning
Re-sanding block paving joints after cleaning is the step almost everyone skips, and it is the one that keeps the paving stable. Once the joints are washed out, refill them with kiln-dried sand.
Wait until the paving is completely dry.
Sweep the sand into the joints, working in sections.
Tap the blocks lightly to help the sand settle.
Top up any joints that drop, then sweep off the excess.
Key Takeaway Re-sanding is not cosmetic. It locks the blocks, stops weeds and moss taking hold in the gaps, and helps water drain away from the surface instead of sitting in it.
Common Mistakes to Avoid
Most failed black spot jobs come down to a handful of errors:
Using household bleach, which bleaches the blocks without killing the growth
Pressure washing too hard and stripping the jointing sand
Cleaning in full sun so the treatment dries before it works
Skipping re-sanding, leaving joints open to new growth
Treating the surface but ignoring the shade and drainage that caused the problem
How to Stop Black Spots Returning
Prevention comes down to light, airflow and drainage. Trim back overhanging branches to let sun and wind reach the paving. Clear leaves and debris so water drains freely. Re-sand joints as soon as they open up.
For paving that keeps regrowing despite your efforts, a professional clean does the job properly. At Cotswold Exterior Surface Cleaning, our trained technicians use professional-grade equipment and eco-friendly products to clear growth at the root and restore the surface. We work with homeowners, holiday lets and commercial property managers, and our customers tell us the difference lasts. As one put it, after years of grime, "the service and quality of work was fantastic!"
Frequently Asked Questions
What causes black spots on block paving?
Black spots on block paving are usually a form of algae or lichen that grows in damp, shaded areas where water sits on the surface. The dark discolouration spreads across the face of the blocks and into the joints, especially on north-facing or tree-shaded driveways. Once established, it clings to the textured surface of the paving and ordinary sweeping will not shift it. Removing it properly means treating the growth itself, not just scrubbing the surface.
Will a pressure washer remove black spots from block paving?
Pressure washing block paving lifts surface grime and loosens some growth, but it rarely removes established black spots on its own. The spots are rooted in the block's texture, so blasting at high pressure either misses them or damages the paving and jointing sand. The reliable approach is to apply a suitable black spot remover first, let it work, then rinse and clean. If you do pressure wash, use a rotary surface cleaner on a moderate setting and re-sand the joints afterwards.
Can bleach damage block paving or its jointing sand?
Household bleach is a poor choice for block paving. It can stain or lighten the blocks unevenly, kill the jointing sand's binding, and wash into borders and drains where it harms plants and aquatic life. It also does not reliably kill lichen and algae deep in the surface. Purpose-made black spot removers are formulated to break down the growth without wrecking the paving or the joints, and they are far easier to control.
How do you stop black spots coming back on block paving?
Prevention comes down to water and shade. Re-sand the joints after cleaning so water drains away instead of pooling, clear leaves and debris that trap moisture, and trim back overhanging planting to let light and air reach the surface. A preventative treatment once or twice a year, timed before the damp season, keeps growth from re-establishing. If the same spots return within months, the underlying drainage or shading problem has not been solved.
Black spots will keep winning if the conditions that feed them never change. That is the challenge most driveways face, and it is one we solve properly rather than temporarily. Cotswold Exterior Surface Cleaning brings specialist jet washing, trained technicians and eco-friendly products to every job, clearing growth at the root and restoring your paving's appearance. Get in touch with Cotswold Exterior Surface Cleaning to book a clean and keep your block paving looking its best.