Outdoor Grout Cleaning: Patios, Algae, Moss and Safe Pressure Washing
On this page
To clean outdoor grout, sweep, treat growth with a biocide or oxygen bleach, then scrub and rinse. If you pressure wash, stay around 1,200-1,500 psi on sound cement grout, use a 25 or 40 degree fan tip held at least 12 inches (30 cm) away, and never use a 0 degree or rotary turbo nozzle on joints.
Key takeaways
- Outdoor grout soils differently from indoor grout: algae, moss, lichen, leaf tannins, efflorescence and airborne grime, all driven by water and shade.
- Kill biological growth before you remove it. A biocide or oxygen bleach dwell followed by brushing is safer for grout than blasting growth off with high pressure.
- Pressure washing is the leading cause of outdoor grout damage during cleaning. Pressure, nozzle angle and distance together decide whether the grout survives.
- Efflorescence on patios is a moisture symptom. Brush it off dry and look at drainage before reaching for acid.
- Clean in mild, dry weather, and do not wash when a freeze is expected within a day or two.
What is on outdoor grout
Exterior tile lives in rain, sun, freeze-thaw cycles and runoff. Cement grout is the most porous part of the surface and stays damp longest, so it is where growth and stains start. Identifying the deposit decides the treatment, so start here.
| What you see | Cause | Where it appears | Treatment |
|---|---|---|---|
| Green film, slippery when wet | Algae | Shaded, north-facing, poorly drained areas | Biocide or oxygen bleach dwell, soft brush, low-pressure rinse |
| Green cushions in joints | Moss | Recessed or eroded joints that hold soil and water | Remove by hand with a plastic tool, then biocide; repair low joints |
| Gray, white or black flat spots that will not scrub off | Lichen | Older, undisturbed surfaces in clean air | Biocide and patience; dead lichen weathers off over weeks |
| Black streaks or speckles | Black algae or mold, airborne soot | Under trees, near roads, on shaded walls | Biocide or oxygen bleach; alkaline cleaner for grime |
| Brown or orange leaf shapes | Tannins from leaves and seed pods | Under trees, where debris sits wet | Oxygen bleach; sunlight fades many tannin stains over time |
| White powder or crust | Efflorescence | Slabs on grade, areas with poor slope or drainage | Dry brush first; see below |
| Orange-brown rings or spots | Rust from furniture feet, planters or fertilizer | Under metal objects, near lawn edges | Dedicated rust remover (stone-safe if needed); see the stain guide |
For staining not covered here, the grout stain removal guide has a stain-by-stain table.
Treating algae, moss and lichen
Growth on a patio has roots or holdfasts in the grout's pores. Blasting it off with high pressure removes the visible part, erodes the grout surface, and leaves a rougher joint that recolonizes faster. Killing the growth first and letting it release is gentler and lasts longer.
Choosing a treatment
- Oxygen bleach (sodium percarbonate). Mixed with warm water and kept wet for 15-30 minutes, it lightens algae and organic stains and breaks down quickly into harmless components. It is a good first choice near plants and on colored grout.
- Outdoor biocide cleaners. Products made for patios and masonry, often based on quaternary ammonium compounds, are applied and left to work. Many are designed to keep acting over days or weeks as rain rinses the dead growth away. Follow the label for dilution and rinsing.
- Dilute chlorine bleach. Effective on algae and mold staining, but it can lighten colored grout, harm plants and lawn edges, and must be rinsed thoroughly. Wet surrounding plants with water first and rinse them afterward, and test on a hidden area. See bleach on grout.
For the pH side of these choices, and why acids are a poor fit for organic growth, see grout cleaners by pH.
Safety: Never mix chlorine bleach with ammonia, with acidic cleaners (including vinegar and efflorescence removers), or with other biocide products. Use one product at a time and rinse between treatments. Wear gloves and eye protection, keep pets and children off the treated area, and avoid letting runoff reach ponds or storm drains.
Moss in joints
Moss grows where grout is low or eroded enough to hold soil. Pick or scrape it out with a plastic scraper or a wooden stick (metal tools damage the joint shoulders), treat the joint with a biocide, then brush and rinse. If the joint is recessed or crumbling underneath, it needs repair before it will stay clean; see spot grout repair.
Pressure washing outdoor grout safely
A pressure washer is useful outdoors for rinsing dead growth and grime off a large area, but it is also the most common way patio grout gets damaged during cleaning. The water jet erodes the cement paste at the joint face, then the sand, and can cut a joint down in seconds when the tip is close or narrow. On weak or old grout it can strip joints entirely, and it can drive water under tile with failing bond.
What decides the force on the grout
Three settings work together: the pump pressure, the nozzle spray angle, and the distance from the surface. A narrow tip concentrates the same pressure on a small spot, and the force drops quickly as you move the wand away. A 1,500 psi washer with a 0 degree tip held close is far more destructive than a 2,500 psi washer with a 40 degree tip held at 18 inches.
| Tip (common color code) | Spray angle | Use on grout? | Notes |
|---|---|---|---|
| Red | 0 degrees (pinpoint) | Never | Cuts grout and can etch tile and stone |
| Yellow | 15 degrees | No | Meant for stripping hard surfaces like bare concrete; too aggressive for joints |
| Green | 25 degrees | Yes, with care | General cleaning on sound grout at 12-18 inches (30-45 cm) or more |
| White | 40 degrees | Yes, preferred | Best for rinsing and general cleaning at 12 inches (30 cm) or more |
| Black | 65 degrees, low pressure | Yes | Applies detergent or biocide; too gentle to clean on its own |
| Rotary or turbo nozzle | Rotating 0 degree jet | Never | Concentrated jet sweeping the surface; strips grout and sand quickly |
Color codes are a widely used convention, but check your washer's manual, since some brands differ.
Safe pressure ranges by condition
The ranges below are conservative starting points for cleaning tile and grout, not ratings from a standard. Grout manufacturers rarely publish a maximum wash pressure, so start low, test a corner, and increase only if the grout shows no erosion.
- Sound, fully cured cement grout (several months old or more): about 1,200-1,500 psi, 25 or 40 degree tip, at least 12 inches (30 cm) away.
- Epoxy and urethane grout in good condition: these tolerate rinsing well; the same settings apply, and there is little reason to go higher.
- Grout under a year old, soft, cracked, or with visible sand on the surface: do not pressure wash. Use a garden hose, a brush and a biocide instead.
- Natural stone with cement grout: stay at the low end, 40 degree tip, and keep extra distance; soft stones like limestone and sandstone erode as readily as the grout.
Technique
- Pre-treat first so the washer only rinses, not scours.
- Hold the wand at a low angle, roughly 30-45 degrees to the surface, and keep it moving. Never dwell on one spot.
- Sweep diagonally across the joints rather than straight down a grout line, which focuses the jet into the joint.
- Rinse toward the drainage path so dirty water runs off the patio, not into the gaps.
- A rotary surface cleaner attachment spreads the spray evenly and keeps a fixed distance, which is gentler on joints than freehand work. Use it at reduced pressure and do not let it sit in one place.
- Stop if you see sand in the runoff, the joint face going rough, or grout edges chipping.
Efflorescence on patios
Exterior tile over slabs on grade, balconies with poor drainage and planters next to patios are classic efflorescence sites. Water moves through the slab and grout, dissolves soluble salts, and leaves them on the surface as it evaporates. Brush the deposit off dry with a stiff nylon brush and sweep it up. Pressure washing or soaking redissolves it and often brings more salt to the surface as the patio dries.
If it keeps returning, the fix is drainage and slope, not chemistry. Hardened deposits may justify a diluted efflorescence remover, which is an acid: use it only on cement grout with non-calcareous tile, pre-wet first, keep the dwell short and rinse thoroughly. The causes and the full treatment are in efflorescence on grout.
Timing, weather and aftercare
- Pick mild, overcast conditions. In hot sun, treatments dry before they work and leave residue. Aim for 50-85 deg F (10-30 deg C).
- Avoid freezing weather. Do not wash or treat when a freeze is expected within 24-48 hours. Water held in porous grout expands as it freezes and can damage joints; see can grout freeze.
- Let it dry before sealing. Exterior cement grout often needs 48-72 hours of dry weather after cleaning before sealer will absorb evenly. Use a breathable penetrating sealer outdoors; film-forming sealers can trap moisture and turn cloudy. See sealing outdoor grout.
- Fix the conditions. Trim back overhanging plants, clear debris regularly, and keep drains and gaps clear. Shade and standing water bring algae back faster than any cleaner can remove it.
- Repair damaged joints. Grout that is cracked or missing lets water reach the setting bed and slab. For choosing exterior grout, see outdoor grout.
Bottom line: Kill the growth first, brush, then rinse. If a pressure washer is used, keep it near 1,200-1,500 psi with a 25 or 40 degree tip at 12 inches or more, and keep it off new, soft or cracked grout entirely.
Frequently asked questions
What PSI is safe for pressure washing patio grout?
For sound, cured cement grout, around 1,200-1,500 psi with a 25 or 40 degree fan tip held at least 12 inches away is a conservative range. Test a corner first and skip pressure washing on soft, cracked or new grout.
Can pressure washing remove grout?
Yes. A narrow tip, a turbo nozzle or holding the wand too close erodes grout quickly, and weak grout can be stripped out of the joints entirely.
What kills algae on patio grout?
Oxygen bleach solutions, outdoor masonry biocides and dilute chlorine bleach all kill algae. Let the product dwell as directed, then scrub and rinse at low pressure.
How do I get moss out of patio grout lines?
Remove it with a plastic scraper, treat the joints with a biocide, then brush and rinse. Moss returns in recessed or eroded joints, so repair low grout afterward.
Why does white powder keep appearing on my patio grout?
That is efflorescence, salts carried to the surface by water moving through the slab and grout. Brush it off dry and improve drainage; washing it tends to bring more salts up.
Should I seal outdoor grout after cleaning?
Usually yes, for cement grout, once it has dried for 48-72 hours. Use a breathable penetrating sealer rather than a surface film.
Sources and standards
- TCNA Handbook for Ceramic, Glass, and Stone Tile Installation, exterior installation methods and maintenance guidance
- ANSI A118.6 and ANSI A118.7, cement grout specifications
- ISO 13007-3, grout classifications (CG2WA for improved, low-absorption, abrasion-resistant cementitious grout)
- Natural Stone Institute, care and cleaning guidance for exterior stone
- Pressure washer operator manuals (nozzle color codes and safe use) and biocide and grout manufacturer technical data sheets (TDS) and safety data sheets (SDS)
Grout Atlas guidance is checked against ANSI A108/A118 and ISO 13007 requirements, the TCNA Handbook and manufacturer technical data sheets. Product formulations change, so confirm details on the current data sheet for the product you buy. Spotted an error? Report a correction.