How to Clean Interlock Paving Stones Without Ruining Them

Interlock paving stones are one of the most popular hardscaping investments in Kitchener-Waterloo — and one of the most easily damaged by well-intentioned pressure washing. The problem isn't the stones themselves; it's the joint sand that holds them stable. Here's how to clean interlock the right way, and what you must do after cleaning to protect your investment.

The Danger of High Pressure on Interlock

Interlock paving stones are connected to each other and stabilized by sand packed tightly into the joints between them. This jointing sand is what prevents the stones from shifting, rocking, and allowing weeds to take root. It is also extremely vulnerable to erosion from high-pressure water.

When homeowners rent a consumer pressure washer and set it to maximum for "a deep clean," they often blast the jointing sand completely out of the joints in a single session. What they're left with is beautiful-looking clean stones sitting in loose gravel with wide-open joints — joints that will immediately fill with weeds, allow stone movement, and create trip hazards.

This isn't just a cosmetic problem. When interlock stones can move, they create uneven surfaces where water pools (causing freeze-thaw damage in Ontario winters) and edges that chip and break. The cost to re-sand and re-compact a large interlock patio or driveway can easily exceed the original installation cost in labour.

Many Kitchener-Waterloo homeowners have invested $10,000–$30,000 in interlock driveways and patios. Protecting that investment during cleaning requires knowing the correct approach.

Correct Pressure Settings for Interlock

The key is using the lowest pressure that still effectively cleans the stone surface. For interlock paving stones, this is 1,200–1,500 PSI maximum. Most electric pressure washers in the 1,500–1,800 PSI range, used with the correct tip and technique, can clean interlock without significant sand displacement.

The nozzle choice is equally critical:

• 40° white tip: The safest choice for most interlock cleaning. Wide fan, lower concentration of force. Start here.
• 25° green tip: Acceptable for stubborn stains on the stone faces only — not for general sweeping passes over joints.
• Never use 0°, 15°, or rotating turbo nozzles on interlock — these will hollow out the joints in seconds.

Distance from the surface matters enormously. Work at 12–18 inches from the stones — farther than you might for concrete. The goal is cleaning the stone face without directing significant force into the joints.

Pre-Treatment Before Pressure Washing

Like any paving surface, interlock benefits significantly from pre-treatment before the water hits it. Pre-treatment loosens organic growth and stains so they can be removed with lower pressure — protecting those critical joints.

For general organic growth (green algae, mildew):

1. Mix a diluted sodium hypochlorite solution (1 cup bleach per gallon of water) or use a commercial patio cleaner designed for interlock.
2. Apply to the pre-wetted surface with a garden pump sprayer.
3. Let dwell 10–15 minutes.
4. Pressure wash at low PSI to rinse away the loosened growth.

For oil and grease stains: Apply a degreaser specific to paving stones (avoid petroleum-based solvents that can damage polymeric sand binders). Let dwell 20 minutes, scrub with a stiff brush, then pressure wash. Oil stains are less common on interlock than concrete but do occur, particularly near vehicles.

For rust stains: Common on interlock near metal patio furniture. Apply an oxalic acid-based rust remover. Brush, dwell, rinse. Test in an inconspicuous area first — some coloured interlock can be sensitive to acid treatments.

For weed killer: Apply a contact weed killer to any weeds growing in the joints 48–72 hours before cleaning. This kills the roots; the cleaning process then removes the dead plant matter. Trying to blast live weeds out with pressure alone is less effective than killing them first.

Cleaning Technique for Interlock

With your surface pre-treated and your pressure washer set correctly, the actual cleaning technique is the final variable. Here's the approach that cleans effectively while protecting the joints:

Angle of spray: Direct the spray at a low angle — approximately 30–45 degrees from horizontal, not straight down. Water flowing at a low angle across the stone face cleans the surface and carries debris away without driving water directly into the joints.

Avoid aiming at joints directly. This is the critical rule. When you're sweeping the wand across stones, try to maintain a trajectory that's along the stone faces rather than across the joint lines. You'll unavoidably wash over some joints, but don't dwell or direct pressure into them.

Work in sections and direct dirty water away from already-cleaned sections. Interlock often covers large areas (driveways, patios, walkways), so work methodically across the surface.

Final rinse: After cleaning, rinse the entire area thoroughly with low pressure (garden hose is fine for the final rinse). This removes cleaning chemical residue and the last of the loosened debris.

"We see expensive interlock installations ruined every spring by homeowners who grabbed the wrong tip and let it rip at full pressure. A little knowledge about joint sand and pressure settings saves thousands of dollars."

— David, D&D Home Services Co-Founder

Polymeric Sand Replacement: The Critical Step Most People Skip

Even with correct pressure settings and technique, some jointing sand will inevitably be displaced during cleaning — particularly in areas where the joints were already partially depleted from natural erosion, snow removal, or previous cleaning. Replacing this sand is not optional; it's essential to the long-term stability of your interlock.

Regular jointing sand is fine silica sand with no binding agent. It stabilizes stones when compacted but erodes easily from rain, pressure washing, and freeze-thaw cycles. It provides minimal weed resistance. This is what was originally installed on older interlock systems.

Polymeric jointing sand (sometimes called "polymeric sand" or "poly sand") is a blended sand with polymer additives. When dampened and allowed to cure, the polymers bind the sand particles together into a semi-rigid filler that:

• Resists erosion from water, pressure washing, and rain far better than regular sand
• Inhibits weed germination by eliminating the loose substrate weeds need
• Deters ants and insects from nesting in the joints
• Maintains stone stability across Ontario's freeze-thaw cycles

If your interlock has regular sand and you're cleaning it, this is the perfect opportunity to upgrade to polymeric sand. The process:

1. Allow the interlock to dry completely after cleaning (24–48 hours).
2. Sweep polymeric sand across the surface, working it into the joints with a push broom.
3. Compact with a plate compactor (rent from a tool rental shop) to settle the sand into the joints. This step is critical — uncompacted polymeric sand will not bind properly.
4. Sweep off all excess sand from the stone faces.
5. Activate by misting lightly with water — just enough to dampen, not flood. Flooding washes the polymers out of the joints.
6. Allow to cure 24–48 hours before foot traffic. Avoid heavy vehicle traffic for 72 hours.

Sealing Interlock: Enhance Colour and Protect Investment

Sealing interlock paving stones after cleaning and re-sanding is optional but highly beneficial for Ontario driveways and patios. An interlock sealer provides:

• Enhanced colour: Sealers bring out the rich colour of the stones, making old interlock look significantly newer. Wet-look sealers are popular for a glossy finish; natural-look sealers enhance colour without the sheen.
• Stain resistance: Sealed surfaces repel oil and other stains that would otherwise penetrate the porous concrete or natural stone.
• Joint stabilization: Many sealers soak into the polymeric sand joints and help lock them in place, further stabilizing the stones.
• UV protection: Paving stones fade in Ontario's sunny summers. Sealers with UV inhibitors significantly slow this fading.

Sealer should be applied after polymeric sand has fully cured (wait at least 48 hours from activation). Apply with a pump sprayer or roller. Back-roll with a short-nap roller to ensure even coverage. Most sealers require two coats, with 30–60 minutes between applications. Allow to cure 24 hours before foot traffic and 48–72 hours before vehicle traffic.

Resealing frequency: every 3–5 years for most residential interlock. You'll know it's time when water no longer beads on the surface or when colours have noticeably faded.

Weed Prevention After Cleaning

Weeds in interlock joints are one of the most common complaints from Ontario homeowners. After cleaning and sanding, the most effective prevention strategies are:

• Polymeric sand (discussed above) is the most effective long-term deterrent — the hardened joints leave no soil for weed seeds to germinate.
• Sealing further helps by preventing the joint surface from accumulating the organic matter (dust, decomposed leaves) that weed seeds use as a growing medium.
• Pre-emergent herbicide: Applied in early spring before weed seeds germinate, products containing corn gluten meal or chemical pre-emergents can be highly effective when applied to clean interlock with intact joint sand.
• Boiling water: For spot-treating weeds in joints, boiling water is an effective, chemical-free option that doesn't disturb the sand.
• Annual spring cleaning: A light annual rinse (garden hose, not pressure washer) in spring removes the winter's accumulation of organic debris before it becomes a weed seed growing medium.

For professional interlock cleaning and re-sanding, our pressure washing team handles interlock surfaces throughout Kitchener, Waterloo, Cambridge, and Guelph. We use appropriate pressure settings and always include polymeric sand replacement in our interlock cleaning service.

D&D Home Services TeamExterior Cleaning Experts

D&D Home Services is Kitchener-Waterloo's trusted exterior cleaning company serving Kitchener, Waterloo, Cambridge, and Guelph.