Overview

Plain rock salt is the best salt for interlock when used sparingly, according to industry guidance for concrete pavers. These are the key points covered below.

• Sodium chloride (rock salt) is the least damaging de-icer for interlock when used sparingly.
• Magnesium chloride and calcium magnesium acetate should never go on interlock, even when the bag says “pet-safe”.
• Skip chemical de-icers completely during the first winter after installation.
• A 12-ounce mug of salt covers about 500 square feet, which is far less than most people spread.
• Use plastic shovels, and keep plow or snow blower blades about 6 mm (1/4 inch) above the pavers.
• Fall joint sand checks and a spring rinse protect interlock more than any product does.

Introduction

Yes, you can put salt on interlock, as long as it is the right kind, used lightly, and not during the first winter after installation.

If you have a driveway, walkway or patio built with interlocking in Ottawa, you have probably wondered about this on the first icy morning. The pavers look tough, and they are. Factory-made concrete pavers are much denser than poured concrete, and they are tested against freeze-thaw cycles in salt water before they are sold. What usually does the damage is the wrong product, too much of it, or joints and a base that were already worn going into winter.

This guide covers which de-icers are safe, how much to use, what to spread instead of salt, how to clear snow without scarring the surface, and what to do in fall and spring so you are not calling around for interlock repair in Ottawa once the snow is gone.

Why Ottawa winters are hard on interlock

The real enemy is the freeze-thaw cycle, not the cold itself. Water that soaks into joints or the soil below expands by about 9 percent when it freezes, and in Ottawa that happens over and over from late fall to early spring. Frost can reach roughly 1.8 metres below the surface in open ground.

Interlock handles this better than a solid slab because it is a flexible mosaic. The stones can shift a fraction of a millimetre as the ground heaves and settle back as it thaws, provided the base underneath was built properly. For a driveway, that means about 200 to 300 mm (8 to 12 inches) of compacted granular base that drains well.

The pavers themselves are made to take it. They are pressed at high pressure with very little water, which gives them a compressive strength above 50 MPa and water absorption of no more than 5 percent. Under the Canadian standard (CSA A231.2), pavers are frozen and thawed while submerged in a 3 percent salt solution, and they must not lose more than 225 grams of surface per square metre over 28 cycles.

Which de-icers are safe for interlock

Sodium chloride, plain rock salt, is the safest choice. The industry guidance from the Concrete Masonry & Hardscapes Association (CMHA) and the Interlocking Concrete Pavement Institute (ICPI) is that it does the least harm to concrete when used sparingly. Other products range from acceptable to destructive.

Magnesium chloride deserves a warning because it is often sold as the “pet-safe” or “eco-friendly” option. It reacts with the cement that binds the paver, turning it into a material with no strength, and it can fracture the stone from the inside. It can also void a paver warranty. CMA does similar damage, and the more magnesium in the mix, the worse it gets.

Rock salt stops working well below about -9°C. On the coldest Ottawa mornings, calcium chloride will still melt ice, but keep it for those days only. Heavy, repeated use can scale the surface over time.

Can you use salt in the first winter?

It is best to skip it. The pavers arrive fully cured from the factory, so the stones themselves can handle salt on day one. The caution comes from everything around them. Hardscape professionals advise holding off on chemical de-icers for the first winter because the system is still settling in.

• Joint sand is still consolidating. Rain and moisture need time to lock the joint sand down to the bedding layer. Strong brine too early can disrupt it and lead to washed-out joints.
• Efflorescence is still working its way out. The white haze that shows on new pavers normally washes away within the first year. Heavy chloride use can interfere with that and leave staining.

This first-winter rule is practical advice from installers rather than a strength limit from the manufacturers, but it costs you nothing to follow. Clear snow promptly with a shovel and use clean coarse sand for grip on ice.

How much salt is enough?

Far less than most people think. More salt does not melt ice faster. It just makes a stronger brine that wears the surface and runs off into local waterways. The Smart About Salt Council and the City of Ottawa point to about 53 to 58 grams per square metre, which is roughly 11 pounds per 1,000 square feet.

A simple rule of thumb is one 12-ounce coffee mug of rock salt for about 500 square feet of driveway. Spread it evenly instead of dumping it in piles.

Salt is there to break the bond between ice and the surface, not to replace shovelling. Once it has loosened the ice, clear away the slush and leftover salt. Do not leave salted snow piled on the pavers all winter, because that keeps the brine in contact with the stone and causes repeated small freeze-thaw cycles on the surface.

What to spread instead of salt

For standard interlock with narrow, sand-filled joints, clean sand is a good option. It gives instant grip without changing the freezing point or touching the concrete. In spring, sweep what is left across the surface and it helps top up any joint sand that washed out.

Permeable interlock is the exception. These systems have wider joints filled with small crushed stone so water can drain straight through. Sand will clog those gaps and stop them draining, so use the same clean, washed stone (ASTM No. 8) that was used to fill the joints. When the ice melts, the stone drops back into the joints.

If you want to avoid both salt and shovelling, electric and heated-glycol snow melting systems can be installed in the bedding sand under the pavers. They go in at the time of construction and heat the surface just above freezing when snow is detected.

Clearing snow without damaging the pavers

Snow removal does more harm than salt if it is done carelessly.

• Use a plastic or polyurethane shovel. A metal edge scratches the surface and leaves tiny steel fragments behind that rust into orange stains by spring.
• Fit plastic, polyurethane or rubber edges on snow blowers and plow blades. Adjustable skid shoes on a snow blower keep the blade just off the surface.
• Keep the blade about 6 mm (1/4 inch) above the pavers. During a deep freeze, moisture in the ground can push some stones up unevenly. A steel blade dragged flush will catch those raised edges, which can chip a paver or pull it out of the bedding sand. A thin layer of snow left behind is harmless and will melt.

The installation standard allows no more than 3 mm of height difference between neighbouring pavers, so any lifted edges you see in winter are worth noting for spring. If you hire a snow service or one of the Ottawa landscapers who plow in winter, tell them the surface is interlock and ask for a rubber-edged blade set above the stones.

Getting your interlock ready in fall

A surface that goes into winter in good shape needs far less help. Walk it in late summer or early fall and check for these.

• Joint sand. Sweep off leaves and debris, then look for joints that have washed out, especially on slopes. Missing sand lets water slip into the base, where it freezes and heaves the surface unevenly. Refill with fresh polymeric sand, compact it into the joints and activate it with water well before the first freeze. Our guide to joint sand problems in Ottawa explains what to look for.
• Low or sunken spots. Anything more than about 6 mm (1/4 inch) lower than its neighbours will hold water and turn into an ice patch that tempts you to over-salt. Minor settling is a straightforward fix: lift the stones, re-level and compact the bedding sand, relay them and refill the joints.
• Drainage. Water should run off the surface and away from the house. A 2 percent minimum slope is the standard design target.

If you are not sure whether a section needs work, Ottawa interlock contractors can assess it before the ground freezes. Anything beyond a few loose stones usually falls under interlock repair, and it is much easier to do in October than in January.

Does sealing help?

It can. Penetrating sealers (silanes and siloxanes) reduce how much water and salt brine soaks into the pavers and joints, which limits freeze-thaw damage and surface scaling. Timing matters, though.

• Wait 60 to 90 days after installation. A sealer applied too early traps the white efflorescence under the film and leaves a permanent haze.
• In Ontario, the best window is late summer to mid-September. The pavers are then fully cured and the sealer has time to set before freezing weather. Sealing late in the fall risks poor curing in cold temperatures.

If the window has passed this year, it is better to wait until next late summer than to seal in the cold.

Spring cleanup

When the thaw arrives, reset the surface. Sweep up leftover sand, salt crystals and winter debris so they do not grind into the pavers underfoot and under tires.

Then rinse the surface to wash out embedded salt. If you use a pressure washer, keep it between 1,400 and 1,800 psi with a fan nozzle held at about a 30-degree angle and 30 to 60 cm (12 to 24 inches) from the surface. A harsher setting will blast the joint sand out. Once it dries, top up any joints that have lost sand and note any stones that heaved over winter.

For permeable interlock, vacuum the joints to remove sediment and winter stone, then sweep in fresh clean stone up to the lip of the paver so water drains freely through spring storms.

Summary

Salt will not ruin a good interlock, but the wrong de-icer or too much of the right one will wear it down. Stick to plain rock salt at about a mug per 500 square feet, keep calcium chloride for the deepest cold, and never use magnesium chloride or CMA. Skip chemical de-icers the first winter, use sand for grip, and clear snow with plastic edges and the blade held slightly above the stones.

Most winter damage starts with something that was already wrong in the fall: thin joint sand, a low spot, or poor drainage. Fixing those before freeze-up, sealing at the right time, and rinsing the surface in spring will do more for your driveway or patio than any product.

If your pavers have sunk, heaved or lost their joint sand, KJ Landscape & Design can take a look at the surface and the base underneath. Learn more about our hardscaping work in Ottawa or call (613) 229-0213.

External resources

• The Effects of Deicing Chemicals on Interlocking Concrete Pavers, ICPI and CMHA technical bulletin 
• Top 3 Tips to Prepare Your Interlocking Concrete Paver Patio for Winter, CMHA