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Polymeric sand and sealing interlock in Cambridge: what each does, when to redo it and how it goes wrong

Polymeric sand is joint sand with a binder that sets when it is wetted, so it stays in the joints, resists weeds and ants and keeps water out of the base. Sealing is a separate, optional step that protects the surface and the joints. Both are maintenance rather than repair, both depend on a dry day, and neither fixes a paver that has sunk.

Last updated October 24, 2026 · Reviewed by the Cambridge Interlocking team

What does polymeric sand actually do?

The joints are what make interlock interlock. The CMHA construction standard for interlocking concrete pavements calls for joints kept consistently between 2 and 5 mm and filled with sand compacted into them, and it is that sand, locked into the joints, that lets each paver share load with its neighbours. When the joint sand washes out, which it does on any Cambridge driveway with a slope and a few seasons of rain, the pavers start to rock individually, edges chip, weeds take root in the open joints, ants excavate the bedding sand, and surface water goes straight down into the base, where it does the damage described on the repair and re-levelling page.

Polymeric sand is ordinary fine joint sand blended with a polymer binder. It is swept in dry, compacted, and then misted with water, which activates the binder so the sand sets into a firm but slightly flexible joint. The standard refers to this class of product as joint sand stabilizers and notes they may help in areas prone to joint sand loss. The result is a joint that stays put, sheds most water across the surface rather than through it, and gives weeds and ants very little to work with.

How often should polymeric sand be redone in Cambridge?

A well-installed joint on a residential driveway lasts several years before the top wears enough to need topping up. Shaded, damp patios in the older parts of Galt tend to go longer because there is less traffic and less washing; steep driveways on Hespeler's hillside streets that shed water fast go shorter. If polymeric sand failed within the first year, the installation was the problem, not the product: it was put down on damp sand, not compacted, or rained on before it cured.

Signs it is time:

• Joints that have sunk below the chamfer of the pavers, or that are open and gritty rather than firm.
• Weeds or moss established in the joints, or ant hills of fine sand appearing on the surface.
• Pavers that rock or click underfoot, which means the joint is no longer carrying load.
• Water sheeting into the joints and taking a long time to drain after rain.

Re-sanding is also the moment to deal with any small area that has started to settle. Re-sanding a dip without fixing the base underneath only hides it until the next rain.

How the job is done right, and why the weather decides

1. Clean. The surface is pressure-washed and the old joint material is removed to depth. Polymeric sand sitting on top of old, dirty sand does not bond and fails in the first season.
2. Dry. The pavers and the joints must be dry. The standard notes that wet joint sand should be dried before use, and for polymeric products a damp joint means the binder sets before the sand has settled. This is the step that gets skipped when a crew is in a hurry, and it is the reason for most early failures.
3. Sweep and compact. Sand is swept into the joints, the surface is run with a plate compactor so the sand consolidates, and more is swept in until the joints are full to just below the chamfer. The standard says filling joints properly may take two or three passes.
4. Blow off. Every grain is blown off the paver faces before water touches them, because whatever is left on the face sets there.
5. Activate. A controlled misting wets the sand through without flushing it out of the joints. Then it is left alone to cure, with no rain in the forecast for the period the product requires.

Wash water from the cleaning step carries old sand, dirt and sometimes detergent, and it should not be hosed into the street. The Region of Waterloo's Sewer Use By-law 21-036 governs what may be discharged to the storm and sanitary sewers, and the City of Cambridge asks residents to keep nearby catch basins clear of trash and leaves. Wash water goes onto the lawn and debris gets swept up, not down the drain.

Should you seal interlock in Cambridge?

Sometimes. A good sealer darkens the colour slightly or a lot depending on the product, makes oil drips and road salt easier to wash off, and slows the surface wear of the polymeric joints. It is worth doing on a driveway that gets salted and parked on, and on a patio where the owner wants the colour held. It is not worth doing on pavers that have not been properly cleaned, on a surface still pushing efflorescence (the white mineral bloom common on newer concrete pavers), or on a day with humidity or rain in the forecast. Sealer is also a commitment: once a surface is sealed it is re-sealed every few years, and switching products later can be a problem.

Sealing comes after the joints have cured, usually on a second visit. Fall is workable if there is a dry, mild window and no frost overnight; late fall and early spring are often too damp. The contractor we book for Cambridge will not seal on a day the forecast argues with. The polymeric sand and sealing page sets out the full sequence.

Haze and the other ways it goes wrong

Polymeric haze is a film of binder set onto the paver faces. It happens when the sand was not fully blown off, when the surface was damp, or when it rained before the joints cured, and it is difficult and expensive to remove. Soft joints that never set were installed over damp sand or were not compacted. White blotches under sealer are moisture trapped under the coating. All of these are avoidable by doing the steps in order on the right day. The City's Property Standards By-law also requires every parking area and walkway to be kept in good repair, free of loose or damaged materials, and open, failing joints are the first step toward loose pavers.

What this means for your quote

Ask what product will be used, whether the old joint material will be removed to depth or just topped up, how the surface will be dried before sanding, and what the plan is if the weather turns. If sealing is proposed, ask which sealer, what it does to the colour, and when it will be re-done. A two-car driveway or a mid-size patio is typically a one-day job for cleaning and re-sanding. Planning ranges are on the Cambridge interlocking cost guide, and you can describe the state of your joints here to arrange a look.

Sources

• PAV-TEC-002-22, Construction of Interlocking Concrete Pavements - CMHA (formerly ICPI)
• Stormwater - City of Cambridge
• Sewer Use By-law 21-036 - Region of Waterloo
• Property Standards By-law 26-021 - City of Cambridge