Concrete Overlay: What It Is, Types, and How to Apply It

A concrete overlay is one of the few home projects where the honest version of the sales pitch is still a good pitch. You take a slab that has gone grey, chalky and boring, and without breaking anything out you finish it so it reads as slate, tile, brick or veined stone. It costs a fraction of tearing the slab out. It goes down in days rather than weeks.

But there is a condition attached, and almost every overlay that disappoints somebody was a job where that condition got skipped. An overlay is a thin decorative layer bonded to the concrete you already own. It is a finish, not a repair. Whatever the slab underneath is doing, the overlay will eventually do too.

That single idea is what this guide is built around, because it is the difference between a patio you are still happy with in five years and a very expensive disappointment. Below is what an overlay actually is, the finishes it can produce, how to tell whether your slab is a candidate at all, and how the preparation and application really work — taken from McKinnon's own published installation instructions and technical data rather than from general internet advice.

What a concrete overlay actually is

An overlay is a cement-based material applied in a thin layer over existing concrete, then textured, patterned and coloured before it sets. Once it cures and gets sealed, it becomes the wearing surface. The old slab stays exactly where it is and does the structural work.

McKinnon's technical data describes its McKrete system as a cementitious acrylic compound that combines with water-based epoxies and polyacrylates to create floor systems for exterior and interior use. The related Deck Coat Overlay product is described as a polymer-modified, Portland-cement-based decorative coating that is mixed with water only and will accept most commercially available colour stains and sealers.

The word doing the work in both of those descriptions is polymer. Plain sand-and-cement mortar spread thinly over a cured slab does not reliably stay stuck; it has no real flexibility and very little bond strength. The acrylic or latex content is what lets these materials go down thin, grip a surface they were not poured with, and survive being walked and driven on.

So an overlay is not a thin pour of concrete. It is a coating that happens to be made of cement.

The question that decides everything: overlay or replace?

This is the part to settle before you price anything, and the answer is written plainly in McKinnon's own product literature. The Deck Coat Overlay data sheet states that structural cracks, and cracks caused by continuing movement of the substrate, may telegraph through the overlay.

Read that carefully, because it is the whole decision in one sentence. An overlay covers a crack today. It does not stop the crack. If the slab is still moving — settling, heaving, shifting on bad fill — that movement carries on underneath and the crack reappears in your new finish, usually following the same line.

The practical test is not how bad the cracking looks. It is whether the cracking is finished:

• Hairline and cosmetic, slab otherwise sound and stable — a good overlay candidate. This is exactly what the material is for.
• Surface worn, stained, discoloured, dated, or previously coated — a good candidate, once the old coating is properly off.
• Cracks with a height difference across them, or cracks that have visibly grown — that is movement. Deal with the cause first, or accept that it will come through.
• Slab crumbling, spalling deeply, or structurally unsound — not a candidate. The data sheet requires a structurally sound surface, and there is nothing sound for the overlay to bond to.

There is a second rule in the same document that gets broken constantly: do not fill existing expansion and control joints with overlay material. Those joints exist to give the slab somewhere to move. Bridging them with a rigid finish just tells the slab to crack somewhere you did not choose. Honour the joints, and carry them through the new surface.

The finishes an overlay can create

Most of the appeal is here. Because the material is applied wet and worked by hand, the same bag can produce very different surfaces depending on technique. McKinnon's page for the system describes designs that resemble marble, stone, granite, tile, Spanish tile and slate, and downloadable stencil patterns are offered for pattern work.

In practice the finishes divide up roughly like this:

• Sprayed and troweled texture. The material is sprayed onto the damp deck and then troweled down. This is the workhorse finish — it gives an even, slightly mottled surface with a bit of tooth underfoot.
• Pattern and stencil work. Grout tape or commercially available stencils are laid out before the coloured coat, then lifted afterward, leaving crisp lines. This is how a slab ends up reading as brick, tile, slate or cut stone. The two photographs in this article are both pattern work.
• Carved or cut patterns. Instead of stencilling, the pattern is cut into the surface, which suits large-format stone looks and one-off designs.
• Marbleized veining. An uncoloured base is sprayed, and while it is still wet a second, coloured mix is lightly sprayed over it and swirled in with the trowel so the colour bleeds into a vein. This is the most decorative option and the least forgiving, and it has its own rules, below.

Colour is added at the mixing stage as a liquid pigment or concentrate. McKinnon's instructions include one detail worth borrowing for any coloured job: add the same amount of colorant to every batch. Colour drift between buckets is the kind of mistake you only see once the whole deck is down and dry.

Preparation is most of the job

An overlay bonds mechanically to the surface it is applied to, which means the bond is only as good as that surface. McKinnon's installation instructions put preparation first and give it more space than the application itself. That ratio is not an accident.

The sequence in McKinnon's own instructions runs like this:

1. Get everything foreign off the deck — mastic, oil, grease and dirt. The Deck Coat data sheet extends that list: the surface must be structurally sound and free of grease, oil, dirt, mildew, dust, wax, curing compounds, paints, sealers and efflorescence, and free of any loose material. Cleaning can be mechanical — sanding, scraping or chipping.
2. Clean and acid etch with McKinnon's Etch & Degreaser. Two details here matter and are easy to get wrong: do not wet the deck before etching — pour directly onto dry concrete — and use a 1:1 or a 3:1 mix depending on the condition of the concrete.
3. Check the pH. The instructions call for concrete at no higher than 9 on the pH scale before you go further.
4. Pressure wash. The instructions are emphatic that it is very important to neutralise all of the acid. If you clean with muriatic acid instead, the Deck Coat sheet says to flush thoroughly and neutralise with a solution of baking soda and water.

Two more preparation points are worth pulling out because they decide bond quality:

A smooth slab has to be roughened. The data sheet is explicit that smooth steel-troweled concrete must be roughened to ensure a good bond. A slick, hard-troweled garage or lanai floor is the classic bond failure, precisely because it looks like the cleanest surface in the world.

An existing sealer is a bond breaker, not a head start. Sealers appear on the must-be-removed list for a reason: an overlay cannot grip through a film designed to repel water. If your slab has been sealed at some point — and many patios and driveways have — that film has to come off first. We covered how to do that, and how to prove it is actually gone, in our guide to removing concrete sealer. The water test at the end of that article is the cheapest quality check in this entire project.

Properly etched concrete has a distinct feel. McKinnon's older technical literature describes it as a sandpaper feel when rubbed, and says that if you do not have it, repeat the operation. That is a better test than looking at it.

Mixing, and the damp-deck rule

Mixing is measured and specific, and it is not the place to improvise. For the McKrete system the instructions call for 18 to 20 cups of additive to a 50 lb bag of dry mix, poured into a clean, dry bucket with the dry mix added slowly to the additive. With the standard additive you mix for about five minutes, let it stand for five minutes, then re-stir before use. If you are using the special additive to regrade or fill holes, stir it for roughly three minutes, let it stand five, and re-stir.

Deck Coat Overlay, which is mixed with water only, runs on the same principle: five to six quarts of water into a clean pail first, then the 50 lb bag of powder added slowly while stirring so it does not go lumpy. Mix until lump free, about three to four minutes, let it stand two to three minutes, then remix. A drill and paddle mixer is faster than a stick.

That stand-then-remix step appears in every one of these instructions and it is not padding. It lets the polymer wet out properly. Skipping it gives you a mix that looks right in the bucket and behaves badly on the deck.

Then there is the rule that surprises people most: always spray onto a damp deck. Keep the concrete damp throughout the installation — but damp, not wet, with no standing puddles. Dry concrete pulls water out of the mix before it can bond and cure properly. Puddles do the opposite and dilute it. Damp is the target, and on a hot day it takes active effort to hold.

Application: the sprayer-and-troweler rhythm

A sprayed overlay is a two-person job, and the two people have to stay in step. McKinnon's instructions describe it precisely:

• Secure the area first. Anything you do not want coated gets protected, because overspray drifts.
• A scratch coat, if you want one. A thin solution is poured onto the damp concrete and troweled out evenly, then left to dry thoroughly. It is also where you fill low spots and cracks before the finish goes on.
• Spray 18 to 24 inches above the deck at about 15 lbs of pressure. Spray no more than you can actually handle at once, judged against how fast it is drying.
• The troweler follows the sprayer, and starts before the material turns dull looking. That colour change is your clock. Once it dulls, your window for working the surface has closed.
• The troweler carries a clean wet sponge to keep the trowel clean. A dirty trowel drags and tears the surface you are trying to smooth.

For marbleized work there are extra rules, and one of them is shouted in the original document. The instructions say never to attempt a marbleized job without groutlines — tape off the pattern first. The colour pass is deliberately sparing: save about a quart of mix for every 100 square feet of deck, spray the uncoloured mix over the whole damp area, then while it is still wet switch hoppers and lightly spray the coloured mix at roughly that quart per 100 square feet. Trowel it with a light swirling motion to bleed the colour into a vein.

The failure mode is doing too much: over-troweling bleeds the colour straight back out and you lose the effect you just paid for. And because you are working on a freshly sprayed surface you cannot walk on normally, the instructions tell both the sprayer and the troweler to wear golf shoes.

Knowing when it is dry, and sealing

The dryness test is a thumbnail. Press the surface; if you cannot indent the material, it is dry. The important refinement is where you test: check the shaded areas, not the parts that have been in sun. Shade dries last, and the shaded corner is what decides whether the whole deck is ready.

Once it is dry, the surface gets cleaned up before sealing. Lightly scrape it, or use a fluted brick, to take off surface imperfections and burrs, then blow the loose material off. On stencilled work this is also when the grout tape comes up.

Sealing is two thin coats rather than one thick one, and each coat must be allowed to dry before the next goes on. Thin is the operative word throughout the sealer instructions — too heavy a coat, or sealing before the overlay itself is dry, is what makes a sealer blister and lift.

Then you wait. McKinnon's instructions allow 24 hours before foot traffic and two days before vehicle or heavy traffic, and note that this depends on temperature. On a cool or humid week, give it longer.

Which sealer depends on the finish and the traffic. McKinnon publishes a list of sealers for use with the system, and the clear ones are used where you want the colour of the overlay to show through, as with a brick or marble look — Crystal Coat is among the clear sealers listed for it. If you are weighing up sealer types more generally, our guide to choosing a concrete sealer works through it by surface, and there is a longer explanation of how penetrating sealers behave.

Temperature and weather limits

Overlays are cementitious, so temperature is a hard constraint rather than a preference. McKinnon's literature says never to apply any McKrete product when the temperature is below 50°F.

The Deck Coat Overlay sheet is stricter and more specific, and worth quoting as written: do not apply when the temperature, materials or substrates are below 55°F before and for 48 hours after installation. Note that it covers the 48 hours after as well — a warm afternoon followed by a cold night is still a failed install.

The same sheet rules out hot, windy or rainy conditions and recommends using sunscreens. Wind and direct sun both flash the surface dry before you can trowel it, which is the same problem as spraying onto a dry deck, arriving from a different direction.

That sheet also gives working figures at 72°F: a bucket life of about three hours, and a minimum dry time of around 12 hours at a quarter-inch thick. Coverage is listed at about 25 square feet per gallon-equivalent bag at a quarter-inch thick, with the note that coverage varies with how smooth the substrate is and how much texture you are building. Unopened material keeps about a year in a dry, sealed bag.

What an overlay will not do

Worth saying plainly, because these are the four things people are disappointed by:

• It will not stop a moving crack. Structural cracks and cracks from ongoing substrate movement may telegraph through. This is in the manufacturer's own limitations.
• It will not rescue an unsound slab. The requirement is a structurally sound surface. An overlay adds a finish, not structure.
• It is not a chemical-resistant floor. The Deck Coat sheet says specifically not to use it for installations requiring acid and industrial chemical resistance. For a workshop or garage floor facing solvents and hot tyres, a resin system such as a metallic epoxy floor is the better-matched product.
• It will not bridge an expansion joint. Do not fill them.

One safety note that belongs with any cement-based product. The Deck Coat Overlay sheet warns that the powder may cause eye, skin or lung injury, that it contains free silica, and that prolonged exposure to the dust may cause delayed lung disease. It calls for eliminating dust exposure and using a NIOSH-approved mask for silica dust. It also contains Portland cement, so flush your eyes immediately and repeatedly with water on any contact and see a doctor, and wash cement off skin promptly. Mixing is the dusty part of this job — wear the mask then, not just when you are spraying.

Where overlays fit

The system is used both outdoors and indoors, and on vertical as well as horizontal surfaces. McKinnon's literature lists pool decks, patios, driveways, sidewalks, basement floors, walkways, foyers, tile floors and walls, and its system page adds interior commercial settings such as hotel lobbies, shopping centres, museums and retail stores.

The pattern across those is worth noticing: the best overlay candidates are large areas of sound, visible, boring concrete. That is where the cost comparison against tearing out and re-pouring, or against laying pavers or natural stone, is most lopsided in the overlay's favour.

If your project is a driveway and drainage is part of the problem you are trying to solve, an overlay is the wrong tool — it is a sealed surface, not a permeable one. We compare the options that do drain in permeable driveway options. And if you are after the look of loose stone locked into place rather than a tile or slate pattern, liquid stone flooring is a different route to a decorative surface.

Getting McKrete for your project

McKrete is sold as a system rather than as a single item: dry mix in bags, four- and eight-bag kits and pallets, a standard or quick-set additive in sizes from a one-gallon can up to a 250-gallon tote, and liquid colour concentrate in quarts. Because it ships as freight, the kits and pallets are quoted rather than carted — the current line-up and pricing live on the McKrete Concrete Overlay System page, along with the gallery and the downloadable installation instructions and stencil patterns.

The full technical data sheets for the overlay products and their sealers are on the technical data sheets page. Read the sheet for the exact product you are buying before you start; the figures in this article are drawn from McKinnon's published documents, but the sheet that ships with your material is the one that governs your job.

For a quote, for help sizing a job, or to work out whether your slab is a candidate at all, call McKinnon Materials at 888-849-0588. Describing the cracking and the existing surface over the phone takes five minutes and can save you an entire failed installation.

Before you start

If you take four things from this guide, take these. Settle the overlay-or-replace question honestly, because a moving crack will find its way through. Spend most of your effort on preparation, and roughen anything smooth. Keep the deck damp and trowel before the material dulls. And respect the temperature window on both sides of the install, not just on the day.

Get those right and an overlay is one of the highest-return projects available on an existing slab. Get the first one wrong and none of the rest of it matters.

Frequently asked questions

How thick is a concrete overlay?

Thin — these are coatings rather than pours. McKinnon's Deck Coat Overlay sheet quotes its coverage and dry time at a quarter-inch thickness, and allows re-levelling or patching small areas with the same material mixed to a heavy paste. An overlay is measured in fractions of an inch, which is exactly why the slab underneath still does all the structural work.

Can you put a concrete overlay over cracked concrete?

Over cracks that have stopped moving, yes — that is normal work, and low spots and cracks are filled as part of the scratch coat. Over cracks caused by continuing movement of the substrate, no: McKinnon's own limitations state that structural cracks and cracks due to ongoing movement may telegraph through the overlay. Check whether there is a height difference across the crack, or whether it has grown, before you commit.

How long before you can walk on a concrete overlay?

After sealing, McKinnon's instructions allow 24 hours before foot traffic and two days before vehicle or heavy traffic, and note that it depends on temperature. Cooler or more humid conditions need longer.

What temperature do you need to apply a concrete overlay?

McKinnon's literature says never to apply a McKrete product below 50°F. The Deck Coat Overlay sheet is stricter: not below 55°F for the temperature, materials or substrates, both before and for 48 hours after installation. It also rules out hot, windy or rainy conditions.

Do you have to remove an old sealer before an overlay?

Yes. Sealers are on the list of things the surface must be free of, along with paint, wax, curing compounds and efflorescence. A sealer is designed to repel water, and an overlay cannot bond through it. Strip it first and confirm it is gone before you etch.

Does a concrete overlay need to be sealed?

Yes — sealing is part of the installation, not an optional extra, and it is applied as two thin coats with drying time between them. McKinnon publishes a list of sealers for use with the system, including clear sealers used where you want the overlay's colour and pattern to show through.

Is a concrete overlay cheaper than replacing concrete?

That is the usual reason people choose one, and McKinnon positions the system as cost-effective compared with pavers, slate, granite and natural stone. Installed pricing swings too much by region and by job for us to quote a figure responsibly here, but the structural comparison is the honest one: an overlay reuses a slab you have already paid for, while replacement means demolition, disposal and a new pour. That saving only holds if the existing slab is genuinely sound.