Nobody notices an exterior slab going. It happens slowly, across a few summers, and then one day the patio looks like the back of the house instead of part of it.
Bare concrete outdoors is not a finished surface. It is an unfinished one that happens to be hard. Left alone through our weather it does three things, in roughly this order: it goes flat gray and blotchy, it starts holding onto whatever grows in the damp corners, and in the wet it gets slick enough that people stop walking across it in socks. All three are fixable, and all three come back if the fix was chosen for looks instead of for the conditions.
Sun takes the color out, rain takes the rest
A new slab has some tone to it. Cement, sand and the local aggregate give it a color, and for a year or two that is pleasant enough. Then sunlight works on the surface paste while rain washes the loosened material off, over and over, and what you are left with is the chalky gray underneath. The patio has not failed. It has just been sanded down a millimeter at a time by the weather, and the part that had any character in it is gone.
The same cycle opens the surface up. More open concrete drinks more water, and anything that lands on it goes in rather than sitting on top. Spilled drinks, grill grease, leaf tannin off whatever drops on it in the fall, rust off a chair leg. Those are not stains on the slab anymore. They are stains in it, which is why pressure washing gets a bare patio about halfway back and no further.
More open concrete drinks more water, and anything that lands on it goes in rather than sitting on top.
Shade is where the trouble actually lives
The part of a patio nobody sits on is usually the part that goes first. Under the eave on the north side, behind a shrub, the strip along the bottom step: anywhere that stays damp after a rain instead of drying out. Open concrete holds moisture like a sponge, and once it stays damp it grows a thin dark film of algae that is almost invisible until you see it against the dry part of the same slab.
That film is the reason an otherwise dry-looking patio can be lethal after a shower. It is slick, it is thin enough to survive a casual sweep, and because it lives in the pores of the concrete it comes straight back a few weeks after you blast it off. You cannot clean your way out of a porous surface in a humid climate. You can only stop it being porous.
So the chalky gray paste is what shows first and the damp shaded corner is what gets dangerous first, and those two things are usually on the same slab at the same time. Which is why we ask about the shade before we ask about the color.
Smooth and glossy is the wrong answer outdoors
This is where people get talked into something that looks excellent the week it goes down and is unpleasant to live with by the following spring. A high gloss, dead smooth coating belongs in a showroom. Put it on a patio and the first thing that happens is water sits on it in a sheet, and the second thing is somebody slides.
Grip on an exterior floor comes from profile built into the system, not from a bottle of additive stirred in at the end of the job. On a patio we build texture in deliberately, with flake or graded aggregate, and we will tell you how it is going to feel under a bare foot before anything goes down, because that is a thing people have opinions about once they are standing on it. A patio that is a little rougher than you expected is a nuisance. A patio that is slick in the rain is a hazard with furniture on it.
UV stability is the other half of the decision
The second question outdoors is what the sun does to the coating itself. Plenty of resins yellow, go milky or chalk off under direct sun, and the places it shows first are the places nothing is shading: the middle of the slab and the edge that catches afternoon light. Then you have a patio in two colors, and nothing short of recoating the whole thing evens it out.
So an exterior build gets specified for exposure, not borrowed from a garage. The topcoat has to be the one that holds its color in the sun, and the texture has to be the one that keeps its grip when the slab is wet. Those two decisions are most of what separates a coated patio around Ellisville that still looks right in a few years from one that needs doing again. The patio coatings page walks through the systems we use outside and why.
The topcoat has to be the one that holds its color in the sun, and the texture has to be the one that keeps its grip when the slab is wet.
What the work looks like on an exterior slab
The prep is the same principle as anything else we do and harder in practice, because an outdoor slab has more history on it. It gets ground open mechanically, cracks get cut out and filled, and anything that spalled or popped off at the edge gets rebuilt before a system goes over it. Control joints get respected rather than coated across and forgotten, because a joint that is still moving will move whether or not there is resin on top of it.
Then it is a weather job as much as a floor job. Humidity moves cure times here, so we plan the day around starting early rather than pushing into the hottest part of the afternoon, and rain in the forecast moves the date rather than the method. Most residential work is a single day of our time: walk on it at 12 hours, put the furniture back at 24. Scroll the finished floors if you want to see how the exterior texture reads next to an interior gloss.
Why waiting another summer costs you prep, not just looks
The thing about leaving a bare slab alone is that the damage is cumulative and it all lands in the preparation bill eventually. A patio that has gone a little chalky grinds back easily. The same patio after several more years of the surface paste washing off has started to expose aggregate, and the edges where water sheets off have usually begun to spall in small shells. Those get rebuilt before any coating goes on, because resin does not bridge a hole, it follows one.
So the slab is never in better shape than it is now. That is not a reason to rush a decision, but it is worth knowing when somebody tells you a patio is fine for another couple of years. What is actually happening in those two years is that more of the day on site moves from coating into repair.
Patio questions we get
Can you coat a patio that already has algae on it?
Yes, and that growth comes off during the grind rather than with a chemical wash. The point of coating it is that the slab stops being porous, so there is nowhere for the film to take hold again in the damp corners.
Will it be slippery when it rains?
Not if the texture is built in. We use flake or graded aggregate to set the profile on exterior work and tell you how it will feel barefoot before we start. A dead smooth gloss finish is the wrong specification for an outdoor slab.
Can I pressure wash it afterwards?
Yes, at sensible pressure. Holding a turbo nozzle in one spot is what causes problems, on coated concrete and bare concrete alike. Day to day a broom and a neutral cleaner handles an exterior floor.
Walk the slab with us
Tell Kevin which parts stay wet, which parts get sun all afternoon, and how you actually use the patio. We will look at it and spec the texture and the topcoat from that. If it has rained in the last day or two, leave the slab alone and let us come see it wet, because where the water goes is half the specification. Patios are everyday work for us in Ellisville, so a date is usually this week.