Laying pavers over an existing concrete slab is a good idea only when the slab is solid, still sheds water away from the house, and has room to rise without crowding a door threshold; a cracked, heaving or water-holding slab should come out so the pavers can sit on a proper compacted base.
An overlay is appealing: no demolition, no haul-off, a faster job. But the slab stops being a patio and becomes a foundation, and everything wrong with it becomes a problem for the pavers on top.
How to tell whether your slab can carry pavers
A slab can carry pavers when it is solid underfoot, has no sections that rock or have lifted, and shows only hairline cracks that have not widened over recent winters. Contractors who handle hardscaping projects regularly will often core or probe a slab before agreeing to build on it.
You can run most of this check yourself on a dry afternoon and again after a good rain:
- Walk every section. Step near cracks and joints. If a piece rocks, sounds hollow or tips, the ground beneath it has moved.
- Look for lifted or sunken edges. A ledge where two sections meet usually means frost heave or settlement, and it will show through the new surface.
- Read the crack history. Hairline cracks that have looked the same for years are normal. Cracks you can slip a coin into, or ones that have grown since last spring, point to active movement.
- Check the pitch. Lay a long level or pull a string line from the house outward. The surface should fall away from the foundation, not toward it.
- Watch the water. After rain, note where puddles sit and how long they stay. A birdbath on the slab will become a birdbath under the pavers.
- Inspect the edges. If soil has washed out from under the slab's perimeter, the edges can crack under the added load and foot traffic.
A slab that passes all six is a reasonable overlay candidate; one that fails the movement checks is a tear-out.
Why trapped water is the biggest risk in a freeze-thaw climate
Water that slips through paver joints onto a slab has nowhere to soak in, so it lingers in the bedding layer, and in a freeze-thaw climate that trapped water can freeze, lift units and flush bedding sand out of the joints.
On a normal paver patio, water that gets past the joints drains down through the base. Over concrete, it hits a hard floor and has to travel sideways to an edge; if the slab is flat, dished or pitched toward the house, it stays put. In a Chicago-area winter, it freezes and thaws repeatedly right under the pavers.
Manufacturer guidance treats this as the main thing to get right. Unilock's instructions for installing pavers on concrete say the slab must shed water away from structures, and where water collects, it recommends drilling small drain holes through the slab in strategic spots and using a thin bedding layer that drains quickly on top. A drainage supplier's guide, Preventing Problems with Pavers Over Concrete (updated April 2026), makes the same point from the other direction: standing water on the old slab persists after the overlay and may get worse, so plan for drainage before the pavers go down.
The height problem at doors, steps and garage slabs
An overlay raises the finished surface by the thickness of the pavers plus their bedding, which can crowd a door threshold, shorten a step or leave a ledge where the patio meets the lawn.
The drainage guide above puts the rise at an inch or more, and with standard-thickness pavers it is often more. Measure before you commit:
- Doors. Check the gap between the slab and the bottom of every door threshold that opens onto it. A raised surface can push snowmelt toward the door sill and may run into local building rules on the step down from an exterior door. Your village building department can tell you what applies.
- Steps. Raising the landing at the base of a stair makes that last riser shorter than the rest, which is a classic trip point.
- Garage and walk transitions. Where the patio meets a driveway or sidewalk, the new surface may sit proud of the old one.
- Lawn edges. A raised perimeter needs edging and a soil or mulch transition so it doesn't become a curb.
Unilock's guidance offers two outs: use thinner pavers designed for overlays, or remove the slab and rebuild with crushed stone below grade so the finished surface lands where it belongs.
Sand-set or mortar-set: choosing the bedding
Sand-set overlays drain and flex with seasonal movement, while mortar-set pavers bond rigidly to the slab and tend to crack along the same lines as the concrete underneath.
In a sand-set overlay, the pavers float on a thin layer of bedding sand or fine chip over the slab, and joints are filled with sand, often polymeric. If the slab shifts slightly over a winter, the units can move with it, and a problem area can be lifted and reset. Unilock's method uses this approach, with edging fastened mechanically to the slab wherever the field isn't bounded by walls or other hardscape, so the pavers can't creep outward.
Mortar-set work glues every unit to the slab. That produces a crisp, fixed surface, but it also makes the pavers part of the concrete. When the slab cracks, the mortar and often the pavers crack with it, and repairs mean chiseling out mortared units. In freeze-thaw country, a rigid bond over a slab that still moves is the riskier of the two choices.
When tearing the slab out is the better investment
Tearing the slab out is the better choice when the concrete has heaved, pitches toward the house or would push the new surface above a threshold, because an overlay inherits every flaw of the slab beneath it. Reading reviews of Mount Prospect hardscape contractors is one way to compare how local crews talk about overlays versus full rebuilds.
Removal adds demolition and haul-off. In exchange, the base depth, slope and finished height can all be set correctly, and water gets a path down into the ground instead of across a hidden floor. If you're weighing the options, it's worth reading through a full-depth paver patio compared with new poured concrete, since once the old slab is gone you are choosing a surface from scratch.
The short version: a sound, well-pitched slab with clearance to spare is a good overlay candidate. A slab with movement, standing water or no room to rise is a base that will keep billing you, and pulling it out first is usually the cheaper decision over the life of the patio.