Short answer
Hairline cracks and flaking on the top surface are surface damage, and patching plus sealing fixes them. A crack that runs all the way through means the slab has moved, and filler only hides it until it opens again. Lay a level across the crack before you price anything. If both sides sit flush and the concrete under the flaking is hard, you are patching. If one side sits lower, or the slab rocks underfoot, the ground beneath it has gone and that section gets lifted or replaced.
Tell surface flaking from a slab that has moved
- Sweep the slab and rinse it with a hose. Wet concrete shows the full length of every crack.
- Scrape a crack open with a screwdriver tip. Note the widest point in inches.
- Lay a level or a straight board across the crack. Run your fingers over it and feel for a lip.
- Tap the flaked area with a hammer. A sharp ring means solid concrete under it. A hollow thud means the top layer has let go.
- Press your weight on each side of the crack and watch the gap. Movement means the support under the slab is gone.
- Look at the exposed edge or face of the slab to see whether the crack carries through the full thickness.
- Watch where rain runs after a storm. Water that sits on the slab or drains toward the house keeps the damage coming back.
If the cracks are hairline, both sides sit flush, and the concrete rings solid, patch and seal it. If you feel a lip, the slab rocks, or the crack runs through an elevated slab, stop and get it looked at before anything is filled.
Repair or replace
| Situation | What to do |
|---|---|
| Hairline cracks and light flaking on the walking surface | Patch and seal. Fill the cracks, skim the flaked spots, seal the whole slab. |
| Flaking across most of the surface, concrete under it still hard | Patch, then put a bonded overlay over the whole slab. |
| Crack wider than 1/4 inch, both sides level | Repair. Use a flexible crack sealant that moves with the slab, not rigid filler. |
| Crack through the full thickness, both sides still level | Repair and watch it. Seal it, mark the width in pen, check again in 6 months. |
| One side of the crack sits lower, or the slab rocks | Replace or lift that section. The soil under it has washed out, and filler lasts one season. |
| Crack through an elevated porch slab, or a crack at a post or column | Hire for this. An elevated slab carries load, and a crack through it is a structural call. |
| Flaking deep enough to show rebar or mesh | Replace that section. Once water reaches the steel it rusts and lifts more concrete every year. |
| Crumbling along the outer edge where cars drive on | Rebuild the edge with formed concrete. Patch material alone breaks off again under wheel loads. |
What it costs
Runhome contractor estimates for patching hairline cracks and sealing a carport slab ran from $310 to $655 as of October 2026, including the trip and assessment.
| Option | Range |
|---|---|
| Patch hairline cracks and seal the slab | $310 to $655 |
Can I do it myself?
Yes, for hairline cracks and light flaking on a slab that sits on the ground. Crack filler runs $10 to $30 and sealer runs $25 to $75, and the work is mostly prep: clean the slab, open the cracks, blow out the dust, fill, then seal when the forecast gives you two dry days. Pressure washing the slab yourself and hiring a pro only for the filling and sealing cuts the labour line.
Hire for this if the slab is elevated, if a crack runs through a porch that holds up steps or posts, or if one side of a crack has dropped. Those are load and soil problems, and a sealed crack on a moving slab tells you nothing about what is happening underneath.
What a contractor will ask you
Have these ready and you skip the back-and-forth.
- A wide photo of the slab showing the damaged area and the concrete around it.
- The main problem in your words: cracks and flaking, a crumbling edge, water pooling, or staining.
- When you first noticed it getting bad: the last few weeks, within the year, or several years.
- The age of the slab, roughly. Under 5 years, 5 to 15 years, or over 15.
- Where the worst damage sits: the parking and walking surface, the outer vertical face, or where posts and steps meet the concrete.
- Whether water sits on the area after rain or just runs across it.
- Whether the cracks are hairline and scattered, or the surface is breaking up in chips and layers.
- The rough size of the slab, how high it sits off the ground, and whether anything is built on top of it.
Common questions
- Can you just patch concrete or does the whole slab have to come out?
Patching works when the slab is still supported and the cracks are narrow. Replacement is for slabs that have dropped, rock underfoot, or are cracked through in several directions. One cracked line in a slab that still sits flat is a patch job.
- Will the patch match the rest of the concrete?
No, a patch stays visible. Fresh patch material cures lighter than old concrete and the line shows for years. Sealing or coating the whole slab after patching is what evens out the colour.
- Is a crack in my concrete porch dangerous?
It depends on what the porch holds up. A crack in a slab poured on the ground is a maintenance job. A crack through a raised porch that carries steps, posts or a roof needs a contractor to look at the framing and footings before it gets filled.
- How long does a crack patch and sealer last?
Expect 3 to 5 years on a sealed slab that drains well. Slabs with water running across them, or with de-icing salt on them in winter, come back sooner. Resealing every few years is normal and is much cheaper than resurfacing.
- The flaking came back a year after I patched it.
The water source was never fixed. Flaking is the top layer letting go after it has been soaked and frozen, so patch material over a slab that stays wet fails the same way. Fix the drainage, the downspout or the grade, then repair the surface.