Concrete Crack and Joint Sealing Guide for Texas Homeowners

Almost every driveway, patio and sidewalk in Central Texas has cracks. Some are harmless. Others are a path for water to reach the soil under your slab, and that soil is the reason slabs sink. This concrete crack sealing guide will help you sort one from the other, choose the right material, and decide whether to do the job yourself.

Short version: sealing is cheap protection when the slab is stable. It is the wrong tool when the slab is moving.

Why concrete cracks in the first place

Concrete shrinks as it cures. It also expands and contracts with heat every day. Builders cut or form joints so the slab cracks in straight, planned lines instead of random ones. Even so, cracks show up.

In the clay-soil suburbs around Pflugerville, Hutto and Manor, there is another cause. The ground under the slab swells when wet and shrinks when dry. Houston Black clay, a soil common in the Blackland Prairie belt, is a textbook example of this. When the ground moves, the slab can crack or shift.

Here are the usual causes, from least to most worrying.

Shrinkage cracks. Thin, short, often wavy lines that appear in the first year or two. These are normal.
Control joint cracks. Cracks that follow the sawed or tooled lines. These are the design working as planned.
Thermal movement. Joints open and close as the slab heats and cools. A summer afternoon slab in Pflugerville can be far hotter than the air.
Settlement cracks. Cracks where one side has dropped or tilted. These point to a soil problem.
Overload cracks. Heavy trucks on a thin driveway, or a slab poured too thin.
Root pressure. Roots lift a section and crack it.

Which cracks to seal and which to watch

Not every crack needs a sealant. Use this simple sort.

Hairline cracks

Cracks narrower than about 1/8 inch, with both sides at the same height, are usually cosmetic. Water can still get in, so many owners seal them. A thin liquid crack filler works, or you can leave them alone and re-check each season.

Moderate cracks

Cracks from about 1/8 inch to 1/2 inch that stay flat are good candidates for sealing. They let in a lot of water and debris. A flexible sealant keeps both out.

Joints

Control joints and the joint between a slab and your house or garage are the most important places to seal. They are straight, easy to reach, and the first place water enters. Open joints also let weeds and ants in.

Cracks you should not just seal

Stop and look closer if you see any of these:

One side higher than the other. This is called an offset. Sealing it will not make the slab flat or stop it from moving.
A crack that keeps growing. Mark the end with a pencil and note the date. If it moves, the slab is moving.
Cracks wider than about 1/2 inch.
Cracks that follow a line toward the house wall.
Stair-step cracks in brick or cracks over doors and windows.

Offset cracks often mean the soil below has settled or washed out. In that case, lifting the slab with concrete slab leveling comes first, and then sealing. If the house itself shows movement, a structural engineer or a foundation repair company should evaluate it. We do not do structural foundation repair, but we can fix the drainage that often feeds the problem.

Choosing a sealant

There are four common types. The right one depends on width, exposure and whether the joint moves.

Liquid crack filler

A thin, pourable product for hairline cracks. It soaks in and hardens. It does not stretch much, so it is best for cracks that are not moving.

Self-leveling polyurethane

This is the usual choice for horizontal joints and cracks in driveways and patios. It flows flat, bonds well to concrete and stays flexible. Look for a product labeled to ASTM C920, the standard for elastomeric joint sealants. The label will show a class number that tells you how much joint movement the sealant can handle.

Non-sag polyurethane

A thicker “gun grade” version for vertical surfaces or sloped joints, such as along a curb or step. It stays where you put it.

Silicone

Silicone handles sunlight well and stays flexible for a long time. It sticks less well to some concrete unless the right primer is used, and it is hard to paint or patch later. Some manufacturers also make polyether and hybrid products. Sources disagree about how well polyurethane holds up in direct sun, so read the data sheet for the exact product you plan to buy.

For Central Texas, where slabs bake in July and August, UV resistance and flexibility both matter. If you are unsure, ask the product maker which type they recommend for full sun on a driveway.

Backer rod

For joints deeper than about half an inch, you push a foam rope called a backer rod into the gap first. It stops the sealant from sinking too deep and gives it the right shape. Sealant works best when it bonds to the two sides of the joint and not to the bottom, so it can stretch.

Step-by-step: how to seal a crack

This is the process we recommend for a DIY job on a stable slab.

Check the weather. Pick a dry day with temperatures in the range printed on the product label. Do not seal in the heat of the afternoon, when the joint is at its narrowest, or the sealant will stretch hard when the slab cools.
Clean the crack. Remove all dirt, weeds and loose concrete. A wire brush, a screwdriver and a shop vacuum work. A leaf blower finishes the job.
Widen very narrow cracks if needed. Some products need a minimum width. A crack chaser or an angle grinder with a diamond blade can open a hairline crack to a clean 1/4 inch. Wear eye protection and a dust mask. Concrete dust is harmful to breathe.
Let it dry. Some sealants tolerate damp concrete. Many do not. Check the label.
Install backer rod in deep joints.
Apply the sealant. Fill from the bottom up and avoid air bubbles. Self-leveling products can be poured from the cartridge. Fill to about 1/4 inch below the surface in joints, so the sealant does not get pulled by tires.
Tool the surface. Smooth with a plastic spoon or putty knife if the product calls for it.
Keep traffic off. Follow the cure time on the label. Even a small amount of rain on fresh sealant can hurt the bond, so watch the forecast.

How long sealant lasts

There is no universal answer. Quality sealants can last several years, but sun, traffic, joint width and prep work all change the result. Plan to look at your joints once a year, ideally in spring before the heavy rain months, and touch up anything that has pulled loose.

The National Weather Service climate summary for Austin lists May, October and June as the wettest months. Sealing in early spring or in the dry late summer gives the product time to cure before the wet season arrives.

Sealing is only half the job

A sealed crack stops surface water. It does not fix water that is soaking the soil from the side, or roof water that dumps near the slab.

If your downspouts end on the lawn next to the driveway, water will keep finding its way under the concrete. Underground downspout drains solve this by carrying the water away from the slab. If the yard slopes toward the garage, regrading or a surface drain may be needed.

Cracks and sinking often show up in the same places. When you see both, treat the cause and the symptom together. Our post on why concrete sinks in Texas clay explains how water, clay and slabs work together.

What about the joint at the house?

The joint where a patio or driveway meets the house deserves special attention. If water runs along that gap, it can soak the soil next to your foundation. Clay near your house shrinks and swells, and an open joint speeds up that cycle.

Seal that joint with a flexible, paintable or color-matched sealant. Make sure the slab slopes away from the wall. If it slopes toward the house, sealing will not be enough. The standard residential building code language calls for the ground to fall at least 6 inches in the first 10 feet away from the foundation, and many slabs next to a house are meant to carry a slight slope away as well. Check how your city applies the code.

DIY or hire a pro?

You can do the following yourself:

Sealing hairline and moderate cracks on a stable slab
Re-sealing control joints on a small driveway or walk
Cleaning and prepping joints before a pro visit

Call a professional when:

The slab has dropped, tilted or stepped.
You are sealing a large area, like a long driveway.
Cracks are coming back in the same places after sealing.
You see water coming up through a crack after rain.
Cracks run from the slab into the house wall.

Our concrete crack and joint sealing service covers driveways, walks, patios and garage floors. In a typical visit, we also look at whether the slab is still level and where the water is going, since sealing a moving slab wastes money.

Before you dig or cut

If a repair involves cutting deep into the ground, call 811. Texas law requires you or your contractor to contact Texas811 at least two business days before digging. Surface sealing does not require a dig, but a drain installation next to the slab does.

Common mistakes

Sealing over dirt. Sealant sticks to clean concrete, not to dust.
Filling a joint to the top. The sealant should be slightly recessed.
Using caulk made for a bathtub. Indoor caulk fails outdoors.
Sealing in the heat of the day. Joints are at their narrowest on a hot afternoon. When the slab cools and the joint opens, the sealant is stretched more than it was built for.
Ignoring the cause. If a tree root or a washout is pushing the slab, sealing hides the sign without addressing it.

A simple yearly checklist

Walk your slabs every spring and fall with a notebook.

Note any new cracks and measure their width.
Check whether any section sits higher or lower than its neighbor.
Look for stained or damp spots on the surface.
Test joints by pressing a screwdriver into the sealant. It should be soft and rubbery, not hard or cracked.
Trace each downspout to its end and make sure the water leaves the area.

If you want a pro to check your slabs, a quick visit can tell you whether you need sealing, leveling or just a downspout fix. Homeowners in Pflugerville and Round Rock can request a free estimate and we will walk the property with you.