How to Caulk Cracks in Concrete and Blacktop

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How to fill cracks in concrete sidewalks and blacktop driveways to extend the life of these surfaces and prevent further damage from occurring.

You can repair small cracks in your concrete walls and sidewalks or blacktop driveway quickly and easily to prevent water and freezing damage that would lead to more costly repairs in the future. Basement walls normally develop cracks as the house settles and as long as they are not displaced you can seal cracks in concrete with durable urethane caulk. This type of repair can be completed in a half hour or less.

Tools and Materials

Caulk gun

Putty knife

Utility knife

Caulking finishing tool or Metal Spoon

Urethane caulk – use caulk that is suitable for concrete and can withstand freezing temperatures if necessary.

Wet-Dry Vacuum

Foam backer rod – open cell is preferred

Mineral spirits

Disposable Latex gloves



As concrete cracks water can get into the space, freeze and expand to make the cracks larger each freeze cycle. Gaps in sidewalks adjacent to your house can direct water against the foundation, leading to basement moisture problems. You should inspect your home once a year and fill the gaps and cracks with urethane caulk to prevent problems. This type of caulk is available at most home centers, hardware stores, and lumber yards. This caulk is very sticky so wear disposable gloves and use a tool to smooth the caulk or use a metal spoon.

For gaps and joints more than 1/4 inch wide you will need to install an open cell foam backer rod to support the caulk. You want the rod to fit tight in the joint, so buy it one size larger than the gap. Open cell foam backer rod is preferred over closed cell since the material does not provide any off-gassing if damaged which can put pressure on the caulk and produce voids in the finished product.

The ratio of the wide to the depth should be about 2 to 1. For instance; a ¼ inch wide gap should have the backer rod installed about 1/8 inch from the surface.


The Ratio of A:B should be about 2:1

Caulking Wide Gaps

1. Clean the crack with an old brush and vacuum any debris with a wet-dry vacuum.

2. Use foam backer rod into the gap with your fingers. Set the rod 1/4 in. lower than the surface of the concrete. If necessary, use a putty knife to push the rod in to the proper depth.

Push the backer rod into the crack or gap between concrete slabs

Use a putty knife to set the backer rod to the proper depth

3. To fill the gap, cut the tip of the tube at a 30-degree angle, making the opening the same size as your gap. Use a smooth, even motion, filling the crack flush with the surface, beveling it if it's against a vertical surface, such as the foundation of your home. Smooth the caulk in wide joints with a caulk finishing tool or the back of an old spoon. Wipe the tool clean with a rag dampened with mineral spirits.

Caulking Narrow Gaps

If you have several sizes of cracks, start with the smaller ones first and cut the tip of the caulk tube to match the size of the crack. Keep cutting the tip as you work your way up through larger cracks.

1. You can caulk cracks 1/4 in. wide or less without using backer rod. Draw the gun down the crack, smoothing the caulk with the tip as you go.


Blacktop Cracks

As with concrete, cracking in blacktop can lead to more serious damage if water enters the cracks and the temperature drops below 32 F, the water can freeze. When water freezes it expands nine percent in volume. This expansion can cause the crack to get bigger or cause more severe damage to the blacktop. The best way to extend the life of blacktop is to keep cracks filled.

There are several products that you can use to repair cracks in your blacktop driveway. There is blacktop crack filler which can be poured into the crack but needs to be used in warmer temperatures. You can also use a blacktop crack filler that is applied with a standard caulk gun. Many of the blacktop repair products shrink in volume as they dry and cure. When the product dries, there is a noticeable depression in the crack. The material may be in a bottle that can be poured into the crack or in a tube that fits a standard caulk gun.

1. Clean all dirt and debris from the cracks using a wet-dry vacuum It's important for you to make sure the crack is cleaned of all loose debris and that the blacktop is dry.

2. For deeper cracks you may need to add material to support the filler. Most people recommend using small stone, but it would be better to use mortar mix and fill the bottom of the crack and let it dry for a day. Then apply the blacktop filler over the mortar base.

3. Caulk narrow cracks by applying an even layer of caulk over the crack using the tip of the tube to push the material into the crack slightly. Go over the crack again to create a slight mound over the crack.

4. Allow the crack filler to drive at least 24 hours before using your driveway.

Tip: Follow the instructions on the blacktop crack filler, most importantly the temperature constraints. Most blacktop crack sealants need to cure in temperatures above 40 F.


Roberta Baxter
Posted on Oct 19, 2011
Francina Marie Parks
Posted on Oct 19, 2011