Filling a Gap Between a Concrete Slab and Wall [7 Steps]

The best way to fill a gap between a concrete slab and a wall is by first measuring the size of the gap. If it is ¼-inch (6 mm) or smaller, it can be filled with urethane caulk. If the gap is larger than 0.25 inches, press a foam backer rod into the gap, then fill the gap with urethane caulk. After caulking, smooth the urethane with a spoon or caulking tool. Finally, use mineral spirits to clean up any excess caulk.

Filling gap between concrete slab and wall

Should You Fill the Gap Between a Concrete Slab and a Wall?

If the gap between your wall and the slab is greater than 0.4 inches it should be filled. Similarly, if the gap is allowing water to leak into your home or basement, it should be filled. Leaky gaps should be filled even if they are less than 0.4 inches wide. If you do choose to fill this gap, it is essential to use a flexible material, such as caulk. Filling the gap with concrete or another hard material can cause further cracks and damage your home.

  • The gap between your concrete slab and wall should be filled if it is larger than 0.4 inches (10 mm).
  • Gaps smaller than 0.4 inches should be filled if they cause water leaks.
  • In most cases, a gap between the slab and wall is intentional, to allow for concrete expansion and contraction due to temperature.
  • Gaps that are 0.4 inches or smaller and are not leaking do not need to be filled.
  • If you fill the gap between slab and wall, use flexible materials, such as foam and caulk.

Houses are commonly built with an expansion gap between the concrete slab foundation and the exterior walls. This gap is intentional. It allows the materials of the house to expand and contract due to temperature changes. Without this gap, expansion and contraction would crack the concrete. These gaps are meant to be a maximum of 0.4 inches (10 mm). If your gap is within these limits and is not causing any leaks, it doesn’t need to be filled.

7 Steps to fill the Gap Between a Concrete Slab and a Wall

To prevent water leaks or other damage, it is sometimes necessary to fill gaps between concrete floors and foundation walls. In order to tackle this task, just follow these simple steps.

Measure the Gap

Begin by measuring the width of the gap. If the gap is 0.25 inches (6 mm) or smaller, it can be filled with caulk. This is great for small gaps that are causing leaks or cracks in your concrete slab. You can use a simple ruler or measuring tape to find the size of the gap. It’s also a good idea to measure several areas of the gap in case it is wider in some places than others.

  • Use a ruler or tape measure to find the width of the gap between your concrete slab and wall.
  • Gaps smaller than 0.25 inches wide can be filled using caulk alone.
  • If the gap is 0.25 inches or larger in width, it must be filled with a foam backer rod, followed by caulk.
  • Measure the gap at several points to determine if some sections are wider and require a foam backer rod.

If the gap is larger than 0.25 inches, it cannot be sealed with caulk alone. You will need to use a foam backer rod to support the caulk and provide a waterproof, flexible seal. Knowing what sort of job you’re up against sets you up for success.

Gather Your Tools and Materials

Once you have measured the gap you will be filling, it’s time to get ready for the job of filling the gap. For this job, you will need:

  • This urethane caulk.
  • A caulk gun.
  • If the gap is larger than 0.25 inches (6 mm), this foam backer rod in a diameter that matches the width of the gap.
  • A spoon or caulk-smoothing tool.
  • Mineral spirits.
  • Disposable gloves of your choice.
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Urethane caulk is the best choice for this job because it resists tearing, can withstand the elements, and adheres well to concrete and stone. Polyurethane is also a good option. Avoid using silicone caulk for filling gaps in the concrete. It is not durable enough for this use.

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Press Foam Backer into the Gap

If the gap you’re filling is 0.25 inches (6 mm) or wider, you must fill the gap with foam backer rod before caulking. To do so, first, measure the length of the gap. Then, cut the foam rod to length. Next, press the foam backer rod into the gap.

  • Gaps larger than 0.25 inches (6 mm) wide must be filled with foam backer rod before caulking.
  • Cut the backer rod to the length of the gap.
  • Press the backer rod into the gap until it is 0.25 inches below the concrete slab surface.

The backer rod must be pressed deeply into the gap. It should be 0.25 inches (6 mm) below the surface to allow for proper caulking and waterproofing. In order to avoid damaging the backer rod, use a screwdriver handle or other blunt tool to press it firmly into the gap.

Caulk the Gap

Before caulking, put on a pair of disposable gloves. Then, cut the tip of your urethane caulk tube at a 30-degree angle to allow the caulk to flow out. Then, load the caulk tube into your caulk gun and fill the gap. Work slowly, from one side to the other. The caulk should fill the gap and completely cover the foam backer rod.

  • Wear disposable gloves when working with urethane caulk.
  • Cut the tip of your urethane caulk tube to allow for application.
  • Load the caulk into your caulk gun and completely fill the gap.
  • No part of the foam backer rod should be visible through the caulk.

The caulk should not overflow out of the gap. Gaps should be caulked to make a smooth, waterproof barrier between the slab and the wall. So, there is no need to overfill this gap.

Caulk Small Gaps

After caulking the main gap in between your concrete slab and the wall, take this opportunity to caulk any other small cracks you see in the concrete foundation. It’s a good idea to clean any dirt or weeds that are growing in these small cracks, then fill them.

  • Inspect the concrete slab for any small cracks.
  • Clean these cracks to remove dirt and plant growth.
  • Caulk the cracks with the same urethane caulk used for sealing the larger gap.
  • Sealing small cracks prevents them from widening.

Caulking small gaps helps prevent water, dirt, and ice infiltration that can widen the crack over time. Catching small cracks before they grow helps to prevent leaks and home damage in the future.

Smooth the Caulk

With the caulk in place, it’s time to smooth the caulk to create a neat, long-lasting seal. The best tool for this job is a spoon. Dip the spoon in mineral spirits, then use the back of the spoon to smooth and spread the caulk to a uniform appearance.

  • Dip a metal spoon in mineral spirits.
  • Smooth the caulked gap with the back of the spoon to create a smooth seal.
  • Clean the spoon with mineral spirits if the urethane caulk begins to stick to it.
  • Wear gloves throughout this step to avoid skin contact with mineral spirits and urethane caulk.

You can use a caulk-smoothing tool for this job, but many of these tools are manufactured with small caulk seams in mind. A spoon works best for a large gap between slab and wall.

Clean Up

Dispose of used caulk tubes and gloves in a sealed trash bag. Then, use mineral spirits to remove urethane caulk from any tools used during caulking. Urethane is stubborn and sticky, but mineral spirits help make cleanup easy.

  • Throw away used caulk tubes and gloves after use.
  • Clean caulk off tools with mineral spirits.
  • Use soapy water or industrial hand cleansers to get urethane caulk off your skin.

It’s important that you do not use mineral spirits to clean urethane caulk off your skin. Mineral spirits can cause skin irritation. If you do get urethane caulk on your skin, wash with soap and water, or use our tips for removing roofing sealant from your hands. These tactics will help clean up caulk safely as well.

How Do You Fill a Gap Between a Wall and a Concrete Slab?

In order to fill a gap between a concrete slab and a wall, simply follow these steps:

  • Measure the width of the gap.
  • If the gap is smaller than 0.25 inches (6 mm) wide, it can be filled with urethane caulk.
  • If the gap is 0.25 inches wide or wider, fill the gap with foam backing rod before caulking.
  • Press foam backer rod 0.25 inches below the slab surface so you can caulk over the foam.
  • Caulk over the gap as well as any small cracks in your concrete slab.
  • After caulking, smooth the caulk with the back of a spoon.
  • Clean your tools with mineral spirits to remove caulk. Use gentler cleansers to remove caulk from your skin.

It’s important to note that houses are typically built with a purposeful gap between the slab and wall. A gap of 0.4 inches (10 mm) or less that is not causing a water leak does not need to be filled. However, large gaps—and any gap that allows water into your home—should be filled using this process.

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