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Gap Between Slab And Foundation Wall: Things To Do

Not aware of the reason behind that gap between your concrete and foundation wall? We’ve got your back, buddy.

Why exactly is there a gap between slab and foundation wall?

The gap between slab and foundation is an in-built design unless the gap is larger than 0.4 inches. If that’s the case, you can seal the gap between the concrete and wall using a concrete caulk. Or by taping the gap using window seal tapes. But make sure to seal it before it cracks up even more.

That’s not all, read our piece to know all about the gap between the foundation wall and concrete floor. 

Why is there a Gap Between Slab & Foundation Wall

Gap Between Slab And Foundation Wall: Things To Do 1

Cracks above door frame, gap between slab and foundation wall, cracks around windows; these are some of the regular scenes in a house with wrong arc

The gap between slab and foundation, also known as an expansion gap, is a common issue. But this gap is generally present while building the house. This is so that the floor can freely move and expand without collapsing.  

Now, what kind of movement does a house or wall need?

The expansion gaps are generally designed to allow the building materials some relief over the stress of movement due to thermal expansion and contractions. Moreover, the concrete of the house shrinks or stretches due to temperature change. This is why having the gap is necessary. 

But there’s a catch- The ideal gap between slab and foundation wall is about 0.4 inches. So if your slab and foundation wall has a gap between 0.4-0.5 inches, there shouldn’t be a problem. 

If you have a bigger gap, it can be a problem. This means the original gap has stretched. This could lead to a bigger problem. So filling the gap between concrete slab and wall is the ideal solution. 

Sealing The Gap

The gap between the slab and foundation wall needs to be treated right away to avoid further damage to the concrete. Too much expansion gap can result in water leak between the slab and wall. This is another big issue which you’ll need to fix similar to a running toilet that’s leaking

Moreover, this gap gives room for dirt build-up. Which can create another mess. Besides, a larger expansion gap also gives rise to unwanted cracks on the concrete.

Gap Between Slab And Foundation Wall: Things To Do 2

Before this gets worse, fill up the gap between the foundation wall and slab. You can seal it following two methods. One is a more permanent fix, while another is a quick fix. 

Method 1 of 2: Caulking the gap

Caulking is the process of filling in cracked concretes or the gap between slab and wall. Now, this is an efficient way of sealing the gap. A large gap can result in water getting into the cracks and making it even bigger. So you must caulk the gap to cover it up. 

Tools You’d Need

Caulking a concrete gap is an easy task and takes only about an hour. So you can easily do this on your own. But before you jump right into the process, let’s take a look at the things you’ll need for this method.

Step 1 of 4: Install Backer Rod

The backer rod is mostly important for the concretes with gaps. It is used to support the caulk. Here’s the thing- the backer rod must be fitted tightly into the expansion gap. So buying a road that is one size larger than the size of the gap is like a rule of thumb.

To install the backer rod into the gap, use your fingers to insert it. Set the rod in such a way that it is ¼ inch lower than the surface of the concrete. 

Step 2 of 4: Tape Before Caulking

Now it is important to tape off the sides before caulking. Use masking tape to line the sides. This will help to control the caulk lines and make the process a lot easier. But don’t forget to remove the masking tape once the caulk dries off. 

Step 3 of 4: Fill in the gap

Now that you’ve completely installed the backer rod in the gap, it’s time to apply the caulk. Though there are many caulk options, urethane caulk works best for expansion gaps.

To apply the caulk, cut open the caulk tube at a 30-degree angle equal to the gap. Use a smooth and even motion to fill the gap between slab and wall. Make sure you smooth out the caulk using an old spoon or something similar to that. 

Caution: Handle the caulk very carefully as it is very sticky. It can also have an allergic reaction to your skin. So, wear gloves when using it. 

However, if somehow the caulk gets on your skin, wipe it quickly using a paint thinner. You can use any paint thinner whether it’s one for Porter Paints or Sherwin Williams.

To clean the caulk off the spoon, use a rag and mineral spirits to wipe it away. 

Step 4 of 4: Cover The Caulk 

Once you’ve sealed the gap between concrete floor and wall, it’s time to cover it. Use a concrete mix to cover up the caulk so the floor looks just the same again. For this, the concrete mix has to be wet to sit on the dry caulk properly.

You might have to look out for problems such as heat from baseboard sounding like water.

Method 2 of 2: Use Window Seal Tape For The Gap

Gap Between Slab And Foundation Wall: Things To Do 3

This is another method to fill in the separation between floor and wall. This one is quite easier than the first method. You can just seal the gap using weather strips or window seal tapes.  But it is a temporary fix and you’ll need to repeat it after a few months or so. 

To seal the gap between the slab and the foundation wall, you just simply push it into the gap with a stripping knife. Make sure the tape is of the same thickness as the expansion gap. Once you’ve placed the tape all over, it’ll expand on its own. And as it has expanded, it’ll hold the gap very tightly. 

What’s the interesting part? These tapes are almost invisible so it’ll not look messy or dirty. 

So get your hands on these tapes to avoid expansion gap issues. Here are our top choices of window seal tapes:

 Window Screen Repair Kit, Black XL(15FT) Check Current Price
 XFasten Transparent Window Weather Sealing Tape Check Current Price
 3M Flashing Tape 8067 – 2 in x 75 ft – Waterproof Check Current Price


How do you know a foundation wall crack is serious?

A wall crack can be serious if the gap gets wider. Make sure to get it inspected by a professional and fixed immediately.

What happens if there’s no expansion gap at all?

Absence of an expansion gap can cause damage to the floor. The floor can bend due to the pressure of expansion. This leads to irreversible damages. 

How do you seal concrete cracks?

Wide concrete cracks can be patched and sealed using a concrete patching compound. And the smaller ones can be fixed using caulk.


A gap between slab and foundation wall is necessary to allow expansion of the floor. But too much of the gap can be bad for the floor itself. So knowing the right size of the gap and how to fill the excess gap is vital.

With our detailed discussion, we hope you’ll be able to fix your concrete gaps and cracks. Try it out and let us know.