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Everything you need to know about basement underpinning


Basement underpinning process

Has someone told you your basement needs underpinning? What is underpinning? Why do you need it for your house? Underpinning is typically something that a lot of investors get nervous about. It is intimidating and a huge job typically you do not see a lot of anything happen just a floor lowering. However, it is a big financial commitment for not a lot of guts and glory. In this article, we are talking about basement underpinning and everything you need to know about it. Keep along with us!


What is basement underpinning?

Basement underpinning is a process of supporting or strengthening the foundation of an existing basement, building, or similar structure. In another word, it is essentially about lowering the floor in a basement to create more living space. 

It is required when the original foundation is no longer strong enough to support the house; this is usually a result of a change to soil structure. Whether due to the type of soil or some external influence. 

Underpinning is accomplished by reinforcing the existing foundation. By extending the foundation the load is distributed over a greater surface area to support the existing foundation.



Strengthened foundation


What is the benefit of basement underpinning?

There would be tons of opportunities coming down concerning underpinning projects and utilizing basement suites. This can be hugely advantageous if you want to create a basement apartment. 

However, if there is not enough ceiling height, you can create more of it in the basement instead of having to take the house down and re-pour a new foundation. You can drop the floor in the basement to create more space. 


What are the first steps of doing basement underpinning?

Before we go further, we want to sure that mention three things. If you are doing any structural work to your house, please take these fundamental steps first:

  1. Hire a structural engineer to oversee the project.

  2. Apply for building permits. Make sure you have inspections along the way and you have the right approvals to be able to do any work you want to do on your property.

  3. Make sure you notify your insurance company. Because your insurance company may have a stipulation in the agreement with them that says that you have to notify them when you are doing any structural work to the home. 




Modern basement renovation

How to do the basement underpinning?

The first thing, you will notice is that the underpinning contractors have gone around the foundation wall, and the perimeter and they have marked out sections.

These will be sections that will be stipulated by the engineer on the drawing and they are usually three to four-foot sections. In addition, the numbers above them indicate when you are going to dig and then pour each section of the underpinning.

They usually marked out number one sections that are going to be excavated, underpinned, and then poured. Therefore, you can see that around the foundation perimeter.

Now, what you do is dig out three or four-foot sections, which are six to eight feet apart. So, you have six or eight feet of foundation that is still intact. 

Then, you can section pull out the dirt from underneath the footing, pour the concrete in, and then the next step is to come in and remove the following section around the foundation.

Essentially, the contractors would knock out a section of the concrete floor and then they dug down, and then dug back.

So, that goes back about two feet or 18 inches, which is the foundation thickness. Therefore, you have to get underneath the entire foundation, dig all way around, and then go all the way down.

We recommend that when you knock on the concrete, keep the slab intact for the most part, and take each little section. Then, that keeps the structural integrity of the foundation walls while they just take out these little sections. 

Once the required sections have been dug out and inspected by the engineer to make sure that they have gone down far enough, and back far enough to support the footings, you can get to form the footings and then pour concrete into those sections.

The forms are put in place to hold back the concrete because they pour that concrete in there. 

Then, they are going to vibrate it to make sure that it gets all of the air bubbles out and it fills all the pockets. However, the vibration of the concrete causes a lot of force. 

So, these footings have to be pinned back to make sure that they do not move while you are pouring the concrete.



Basement construction work


What are the different methods of basement underpinning?

There are a couple of ways for underpinning a basement:

  1. Flush pour

That means that they are going to pour the concrete flush in the line with the existing foundation. 

  1. Step foundation

In the step foundation method, the concrete would come out a little bit from the existing foundation and widen it a little bit. 


Note:

There are also different ways to pour the concrete as well that you can choose according to the engineer’s suggestion. 

You can either do an over-pour or you can pour a couple of inches short of the existing foundation wall. Then, you can fill that gap with what is called non-shrink grout.

That is all stipulated on your drawings from your engineer, and all stamped by the city. So, you know exactly which way you can tell your contractor that everything needs to be done.


How to get the dirt out of the foundation?

As you can imagine, there is a lot of dirt that comes out of each one of these individual holes. So, the biggest part of underpinning is figuring out how to get the dirt out of the foundation.

If you have access to the backyard, this job would be relatively easy for you. Because you are going to be able to get the bins right at the back of the house. The contractors can knock a hole in the back wall of the foundation and bring the machinery right in. 



Underpinning support beams

Which machinery do you need for the basement underpinning?

Depending on your underpinning contract, they may use various forms of machinery or might just do it by hand. Maybe, the company you hired uses a lot of machines. This would be very effective and fast as they will be able to move so quickly.

For example, they might use small machines and excavators that are used to remove the dirt. They put it on the concrete floor and another machine comes and grabs it. 

What they do is they take it all to a designated space and pile it at the back exit. Then, the other excavator outside grabs the piled dirt and throws it in the bins. The workers keep exchanging out the bins. It is an efficient process!


Removing the center section in a basement underpinning project

To remove the center section in your underpinning project, you can take up the slab and start to remove the dirt. Because now the entire perimeter footing has been dropped down to the level they are going to end up at. 

If a center section of a basement is really wide, you are not able to remove it until you got a load-bearing wall right down the middle. 

In this case. You have to underpin all the sections we mentioned before and let them cure. In addition, you need a wall (suspended kind of midair) indicator of where the floor height used to be and what you have gained in floor height now. 

This wall is not picking any structure. Otherwise, the contractors would not have never removed what is underneath it.

You get the indication too of high the windows are going to be. You are coming up a foot from the floor. For example, six inches of gravel will essentially line up with the footing you put in.

Then you have to get two inches of insulation and then four inches of concrete. Therefore, you are going to come up a full foot from where you are. However, you need to end up with about nine for example, around nine-foot ceilings down there.

After you put in the bearing wall and figured out the structure, you can remove all the center sections including all the brick walls, which are not the bearing. 

Then, you can strip the footing down the middle. After that, you can restructure the framing to be able to open up the entire space and blast the back walls to put in the addition.



Basement construction work


Why do you put the bench footing in the basement underpinning?

The bench footing is not always necessary but you are going to put it in if you want to do a structural wall to pick up the load for your floor joints. 

If you would not have the bench footing, you will restrict your options. Either by putting the bench footing in, you can pick up the structure of the floor joints with a structural 2X4 studded wall, or these joints are going pocketed into the brick. 

However, the bench footing gives you multiple options and it allows the engineer to decide whether they want to put in a structural 2X4 wall or they want to pocket the floor joints in and set them on the foundation.

You can use temporary jack posts and the picking up the structure and the load above that. So, you will be able to remove the entire brick walls and take out all the remaining dirt in the basement.



The interior waterproofing of a basement underpinning project

In the case of waterproofing your underpinned basement, you can use a delta membrane that goes on the inside of the walls. 

So that, if water penetrates through the foundation, it hits that membrane goes all the way down to the foundation and then goes right into the weeping tile system.

The weeping tile system goes all the way around the perimeter and then it empties into a sump pit. Then, the sum pit ejects the water to the exterior. 

The reason why you are doing it on the inside is that the foundation is all exposed and you can do this around the interior of the perimeter as opposed to doing it on the exterior.

You should also have a strip footing going right down the middle of the property. This way, you can set a structural 2X6 wall down the center of the property. That will essentially pick up the entire load from the floor joints above that.

When the insulation process has been completed, you should lay down the radiant heating pipes on the two inches of rigid foam insulation. Those radiant floor pipes go all the way out and around. 

There will be different zones that are connecting to one single thermostat. However, each of them zoned differently. All of those pipes lead back and manifold in one place, which will connect all of the radiant heating floor systems. 

Radiant heating is not required in a basement underpin but it is a great value add if you are doing a underpin and you got the ability. 

It is probably just as easy to run the pipes even if you do not use them, it is great to have them there just in case. Then you will be ready to pour concrete and finish up your basement underpinning project. 

While you pour the concrete on the floor, you should keep the radiant in-floor pipes pressurized. You should have about 60 PSI (pound per square inch) on the pipes. 

So that, if any one of them gets nicked or burst, you would immediately know that and you can stop the pour of the concrete, fix that pipe, and then keep going.

The last thing you will want is to pour all the concrete over the radiant heating pipes and then one of them to leak or burst. That is why you are going to keep them pressurized during the pouring of the concrete.


Underpinning support beams

What is the cost of the basement underpinning?

Every contractor is going to have different pricing. However, around 400 dollars per linear foot of your foundation wall is pretty consistent.

That included digging out the sections and underpinning, the gravel that you put in, and then pour down the new concrete pad that does not include the radiant floor heat.

So, the cost is going to depend on how easily accessible the site is; how quickly and effectively they can get the dirt out and concrete back in. 

The whole process should take around four to eight weeks to complete an entire underpin. If there would any complexity with adding the addition, the process would take a little bit longer.


The Bottom line

Here, we talked about the entire process of basement underpinning and everything you need to know about it. Please feel free to leave us comments and questions below. We would be happy to answer your technical questions about underpinning the foundation to the best of our knowledge. Good luck!


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