How to Dig Out a Basement from a Crawl Space-Bench Footing or Underpinning?11 min read

How to Dig Out a Basement from a Crawl Space-Bench Footing or Underpinning?11 min read

Have you wondered why someone would leave 5 feet and build a crawlspace? I had the same thought when I bought a house and found the crawlspace to be in-between and just needed that extra 3 feet to make it habitable.  Let’s see how to dig out a basement and repurpose the crawlspace and eventually lower the floor to make it livable.

Very simply, the basement conversion project entails a few critical steps. First, you must assess the current structure thoroughly, and then dig out the basement to pour in a new footing – under or beside the existing foundation. Then excavate rest of the floor, waterproof and put in a new floor.

It just came out simple but there are a lot of critical steps involved, let’s dive deep – shall we?

How to lower a basement floor 101

In simple terms your dig out the dirt to make enough headroom, extend the foundation all the way below, fix the drainage system, put in a floor and finish up the basement.

Check my blog post on an In-depth guide on flooring options, once you are done lowering the basement.

But there are two widely used structural methods with the foundation wall.

Underpinninghow to dig out a basement-Numbered sections and finished underpinning foundation

Essentially, when you dig out the crawl space into a basement you would have to do as you have to do a full perimeter excavation.

Once you do the excavation you must stabilize the structure on top of it temporarily till construction is complete.

And then the 3-foot crawlspace wall we going to have to pull another 6 to 9 feet of foundation wall underneath the existing crawl space.

Bench Footing

How to lower floor-using bench footingBench footing is the exaction and construction of a small retaining wall, that looks much like a bench.

Once the concrete bench structure is built, then the rest of the required basement floor space is excavated.  The needed floor space is then excavated and a new concrete floor is poured.

Both have its pros and cons. Usually, it boils downs to the risks and budget.

Dos and Don’ts with turning crawlspace into a basement

Do not start the underpinning as a DIY project.  You most definitely need the help of a structural engineer.

Do pull in a permit – In fact, this is one of those projects where you need to pull a permit and it’s for your own good.  I am not saying you don’t need for others that your city enforces, or IRC recommends.

Here is a document, published by DC government, providing guidelines for underpinning permit. Very handy, use it.

Do have a structural engineer on your team anyhow to get the inspection cleared.  Find an engineer you are comfortable working with.

Reasons for underpinning (and bench footing)

To increase the livable space

When you are in a situation where you can’t extend or pop-the-top but need more livable space underpinning the crawlspace is a great option.

Create additional apartment for in-laws or make rental income space

When you want to keep things in a traditional way, especially make use of the space and make it livable for additional rental income, or wanted to have that in-law suite, then underpinning is not of an option.

Correct any structural deficiencies

When you have a certain structural issue and have to work on the basement, might as well, fix and use that time and expense to stretch it further, so you get a nice living space.

In most places I know, the taxable living area is anything above grade.  So, this is a wise option to increase the living space but not increase your tax bill legally.

Lowering Basement floor with Underpinning a Wall in Crawlspace– How is it done?

Each marked section individually excavated and foundation poured When the job begins, the foundation wall is marked. This will be in accordance with the underpinning crawl space drawings provided by the engineer.

To lower the concrete floor, your contractor will then mark numbers on the wall (1,2 and 3). The length of each of those sections would be 3 or 4 feet wide.

Once the sections are marked out, the earth is then Dug and excavated.  The excavation happens in sections. Each numbered section is excavated all the way down to the desired depth needed for the project or per your engineer’s recommendation.

Then the underside of the excavated area is cleared of all debris and the footing is cleaned. Then it is inspected by the engineer and the city inspector.

Once approved you can put the concrete forms in place and start pouring the concrete and reinforce it with steel bars.  Concrete, when poured, will be vibrated to make sure the distribution is even and there are no air holes or pockets.

When the underpinning sections are poured and cured the rest of the dirt is excavated from the basement.  The excavation happens till the depth of the footing.

Waterproofing part of the project takes place after that. Normally gravel and weeping tiles are set.

If the basement is going to have rooms that need plumbing – Kitchen, wet bar, bathrooms etc. – then basement plumbing will happen before the new concrete floor is poured.

Lowering basement floor without underpinning – i.e. With Bench footing

When underpinning the foundation is not the desired approach to put in a basement, bench footing work is performed.

When you are in a row house or a townhome like structure and have no consent from your neighbor to excavate underneath, then benching a basement foundation is the only option.

Because you will be excavating the dirt from beneath your house rebar reinforced bench footing preparationalone.

Again, check with your local authorities and most importantly your HOA for the bench footing basement allowances.

If the engineer looks at the foundation and for whatever reasons thinks it’s not structurally safe to underpin, then bench footing is how you will be advised to lower the basement floor.

Benching is essentially this. There is a load bearing soil underneath the footings. It’s left undisturbed and in place as it.  What you do is, pour a thick concrete bench in front of the foundation.

This bench basically encapsulates the soil under the old footing thereby not disturbing the original structure, leaving it undisturbed.

Before the concrete is poured the bench is reinforced with rebar and anchored into the existing foundation wall, so it is tight and doesn’t have any give.

Think of this as a retaining wall for the for the dirt beneath the foundation.

After the bench pouring is completed and cured, around the perimeter of the interior of the basement foundation, dirt excavation happens in the basement.  Again, this is done to the depth of the poured bench.

From here on, the rest of the project is the same as the underpinning method, with waterproofing, gravel, weeping tile, plumbing and concrete floors.

Structural Issues with Converting Crawlspace to Basement

Issues with underpinning wall seams

When you do the underpinning, it creates a seam. Because it’s one foundation on top of another.  The seam is between the two foundation walls.

And in fact, that’s the most vulnerable area of the foundation that we need to really pay attention to.

Number one issue is since concrete is porous materials, to begin with, the material itself isn’t airtight and now you have the seam, that might further worsen the issue.

Issues with drainage worsen the seams

And if you don’t have a good drainage system around your foundation to move the water away from the foundation, it will experience hydrostatic pressure and that’s not good for the foundation.

With a poor drainage system, your house will have a false water table.  And that is going to raise around the home and increase the hydrostatic pressure even more. From the sides of the basement wall and from the bottom of the floor.

That is when you see the hydrostatic pressure the water right through the foundation wall. Basements are generally considered humid, to begin with – check this post to understand the correct basement humidity level.

When the water table rises due to poor drainage, you will start to see basement flooding.  The problem isn’t just that.  There is an even terrible problem that you would have created. A weak point is created between those 2 walls – The old concrete and the new poured concrete.

Poor soil conditions will worsen the problem even further

The problem will actually magnify if you have a soil that expands very much and that is very volatile.

If you are dealing with clay or bentonite soil, for example, you will have to be more careful in planning and executing the crawlspace to basement conversion project.

These soils expand when wet and contract when dry making this a really tough soil to have a basement wall in.

Again, with these soil types if you start to experiment a little pressure right on that seam and you’re going to see a lateral movement.  And that’s when you will be running to stabilize your foundation wall from a lateral moment.

That, in my opinion, is too late to work on that problem. Bring in the expert, do the soil test and do it right the first time.

How much does it cost to dig out basement and lower a floor?

The cost to deepen a basement can vary depending on many factors.   Mainly, it depends on the engineering concerns, the type of soil that the house is in, the site condition where the work happens and such.

I am going to assume you have a 500 sqft crawl space, which is 6 feet deep and you want another 2 feet dug to make it livable.

Using broad strokes in my calculation, I would say, the cost to dig a basement deeper using underpinning is going to be anywhere between $20,000 to $30,000.

It’s hard to put an underpinning cost per foot calculation but based on the above number that would translate to $40 to $80 per sqft. I think a cubit feet calculation is more ideal when you are dealing with excavation though.

Benching, as we know, takes less work and is going to cost less.

By how much?

I would say, it is going to be around 60% – 70% of the cost of underpinning.

But if you consider the floor space that you get for the expense, you are going are not going to be too far. Since Benching takes a good bit of the floor space but costs less, you end up with a smaller floor space for a less cost.  Which equates to not a lot in cost savings though.

I didn’t include much of the cost for plumbing and any additional reinforcing costs for the turn-of-the-century building.

Is lowering a basement floor and bench footing a DIY project?

In my opinion, any structural work should not be a DIY. If at all you want to venture out on your own, you must consult a structural engineer.

One of my friends managed a property once.  We saw a vertical push in one of the retaining walls.  Mainly due to a lateral pressure from the other side and the wall started to give.

I was able to slide my pinky in.  He went to sell, and the home inspector suggested a structural engineer inspect and draw a plan for the fixup.

Even though he and I could have fixed it up, we couldn’t. It was a known issue and he was told by the realtor that the issue is public and can eliminate the issue only if it’s done by a licensed contractor and signed off by a structural engineer.

Digging out a basement by hand

You might be an odd situation where you can’t get a small excavator in to dig the trench or the basement.  You have no option but to put a lot of elbow grease.

There are other situations where the crawl space is only 3 feet below the ceiling and there is no way you can excavate using heavy equipment. Your only choice then is to shovel the dirt by hand.

At least until you can make way for the excavator to slide its bucket in.  But if you want to DIY this project and do it all by hand, it’s tough.

Especially if you are in an area where you constantly hit rocks – good luck.   Definitely get a lot of help from your family.

Set up some semi-mechanical stuff like conveyor belts to pull the dirt out. Use jackhammer to break anything difficult to excavate by hand.

Whatever you do – Safety First. Whether you do it by hand or using machines, make sure you have enough vertical structural support, so the house doesn’t sink in by accident.  And also make sure to take precautions while doing this whole project.

In Conclusion

If you are looking to dig out a basement from a crawlspace you are taking on a big project and in fact a complicated one too.   To underpin a house foundation, you must first excavate the soil below the current footing and build a concrete wall.

Once that’s cured you will excavate the basement floor, waterproof, do necessary plumbing, install a heating system and finish the basement floor.

This is just the beginning.  You just have opened a can of worms with the crawlspace lowering project – in a good way.  You will have tons of other small projects that you will have to do to your basement (no more crawlspace) to make it an excellent and useful living space.

If you have come this far, don’t just leave it as a storage space, start finishing up the below grade space.  If you can handle this, all others are not that complicated.

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