A basement knee wall is a poured concrete wall, with a height of 2 to 3 feet, built to protect damaged basement walls. They are most commonly built in older houses where the existing basement wall is starting to crack or bow due to water seepage or lateral load from the exterior of the basement.
What is a Basement Knee Wall?
A basement knee wall or an engineered knee wall is a short-height poured concrete wall installed on the inner side of the basement foundation wall.
The height of a knee wall ranges from 2 to 3 feet. This in effect doubles the thickness of the foundation wall.
A knee wall supports an existing damaged basement wall and serves to protect it from further structural damage.
They reinforce the foundation wall by filling existing cracks and fissures and offering additional support to the basement wall.
Normally, new constructions do not require a basement knee wall and they are commonly constructed in older homes.
The basement in older houses is made of bricks, concrete blocks, and sometimes even with stones.
Such foundations have hundreds of joints and these deteriorate over time.
To maintain structural integrity, a basement knee wall can be built inside the existing basement wall.
This strengthens the old walls and improves their structural integrity.
They are called knee walls because their height is generally around the height of a human knee.
However, it is not absolutely necessary that knee walls are made at that height. They can be taller or shorter depending on your need.
A permit from the local building department is necessary to add a knee wall to a basement.
Need for Knee Walls in Basement
Basement walls are the most important and integral part of your structure and give support and strength.
A leaking basement and cracked foundation is a risky situation.
If you have a leaky or flooded basement or a cracked foundation then a knee wall can be a good solution.
Cracked or bowed basement walls are a sign of damaged basement walls and need immediate action to avoid any further structural damage.
Poor water drainage and lateral pressure are the most common reasons for cracks in a basement wall.
If you have a poor drainage system around your home then this situation can get aggravated.
Water creates hydrostatic pressure on basement walls and pushes them.
If you have zig-zag cracking patterns in your basement wall then you can be almost sure that water is the issue.
When soil absorbs water, it expands and creates lateral pressure on basement walls. Most old basement walls are not designed to withstand lateral pressure and this causes deterioration.
For both finished and semi-finished basements, knee walls offer a good structural option to keep the basement dry.
Basement knee walls are important for walkout basements which are more prone to flooding.
Sometimes the soil around a home can be super absorbent. It absorbs water heavily and expands abnormally.
Another risky situation is when the roots of nearby trees encroach through your basement walls. As these roots expand, they create cracks in basement walls.
A basement knee wall can be an economical solution for all these issues.
When renovating your basement for structural reasons, constructing a basement knee wall is one of the most important steps, the other being reinforcing or replacing your basement beams.
How to Build a Basement Knee Wall
Building a knee wall can be a bit tricky but it's possible to construct it perfectly if you understand the process.
Here's the step-by-step process of constructing a basement knee wall.
Step-1 Remove any obstructions
As the first step clear any hindrances near your basement wall.
These obstructions may be plumbing pipes, water pumps, or any other utilities.
Plumbing lines have to be reoriented if they are active.
Take the help of a plumber if necessary.
If you have a jack post or lally column near your basement wall then that should also be either removed or reoriented.
All these steps are to be done before you can accommodate plywood forms.
You have to make sure there's no obstruction within 12 inches of your existing basement wall.
Step-2 Determine the Height of the Knee Wall
This step is crucial. You have to observe up to what height your basement wall is damaged.
After determining the damaged height of the basement wall you can plan your basement knee wall’s height.
Normally, the height of a basement knee wall ranges from 2 feet to 3 feet.
However, there's no specific limitation on the height you can have.
Step-3 Installing Plywood Formwork
Next, you need to install formwork near your basement wall for pouring the concrete. Wooden ply boards of sufficient strength should be used.
If you are using plain lumber boards then their thickness should not be less than 1 inch.
If you are using plywood boards for concrete forms then the thickness of the ply should not be less than ¾ inches.
Orient these boards in a straight direction and support them with vertical wooden posts at every 4 feet.
This will provide sufficient strength to boards and prevent them from collapsing after pouring concrete.
In addition, you'll need proper reinforcement of these boards. Inclined wooden posts should be installed at regular intervals.
These inclined wooden posts should rest on previously installed vertical posts.
Step-3 Pour Concrete
Opt for either ready-mix concrete or prepare the concrete mixture on your own by adding sand, gravel, cement, and water in the right ratio.
Transport the concrete to the basement and pour it into the gap between the wall and forms.
Compact the concrete with a concrete vibrator. These vibrators will help you to achieve better compaction and eliminate the voids or entrapped air from the concrete.
After filling the concrete up to a required height, smooth the top surface of the concrete.
Step-4 Remove the Forms and Cure your Concrete
The curing of the concrete is very important for achieving good strength.
Remove the forms 7 days after pouring the concrete.
If you want, you can keep them in position for a longer duration but a minimum of 7 days is required for concrete to set.
After removing the formwork, the whole surface of the concrete knee wall will be exposed to the elements.
Moisturize the exposed surface of the concrete with water for the next 28 days.
Step-5 Framing of Your Basement Knee Wall
This step is optional. If you want to frame your basement knee walls with wooden frame then you can follow this step.
First, you have to insulate your knee wall with proper insulating material.
In the second step prepare a wooden skeleton according to your knee wall and stiffen it on the top and bottom.
Again framing of a basement knee wall is not necessary until you want to do it.
That's how you construct a basement knee wall to prevent further damage to your basement walls.
What height should a knee wall be?
Generally, the height of knee walls ranges from 2 to 3 feet. However, this height is not fixed and can be changed according to your requirement.
Knee Wall vs Pony Wall
Knee walls and pony walls are two different things.
Pony walls are short walls used to separate spaces inside a house.
These 2-3 feet high walls can separate different areas of your house without blocking the passage of air.
Most importantly pony walls are non-structural walls. This means they are not load-bearing walls.
On the other hand knee walls are generally load-bearing.
If you are building a knee wall in the basement or attic then it is most likely a load-bearing wall.
Alternatives to a Basement knee wall
There are two common issues that affect a basement wall.
One is the cracking of the basement which may be a horizontal or vertical crack.
The other is the bowing of the basement wall.
Note that hairline cracks are not very dangerous if their size stays the same.
But if you have horizontal cracks in your basement wall then it is a serious concern for your basement wall.
Bowing of the basement, even in small amounts, is a matter of serious concern.
While a basement knee wall can be a solution to these issues, there are alternatives that you can consider.
1. Liquid Polyurethane
Most basement wall cracks can be repaired from the inside.
Liquid polyurethane is a chemical compound that can be injected inside the cracks of a basement wall.
Once injected, when this compound meets water inside the cracks, it expands and forms a closed-cell foam.
This closed-cell foam fills the entire crack and makes them leakage-proof.
This works only for small cracks through which water seepage occurs.
For large-width cracks, this solution is not ideal.
However, for small cracks, it is a clean, fast, and economical solution for avoiding water leakage into the basement wall.
2. Carbon Fiber Straps
Carbon fiber straps are an effective solution for damaged basement walls.
They are easy to assemble and probably the least expensive solution.
These straps can be placed on the wall and sealed with epoxy. The top and bottom edges are then secured with the help of anchors.
They can be installed from the inside of the basement and require no excavation.
The spacing between two straps is typically 4 feet. After completion of installation, you can paint them with your desired color as well.
Straps provide additional support to damaged basement walls and if there are micro cracks you can fill them using liquid polyurethane or hydraulic cement.
Overall, carbon fiber straps and liquid polyurethane in combination can be an effective alternative to a basement knee wall.
3. Wall Anchors
Wall anchors are another alternative to strengthen a deteriorated basement wall.
They are more expensive than carbon straps. Wall anchors require 10 feet of usable space outside the basement for perfect installation.
A steel plate or channel will be mounted on your basement wall and another plate is buried in your yard.
These two plates are then connected through a steel rod.
This rod, when tightened, creates a pull to hold both the plates and provide additional support to your basement wall.
Usually, the spacing between two anchors is 5 feet.
While these are great alternatives to building a basement knee wall, you should also try to improve your home's exterior.
A better exterior can be a great way to prevent water from seeping into your home which will make your basement knee wall much more effective.
Plus it's also a great way to improve the value of your home!
Frequently Asked Questions
Are knee walls structural?
When knee walls are installed in the basement or attic then these walls will be load-bearing and can be called structural walls.
In basement knee walls, they absorb the load of super-structure in conjugation with basement foundation walls. It will transfer a load of superstructure to the ground.
Knee walls in the attic will support the loads coming from rafters and transfer them to the foundation. Any knee wall apart from these two applications will not be structural.
Are pony walls structural?
No, Pony walls are used only to separate spaces. They are not structural as they do not support any loads.
There is no specific height or width for pony walls and their height is determined based on architectural requirements.
How to remove knee walls?
Check if your knee wall is structural in nature. Removing a structural knee wall can be dangerous.
If your knee wall is non-structural, you can remove them without any worry.
Take the help of a licensed engineer or a contractor for evaluating whether the knee wall is structural or non-structural.
Are knee walls compulsory in a structure?
If your structure is technically sound, then there is no need for knee walls.
If you are living in an old structure then there's a possibility of issues with basement walls.
In such a case installing a basement knee wall can be an effective step.
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