How to Add Subflooring to Your Basement

Basement subflooring is an important part of your overall home design. Not only does the basement subfloor provide support and structure for the floor you walk on, but it also offers moisture control and creates a flat, level surface for the entire rest of the basement.

The subfloor must be able to withstand flood damage. There’s a lot it has to do, and that’s why you need to know how to add subflooring to your basement.

What Is Subfloor?

Like the name says, subfloor is the floor that goes under the floor of the basement. It sits directly on top of the foundation of the house and under the carpet or laminate or whatever type of flooring you have for your basement.

Subflooring keeps the basement warmer and more moisture-tolerant, and it’s really a necessity for a finished basement. For many types of flooring, subfloor is strongly suggested. You want the insulation and moisture protection of subflooring to keep the basement dry, warm, and more hospitable.

If you’re going to have a bedroom, a living room area, an entertainment space, or any room or section of the basement meant for spending time in, you should strongly consider using a subfloor before placing a floor for the basement.

The subfloor sits about 8 to 10 inches high, which keeps it up above moisture from the ground below in most cases. There are different types of subflooring that sit at different heights.

The subfloor you choose to install depends on the size of your basement, the height of your ceiling, the budget you have to work with, and your own preference. Don’t let DIY skills, or lack thereof, affect your subflooring decision.

Even if the work is tedious and difficult at times, installing subflooring is not a task that can’t be tackled by even a fairly novice DIYer.

First Steps

Before you install any type of subfloor, make sure you’re starting with a clean and level foundation. First, thoroughly sweep and mop the foundation to get it clean. Inspect for any signs of damage, such as cracks, and repair these areas first. Make sure the floor is level and repair any uneven areas of the foundation.

Use long levels across the floor to ensure that the floor is mostly level. You can use shims and sanding techniques to get the subfloor perfectly level if the foundation is a little off, but big differences in the level of the foundation will have to be repaired before the subfloor is installed.

Once everything is prepped, you can begin the installation of your subflooring and then go on to finish the rest of your basement.

drilling wood into subfloor

Types of Subflooring

There are four types of subfloor for you to consider for your basement. Weigh each one of these options carefully and consider factors such as ease of installation, the size of your basement and how much thermal and moisture protection you want for your basement floor.

Once you know more about the different types of subflooring for your basement, you’ll be able to choose the perfect one for your home.

Floating Plywood Subfloor

A floating subfloor isn’t attached to a lower layer with nails or screws. This is the most affordable of the types of subflooring you can choose for your basement because it takes very few materials. To install this floor, you need only to place an underlayment and a vapor retarder on the concrete foundation and cover this with plywood.

Nothing else is needed to secure the plywood. The weight of the floor on top of the subfloor will be enough to hold it in place. This is a very straightforward installation that’s relatively hassle-free, compared to some of your other subflooring options.

The underlayment for the floor can be polyethylene, a type of plastic, or foam. This creates a small air gap that creates a thermal layer to insulate the floor against the cold. The vapor barrier adds moisture resistance to the construction.

Rigid Foam

insulation pad on basement floor

A rigid foam subfloor insulates your basement floor, which helps prevent heat loss. It’s also easy to find rigid foam anywhere. This is a common item sold at home improvement stores, so you won’t have to look very far or make a lot of trips to get all the foam you need to create this subfloor.

To install this subfloor, first place plywood across the foundation. Then, you’ll cover the plywood with rigid foam about one and a half-inch thick. Once the foam sets up, you can place your vapor barrier and plan to place the flooring you chose for your basement.

Sleeper System Subfloor

A sleeper system subfloor is built with two-by-fours and topped with plywood. It sits up off the floor several inches, creating distance between water and your flooring. To build a sleeper system subfloor, you will install two-by-fours every 12 to 16 inches across the entire floor.

Rigid foam insulation is placed between these “sleepers.” Everything is then topped with a vapor retarder, which protects against moisture. Plywood sheets top everything off.

The two-by-fours used to create this subfloor should be placed flat on the foundation when placed. After the moisture barrier is laid out on top of the foundation, begin laying out the two-by-fours in straight lines.

Use a drill and screws to secure the two-by-fours in place. Use screws and the drill to attach the plywood on top. You should add screws every eight inches across the sheet, securing them through the plywood to the floor joists you created with the two-by-fours.

Because you’re placing plywood on top of two-by-fours and then the flooring on top of that, this is a tall subfloor compared to your other options. You need to make sure your basement has the height for this flooring choice.

Make sure there will still be at least eight feet of clearance above the subfloor and finished floor when the basement is complete. Otherwise, the space is going to feel cramped, and it will have very little ventilation.

Subfloor System

You can cut out some of the work and purchase a preassembled subfloor system, which is really a module. This already has all the layers of the subfloor assembled so you don’t have to do anything but put the flooring in place.

Usually, these systems come in the form of tiles or panels that fit together. The pieces essentially click together. All you have to do is place them.

This is a very fast and easy system to use, but of course, it is the most expensive option because there’s so little DIY involved. This is a very attractive option for those who don’t want to take on too much DIY but the price may be a sticking point.

Subfloor systems are sold in price per square foot, so you will know exactly how much the floor will cost you after you get your measurements and do the math. If a subfloor system fits into your flooring budget, you might want to consider this and save yourself some time and effort.

Otherwise, the other subflooring option are more affordable. They will only cost you more elbow grease.

Things to Remember

plywood floor

When it comes to subflooring, there are certain construction standards you must adhere to in order to ensure the subfloor is structurally sound and meets minimum requirements for weight tolerance.

After all, you need your floor to be safe above all. It must support the weight of human feet walking, not to mention all the furniture and the floor you plan to place on top of it.

The minimum thickness of the plywood you use when building a subfloor should be five-eighths of an inch. However, the thickness of the plywood varies based on the spacing of the joists underneath, when present. If your joints are 16 to 19 inches apart, use plywood that is three-fourths of an inch thick.

When joists are spaced more than 19 inches apart, use plywood that is seventh-eighths thick. Get plywood that is thick enough for the type of subflooring you are installing. Otherwise, the integrity of the flooring will be compromised.

Don’t forget about the cost of subflooring. Measure your basement and make sure you can purchase enough materials to cover the floor of the basement. Plywood, for example, costs about $1.50 per square foot.

You’ll also need to factor in the cost of a moisture barrier and insulation, if used, along with all nails and other materials you might need to place the subflooring.

Depending on the subflooring you choose, you may also need two-by-fours and screws. Consider the cost of the entire project and don’t forget that you need some of your flooring budget left over for the floor you plan to place on top of the subfloor.

You should also consider putting down a layer of building paper, 15-pound weight, on top of the subfloor once it’s installed and you start to place the flooring on top. This paper will prevent squeaky floors, something that everyone hates.

You can also use foam or plastic filament to prevent squeaking floors. Whatever you choose, remember this crucial step, and don’t forget to factor this into your flooring budget as well.

How to Add Subflooring to Your Basement

It can be time-consuming and a bit labor-intensive to install subflooring, no matter which option you happen to choose, but this is still a project that anyone can figure out how to do. With some hard work and effort, you will be able to get this project done on your own.

Work slowly and carefully when placing a subfloor. Remember, the subfloor will support the floor and everything on it. You want to build something that’s going to be strong, so take your time and get this done right. Once the subflooring is placed correctly, it should last for the duration of the home unless serious damage occurs.

Subflooring FAQ

subflooring material

How Long Does It Take To Install Subflooring?

Installing subflooring does take several steps but if you work steadily, it does not take too long to complete this project. You can get 10 to 20 feet covered in a couple of hours, maybe three. You’ll start to work a little faster as you get used to laying out the subflooring.

Depending on the size of your basement, you can knock out a subfloor installation in a few days. If you have help placing the subfloor, you can cut the time of installation down by as much as half.

Work safely, taking frequent water breaks. Make sure you have ventilation while you work and wear protective equipment. Eye protection and good footwear are always essential no matter what type of DIIY project you’re doing.

What’s the Best Subfloor for the Flooring You Want?

Choose your subflooring based on your own DIY skills and budget, along with personal preference.

All subflooring options serve the same purpose of creating a thermal and moisture barrier between your floors and the foundation of the basement. This means that any type of floor can be placed on any type of subfloor, so the choice of which one to choose is entirely up to you.

Can You Install Subflooring Alone?

There are many types of DIY projects you can tackle yourself, but subflooring is a pretty big project. Is this one that you can tackle all by yourself? While it’s always easier to have someone else to help you with any DIY project, especially the big projects, laying subfloor is something that you can do yourself.

It will take longer when you work alone but this can be a strictly solo DY project.

Further Reading

Basement Flooring Solutions For A Dirt Floor

Basement Floors: How To Test Your Floor For Moisture

DIY Flooring Systems & Installation

Flooring Do’s And Don’ts

How To Install Plywood Underlayment

How To Level A Basement Floor

How To Prevent Uneven Floors

How To Repair A Basement Subfloor

How To Replace Floor Joists