How To Add An Extra Layer Of Plywood Over Subfloor In 5 Easy Steps

When doing the flooring in your house, whether you’re remodeling it or doing a brand new floor, it is important to think about stability and security. Of course, you’ll want to choose quality materials that will last and complement your interior.

However, the emphasis should be on the foundation you lay the flooring on, as it will determine all the above factors. Without a quality subfloor design, you may experience many issues in your flooring, both functional and aesthetic.

Adding an extra layer of plywood over the subfloor is the best way to create a secure foundation for your flooring. 

Why add an extra layer of plywood over a subfloor?

Before you decide to take this step and install plywood over the subfloor, you may need some more information about the main function of plywood actually in regards to floor safety and functionality.

The main role of plywood in this scenario is to provide a perfect level foundation for your flooring – no matter what kind of flooring you go for. Oftentimes, the subfloor you already have isn’t a good foundation to lay your flooring, which can create many issues.

Not only can an uneven subfloor give you many headaches while installing the flooring, but the problems may as well continue once the flooring is all set up. No matter how good the flooring installation was, it is the quality of the foundation that makes all the difference.

When you think about it, any project requires a proper base above anything else. An adequate base allows for the later stages, including the installation of flooring, to be executed perfectly.

Plywood is essentially a building material made from three or more layers of wood (so-called veneers) glued together. Since it is quite stiff and strong, many people choose it to fortify their subfloor prior to the flooring installation. 

Another advantage of plywood is that it can absorb impacts, which is particularly important for flooring foundations. It usually comes in the tongue-and-groove design which is an excellent choice for the subfloor. 

Basically, plywood is assembled by fitting the tongue side of one panel into the groove side of the other panel. This kind of connecting mechanism provides extra security, and it is much more reliable than simply butting the plywood panels together. 

However, you can also use plywood sheets that don’t have the tongue-and-groove mechanism. A more important factor when choosing your plywood is the quality grade.

For subfloor fortifying, you want to choose a sturdy, stiff kind of plywood, so the actual aesthetics of it doesn’t make much of a difference. 

We’re only pointing this out because A-grade plywood, even though it is the highest quality, is commonly used for furniture and not subflooring. This is essentially better-looking wood which is often sanded.  

Depending on where you’re going to use the plywood, as well as other important factors, there are four kinds of plywood to choose from:

  1. Interior plywood is an excellent choice for a base for floorings such as carpeting, vinyl, tiles, hardwood, and laminate flooring. It is meant for spaces that aren’t exposed to much moisture, but you can also use it in kitchens and bathrooms.
  2. Exterior plywood is a safer choice for kitchens and bathrooms since it is designed specifically for rooms with extra moisture. This is a safe bet for floors that you know will get wet often.
  3. Marine plywood, as the name itself implies, is designed for wet surfaces constantly exposed to moisture. However, this kind of water-resistant plywood is almost never used in homes, as it is more suitable for yachts, boats, etc.
  4. Structural plywood provides more structure and stability than any other type of plywood, and it is designed to endure heavy loads. Due to its specific design, it is rarely used in traditional homes, and it is more suitable for factories and such spaces.

How does subfloor leveling work?

Before laying plywood over a subfloor, it is also important to understand the structure of your floor and the purpose of each layer and element. This will also help simplify the process of the plywood installation later on.

Even though they may seem like flat floor layers, joists are in fact structural pieces, and they support the entire weight of the floor. Made from dimensional lumber, they are necessary to support the floor, unless the floor is a concrete slab.

You can install plywood subfloor over joists, but if there’s a concrete slab, we consider the slab itself a subfloor. This is the base of the house and the flat surface on top of which we install all of the other floorings.

In case you want to add some extra protection to your subfloor, since it has to endure the weight of your flooring and furniture, it is fortified with plywood. The subfloor can also be made of planks or OSB (oriented strand boards). 

The layer that goes on top of the subfloor is underlayment, and it isn’t a structural element, which means it won’t add any extra security to your floor. However, the purpose of the underlayment is to make the surface smooth and flat for the next layer.

Different types of flooring require different kinds of underlayment. For hardwood floors, you want to use plywood underlayment, while stone and tile flooring requires a cement backer board. Laminate, on the other hand, will look great on top of a thin layer of foam. 

The final layer is the floor covering, which is entirely up to you. There are many designs and varieties you can choose from, depending on your preferences and the overall interior design of the house. The purpose of this floor element is purely aesthetic. 

Floor covering can be carpet, laminate, hardwood, tile, parquet, vinyl, or anything else you desire. However, make sure the floor covering you choose is of high quality, in addition to being aesthetically pleasing. 

How to install plywood over an existing subfloor

In this article, we’re focusing on adding plywood to the subfloor that you already have in your house. As we’ve explained, this step is crucial in making your floors durable, sturdy, and ready for any kind of load such as furniture. 

Before we go into the actual process, it is necessary to gather all the required tools and materials, so here’s a quick cheat sheet:

  1. Tape measure;
  2. Safety goggles;
  3. Circular saw (or any other saw that you find suitable for wood cutting; keep in mind that plywood isn’t hard to cut and this step shouldn’t be too complicated);
  4. Galvanized staples;
  5. Pneumatic stapler.

Once you’ve gathered all the necessary tools, you can proceed with adding an extra layer of plywood over the subfloor by following the steps below.

1. Prepare the underlayment

Underlayment requires some acclimation, so you will have to store it in the room where it will be installed 72 hours before actually installing it. This is a necessary step that will help the underlayment adapt to room temperature. 

In case you neglect this step, the underlayment could expand after flooring installation and ruin all your effort. Therefore, make sure to keep it in the same room where it will be laid 72 hours before you install the flooring. 

2. Prepare the subfloor

Before adding an extra layer of plywood, whether you’re adding ½ plywood to the subfloor or any other width, it is necessary to prepare the surface by cleaning it. Often there will be a lot of dust and grime on top of the subfloor that you want to remove thoroughly. 

A clean surface is not only easier to work on, but it also ensures a clean application and a flat surface later on. Depending on the amount of dust, you may want to use a shop vac, but for some cases, a broom will do the trick. 

However, if you notice that the broom isn’t picking up all the dust and it is flying around, you may want to mop the area. A mop will trap the dust and leave a clean, smooth surface for you to work on.

3. Plywood installation

Once you’ve prepared the surface, you can start installing the plywood, which is also the most important stage of this process – so make sure you take your time with this. 

First, you want to lay the plywood underlayment sheets one by one, making sure they’re positioned horizontally to the subfloor sheets. You’ll know the placement is correct if the plywood seams are meeting over the subfloor joints. 

4. Place the staples

Now, you will secure the boards into place by adding staples every two inches all around the edge of the plywood sheet. Also, the staples should be added every four inches on the interior. 

In case you’re using screws or nails instead of staples to secure your plywood sheets, make sure to position them farther apart, and slightly below the top of the plywood sheet. 

5. Finishing touches

Finally, you should fill the gaps between the plywood sheets, and you can do so by using a seam filler. This step should be completed before adding the top layer. 

F. A. Q.

1. Can I add a second layer of plywood?

In case you’re looking to add a second layer of plywood for extra security, it is necessary to let the first layer on the subfloor dry completely. The second layer of plywood would actually serve as an underlayment. 

In fact, some people choose plywood as an underlayment in the first place, so this isn’t an exception. However, when installing one layer of plywood on top of the other, remember that the top one should be positioned horizontally to the layer below. 

In addition, you should not be using glue to bind the two. Wood screws will do just fine!

2. Can old floor covering be used as underlayment?

Yes, in theory, this can be done. However, it is necessary to ensure that the old floor covering is in good shape, and it is smooth enough to act as an underlayment.

Remember that underlayment has to be a smooth, firm surface for the floor covering to come on top.

Also, you should never use carpet floor covering as an underlayment, as it won’t provide the necessary structure, and it isn’t water-resistant.

3. Do I need to hire a professional to add plywood over the subfloor?

No, this isn’t a project that strictly requires a professional. As long as you have some experience in similar DIY house repair tasks, this shouldn’t be an issue for you. Also, make sure you follow every single step of our guide.

However, if you have no experience dealing with flooring, or you’re facing more serious issues, such as floor rotting, it is better to rely on a professional to do the job. After all, flooring is something that you want to be done perfectly, as you’ll use it every single day.

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