How to Install Plywood Over a Subfloor

When you are installing a finished flooring like hardwood or carpeting, you will need an underlayment.

Underlayment smooths out imperfections in the subfloor and gives you a solid base to lay flooring onto. By adding this layer, you can be sure that your finished flooring will be level and even.

Underlayment can also help protect the finished flooring from moisture, mold, and mildew.

Plywood is a great underlayment to install over a subfloor. It is strong and durable and often very smooth without a lot of imperfections or knots.

In this article, we have laid out the steps to installing plywood over a subfloor. By following these steps, you can get your flooring ready for its final form.

Key Takeaways

Here are the basic steps:

Choose plywood made from multiple strands if possible.
Remove the molding and baseboards and remember where they went.
Sweep and dust your subfloor and remove moisture. Check for cracks.
Bring plywood into the room so it can acclimatize. Leave for 72 hours to avoid swollen flooring.
Lay the plywood on the subfloor with screws 18 inches apart.
Leave around an eighth-inch between the wall and underlayment.

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Installing Plywood Over a Subfloor: Basic Idea

Installing plywood over a subfloor is relatively simple and doesn’t require much expertise. However, there are a few steps that you should follow to make sure it’s done properly.

These steps are:

Remove baseboards and other trim
Prepare the subfloor
Acclimatize the plywood underlayment
Lay down the underlayment

Best Type of Plywood for Installing Over a Subfloor

There are a few different types of plywood and you will have to choose one for your underlayment. Knowing what these different types are made of and how they perform can help you make your decision.

Most higher-end plywoods are used for things like cabinet making and other carpentry applications. While this will be the best, most durable type, it may not be cost-effective.

Instead, you can use plywood that has been put together with several different strands of wood. These strands are glued or waxed together and then pressed into boards.

This type is called OSB, or oriented strand board. While it is not as pretty as standard plywood, it is still fairly strong and will make a good underlayment.

Since you are not going to be seeing this underlayment, it doesn’t really matter what it looks like. You’ll be better off saving a few dollars and using OSB, then installing your finished floor.

Remove Baseboards and Moulding

Your first step to installing plywood over your subfloor is to remove any pieces of trim. Depending on how high your plywood is, you may have to reinstall these pieces.

For example, your baseboards may have to be reinstalled ¾” higher than they were. This is to accommodate the height of the plywood underlayment.

Removing these pieces will also give you more space to work when installing. It’s important that you are able to reach all the edges and make sure the underlayment is flush.

Be sure to remember which trim and moulding pieces go where. This will help you when you go to reinstall them.

Prepare the Subfloor

After you have removed all the trim pieces, you will have to prepare your subfloor.

Generally, this means sweeping and making sure that all dust and debris is removed. If this isn’t done, you could end up with an uneven underlayment that isn’t level with the subfloor.

You should also take the time to make sure that there is no moisture that has seeped up.

Sometimes, groundwater can make its way through the subfloor over time. Make sure any moisture is dry before installing the underlayment.

Check for any cracks or broken planks in the subfloor. Make these repairs before you move on so your underlayment has a solid base to attach to.

Acclimatize the Plywood

After you have prepared the subfloor, you should acclimatize your plywood. This means bringing the wood into the room and letting it get used to the climate.

This is an all-important step. Because plywood is sensitive to things like moisture and temperature, it tends to change with fluctuations.

Bringing the plywood into the room where it will be laid down will help it get used to the atmosphere.

Leave the plywood in the room for at least 72 hours. This will give it enough time to warp as needed and get in its final state.

If this step is ignored, you could end up with cracked or swollen flooring after installation.

Lay Down the Plywood

Now that the wood is acclimatized, you can install it.

Secure the plywood on the subfloor using wood screws. Place these screws no more than 18 inches apart to keep it from warping.

You should also make sure that you are leaving about ⅛” between the underlayment and the wall. This will give the wood room to breathe if it needs to and prevent the finished flooring from warping.

After your underlayment is installed, you are ready to lay down your carpeting, tile, or hardwood flooring.

F.A.Q.

Is MDF stronger than plywood?

MDF is not stronger than plywood. MDF is made of many particles that are pressed together.

Plywood, on the other hand, is made of long sheets that have a more cohesive structure.

How can I waterproof plywood?

You can waterproof plywood by sealing it with an epoxy sealant.

While you don’t need to do this if you are using plywood as an underlayment, you absolutely can. If nothing else, it will help block moisture from coming up through the top or getting underneath.

Is it better to nail or screw a subfloor?

It is always better to screw in a subfloor. Nails do not stand up to warping very well.

Screws give a better hold and your local building codes may even require their use.

Conclusion

Installing a plywood underlayment over a subfloor is a great way to give your flooring a solid base. It can give you many more years of longevity with your floor.

By following the above steps, you can be sure it’s done correctly and won’t warp or crack over time.