How To Fill Gaps In Plywood Subfloor: Here’s How

Our floors are the surfaces that receive the most traffic compared to all the other ones. Because you’re constantly moving across it, it’s easy to notice when something doesn’t feel quite right under your tread. Perhaps you’ve discovered gaps in your subfloor.

Fortunately, you can easily fill large gaps in your plywood subfloor using cement-based caulk, wood strips, shims, or rope. Alternatively, you could also rearrange the floorboards to bridge large gaps and fill the rest using gap fillers. You can also replace the subfloor if push comes to shove.

But how do you fill it? And why does it even happen? Well, the rest of the post will look at those questions exactly.

How To Fill Gaps In Plywood Subfloor

Finding a gap in your subfloor is a common occurrence. While these openings usually don’t have a detrimental effect on your house’s structure and floor’s reliability, they can be annoying once you know about them. 

You want to think about filling the gaps in your subfloor because you run the risk of mold problems or bug infestations. 

Method 1 — Apply Fillers

Filling small gaps in your subfloor is an easy task. You can use various items to seal the hole, especially if it isn’t a big one. 

How To Fill Gaps In Plywood Subfloor: Here’s How

To cover larger areas or lots of minor damage, you can use a cement-based self-leveling compound to cover the floor. It’s simple to handle and can do a quick job of filling your subfloor. 

You can also use caulk to fill minor gaps, especially if your subfloor is in relatively good condition. 

Method 2 — Use Wood Or Other Materials

Filling the gaps in your subfloor before caulking over is another way to approach the situation of uneven floors — albeit not the most aesthetic of the ways you can fix it.

You can use wood strips cut down to size, precut wood shims, or even pieces of rope. You want to ensure that you install these fillers during the warmest part of the day. 

The gaps in the subfloor will be narrower during this time of the day, allowing you to install what is necessary and prevent damaging your subfloor when it’s cooler. 

What’s nice about using wood fillers is that you can easily glue them in place, sand them down to your subfloor level, and stain/paint them the same color as the existing floor. 

If you don’t want to go the natural route, you can also use industrial-grade fillers to fill the gaps in your subfloor — but these won’t be the most aesthetic option. 

Method 3 — Rearrange The Floorboards

A common problem with laying subfloors is that it’s positioned and nailed down without leaving a gap. 

Allowing an ⅛ inch gap between the boards will allow them to expand in varying weather conditions freely. Stacking them too closely could cause uneven pressure, which may cause splintering. 

How To Fill Gaps In Plywood Subfloor: Here’s How

There’s also the risk of leaving too large a gap between the boards. As you use the floor, these gaps can become wider over time. 

A simple solution to both too small and too large gaps is to rearrange the floorboards to have ⅛ inch gap between each board. 

If there’s a gap larger than ⅛ inch after rearranging the boards, you can use method 1 or 2 to fill the remaining gaps. You also want to secure the boards to avoid unnecessary shifting. 

Method 4 — Invest In New Floors

If the gaps in between the floorboards are too large and the boards look pretty battered, it may be wise to consider new floors. 

You can repair the subfloor yourself by following the instructions in this post, or you can hire a contractor to install the new subfloor for you. 

Replacing the subfloor can increase your home’s value, especially if you intend to sell it later on!

Top 5 Reasons You Have Gaps In Your Plywood Subfloor

The following 5 reasons are the most common explanations for people seeing gaps in their plywood subfloor. 

While remedying the problem is simple, installing your subfloors knowing what could cause gapping down the line is best. 

Another reason you want to know the cause of the subfloors gaping is to remedy the problem correctly. 

  1. Your Subfloor Hasn’t Acclimated

Woodis a hygroscopic material — this means that it will naturally absorb and expel moisture in the air. It does this to balance itself with the surrounding environment. 

The time wood takes to adjust to your environment can vary, so it’s a good idea to store your subfloor boards somewhere near your house. In doing so, the boards will have time to get used to their new environment and not cause damage later along the line. 

  1. The Boards Were Left Too Long Without Repair

The subfloor isn’t a space we often see for extended periods. However, when you note a change in the gap between the boards, you want to time your repairs right. 

The number of gaps, cracks, and holes you notice in your floor will help you decide which repair approach to utilize. 

If the damage changes with the weather, you usually don’t have anything significant to worry about — these changes are expected. 

However, if there are large areas of persistent damage, you want to look into ways to fix it. 

You perhaps want to look into hiring a professional to evaluate and confirm your suspicions for either of these observations and maybe even fix it for you. 

  1. Your Plywood Subfloor Didn’t Have Enough Drying Time

Whether you had an accidental leak or you’re just installing the subfloor, you want to ensure that you allow the boards to dry out completely before adding the finishing layer. 

This way, you’ll be able to notice any large gaps that surfaced after the drying process that you can remedy before you add the final layer of flooring. 

  1. You Didn’t Test-Walk Your Subfloor

As silly and simple as it sounds, you want to walk over your entire floor before adding the finishing layer. 

Walking over the existing boards and ensuring that they feel stable under your feet can be super beneficial. 

For one, you’ll be able to feel for any unevenness or gaping and remedy the problem before adding the final layer.

You’ll also be able to listen for any squawking and creaking that may bug you after the fact. 

  1. You Used The Wrong Type Of Nail For The Job

One of the leading causes of uneven, unstable, and squeaky floors is using the wrong nail. 

To avoid the problem of nails popping out and causing an uneven surface, you want to ensure that the nail you use penetrates completely into the floor joists. 

You want to get the right type of nail from your hardware store and make sure to check the package instructions on how to use them before you buy them.


Now that you know exactly how to fill large and small gaps in your plywood subfloor, you’ll be able to fix the subfloor in no time flat. 

Frequently check your subfloors for gaps, and don’t leave the problem for too long before fixing it. 

As a main rule, keep your subfloor dry and nailed down securely to prevent future gaping, and call a professional if you suspect any damage. 

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