How To Fill Large Gaps In Subfloor: 4 Easy Ways

Our floor is the most used surface in the house, that’s why it’s easy to realize when something’s not quite right with it

If you feel something unusual when walking over the floor, or if you recently uncovered it to see what’s the condition underneath, you may have noticed some gaps in the subfloor.

It’s not as concerning as it may appear unless your subfloor is in overall very bad conditions. Large gaps in the subfloor are a common occurrence that can be easily fixed without professional help.

In this article, we will go through the possible causes and solutions available to fill large gaps in the subfloor. Let’s dive in right away!

Is it ok to have gaps in the subfloor?

Generally speaking, it’s fairly common to have gaps in an old subfloor, and they usually don’t change much in terms of the stability and reliability of your floor.

However, the main concern when you have holes in the subfloor is vermin, which not only eat through wood but also leave their excretions in the subfloor

In the long run, the subfloor becomes less and less stable and your house starts smelling unpleasant.

Another problem with having gaps in the subfloor is the mold, especially if you live in a very humid area. 

Molds multiply in the darkness, where there’s no air circulation, so the underlayment of house floors is their favorite place to be. When mold starts spreading far and wide, they also release a foul smell.

Filling subfloor gaps won’t guarantee that you will never have mold problems, but it can greatly reduce the possibility. It will also prevent the infestation of vermin.

Whether you installed a new subfloor recently or you’re thinking of uncovering an existent one, it’s useful to know what could have caused the large gaps in your subfloor before you proceed and fix it.

1. Your subfloor didn’t have time to acclimate

When you want to install wood subflooring, it’s very important to let the material acclimate to your home first.

That’s why before you even think of laying a sub-layer in your home, you should store the boards somewhere safe for a few days and let the material get used to the environment.

If you want to be extra careful, you could also measure the level of humidity in your home. That way, you will know what to expect when you install your wood subflooring since this material is extremely sensitive to moisture.

2. You waited too long to do repairs

When it comes to gaps between plywood subfloors, timing is essential. You should keep an eye on the condition of your flooring and note every change in it.

The number of gaps/cracks and their size will determine how the floor feels under your feet, and most of all the proper way to repair them.

Usually, if you notice that these flaws appear and seemingly disappear with changing weather conditions, it’s fairly normal and there should be nothing to worry about.

However, if you’re concerned, you should make sure to ask for professional advice and see if there’s anything to be done about your floor.

3. Subfloor didn’t dry out properly

Whether you’re using plywood boards or OSB (Oriented strand boards) to lay down your subfloor, remember to let them dry before you add your finishing layer.

This way, you can have enough time to check if the dried-out boards display any changes and do the required fixes before applying the cover.

4. Wrong nails

Improper nailing is one of the main causes of the squeaks and noises you might hear when walking over your brand new flooring.

To avoid all the common problems of improper nailings, such as nails pulling out, popping, or just barely hitting the floor joists, you need to make sure the nails penetrate completely into the joists.

Since different types of nails and flooring might require different installation, make sure to check nail-related instructions on the construction drawings

5. You didn’t check the subfloor before finishing

This might be an easy trick, but also one you never thought about before. Inspecting the subfloor before applying the final layer could help you avoid many problems in the long run.

Walking over your fully dried subfloor to check for improper nailing, squeaks or slumps is the best way to make sure your soon-to-be floor is perfectly stable, stiff, and resistant.

Many people wonder whether is it safe to walk over your subfloor. Not only it’s absolutely safe, but if you cannot walk over your subfloor, it means your house will never be safe to walk around.

How to fix large gaps in the subfloor

The best way to fix gaps in the subfloor is to prevent them. This might seem obvious, but once you cover up everything, it becomes impossible to check on your subfloor or to become immediately aware of any changes or small damages.

However, it’s not always possible to prevent this type of issue in your home. Luckily, it’s something that can be fixed with a few useful tips.

Whether you’ve installed a new subfloor or you’ve uncovered the one you bought with your home, you might find out that there are big gaps in the subfloor that could potentially become a problem in the long run. Follow our advice to fix the problem right away.

1. Move the floorboards

Some people will tell you to stack the floorboards as close to each other as possible when you lay down your subfloor layer. However, this isn’t exactly true.

The first thing you need to know when you do this kind of housework is that there should be a ⅛-inch gap at all ends and edges to ensure that your subfloor has enough space to expand in different weather conditions.

All the same, leaving too much space around the boards could result in gaps between the boards that only grow wider and wider in time.

The easiest fix you can do in order to fill large gaps in the subfloor is to move the floorboards close to each other and see how much space is left around the edges. If it’s over ⅛-inch you should add some filler to reach the desired gap.

If you lay a new subfloor, all you need to do is butt the floorboards together and fast them to make sure they cannot move away from one another.

2. Use fillers

There are several types of products you can use to fill holes in the subfloor. Keep in mind that when the subfloor is heavily damaged or there are too many spots to fix, it might be better to replace it entirely.

We recommend the use of specific products to fill a small hole in the subfloor, or larger gaps as long as there’s just a handful of them.

When it comes to subfloor fillers, you usually have two options: 

  1. For spot filling, it’s better to use the ever-popular Fix-It-All, which allows you to reach a stiff consistency with less effort, and it’s perfect for multiple applications on wood.
  2. To fill larger areas, you should use a cement-based leveling compound, which is easier to work with and has a longer open time.

Another option would be to use caulk, but this material works better with small gaps or in general with subfloors that are in good conditions, to begin with. 

The advantage of caulk is that it prevents the gaps from widening, and it’s also really easy to find and apply.

3. Use strips of wood or other materials

Using fillers is perfect to fill gaps in plywood subfloor or OSB subfloors, but it’s certainly not the most aesthetic way to fix your floor.

If you wish for something as efficient, but more natural and permanent, you can use other materials such as wood strips, wood shims, or ropes as a subfloor gap filler.

In case you decide to go with this type of installation, make sure to do so when the weather is most humid. The gaps will be smaller and you will avoid damages to your repairs when the boards expand next time.

You can also add glue to ensure more security, sand them and paint them like the rest of the subfloor as a finishing touch.

4. Replace floorboards

It’s not always possible or wise to repair your subfloor. Sometimes the problem isn’t limited to filling the gaps, as there are small cracks all over the surface as well.

Even if it’s possible to fill them, you should consider whether this kind of work is worth your time and money.

If your subfloor is not in overall good conditions, filling the gaps and repairing the cracks is just postponing the inevitable.

Replacing your floorboards and installing a new subfloor could be an investment for the longevity of your home. On top of that, if you’re planning to sell your house in the future, it could also raise its overall value.

F. A. Q.

How do you fill a low spot subfloor?

If you want to fix a low spot subfloor you will need specific products made for underlayments, such as a floor leveler or a floor patch. 

A floor leveler is a cement-based product that either comes pre-mixed or can be mixed with water; a floor patch is usually used to fill small holes, gaps, and cracks.

You can apply these products as you would with any other subfloor filler. Make sure to let it dry overnight and sand down any imperfections or high spots. 

How big a gap can you fill with caulk?

Generally speaking, a bead of caulk can fill ¼-inch gaps. In case the gap is bigger, push the caulk deeper into it and don’t flush it. Wait until the caulk is completely dry, then add another bead on top of it.

However, for much larger gaps we would recommend using a different filler.

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