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How Thick Should Subfloor Be?

The subfloor provides a structurally sound surface to hold the flooring material. But how thick should it be? We researched the topic to give you an answer.

Disclosure: We may get commissions for purchases made through links in this post.

The minimum thickness for plywood subflooring is 5/8 inches, while that of the Oriented Strand Board (OSB) is 23/32 inches. The thickness requirements increase to at least 7/8 inches for plywood subflooring and 1 inch for OSB subflooring as joist spacing increases.

Concrete slab subflooring in residential properties should be at least 4 inches thick and about 5 or 6 inches in areas where heavy vehicles will frequent. Also, note that the choice of underlayment influences the subfloor's thickness.

Do all subfloor materials have exact thickness requirements? What are the characteristics of a sound subfloor? Find the answers to these questions when you keep reading. 

Skilled worker preparing the subfloor base using a hand tool, How Thick Should Subfloor Be?

What Factors Determine Subfloor Thickness?

The different layers in your flooring system work together to make your home structurally sound. You want to ensure that the floor has sufficient load capacity to support its functions.

A high floor load capacity ensures that the floor can support its weight, the weight of fixtures attached to the structure (such as walls, beams, and roofs), other items inside the house (like appliances and furniture), and people.

Typically, the flooring system comprises joists,  subfloor, underlayment, and floor covering. The joists form the foundational flooring layer - they help to distribute the structure's load (weight acting on the floor).

It is important to note that some floors, such as concrete floors, do not have joists. Instead, they have a base made of crushed stone material. This base promotes drainage and absorbs the soil's differential movement, thus anchoring the flooring system.

Builders then install the subfloor as the middle layer. Sometimes, they add an underlayment before installing the floor finish.

The subfloor influences the floor's longevity and how well it functions. It is also your home's first line of defense against the moisture damage that comes from beneath the house.

These factors will influence subfloor thickness:

Joist Spacing

Joist spacing influences the building's structural integrity. So, to provide adequate load support, the further apart the joists are,  the thicker the subfloor. 

Below are subfloor thickness guidelines for different materials at a given joist spacing.

Plywood Subfloor

If you are installing plywood subflooring in a single-story building with 16-inch joist spacing, ensure that it is at least 5/8 inches thick.

But if the joist spacing ranges between 16 and 19.2 inches on the center, use plywood with a 3/4-inch minimum thickness. If the joist spacing exceeds 19.2 inches on the center, the plywood's minimum thickness should be 7/8 inches.

Oriented Strand Board (OSB)

OSB subfloors do not hold fasteners as well as plywood in mechanically fastened floors. So, the OSB subfloor should be thicker than the plywood subfloor for the same joist spacing. 

The minimum thickness for the OSB subfloor for joists 16 inches on the center is 23/32 inches. This thickness increases to 3/4 inches if the joist spacing increases to measurements above 16 inches but below 19.2 inches on the center.

However, if the joist spacing is more than 19.2 inches on the center, it is best to use an OSB subfloor that measures at least 1 inch.

Concrete Slab

Hand builder with trowel leveling concrete, spreading poured

If you are installing concrete slab subfloors on your residential property, ensure that it is at least 4 inches thick. However, if the concrete will be subject to heavy vehicles or motor homes, it is best to increase the slab's thickness by one or two inches.

Choice Of Underlayment

The underlayment is a smooth surface on which the finished floor often sits. It provides an extra line of defense for your flooring against moisture damage.

Generally, floor finish manufacturers specify the underlayment their products will need.

You can use underlayment to meet sound insulation properties for multi-floor buildings. Furthermore, the choice of subfloor material influences your floor's heat insulation properties.

You want to install a flooring system with good insulation properties to minimize heat loss through the floor, thus reducing your home's heating expenditure.

You can substitute subfloor thickness for underlayment membranes with high insulation properties. That said, you must meet the minimum subfloor requirements. 

What Are The Characteristics Of A Good Subfloor?

The subfloor significantly influences the final floor installation and outcome. You want to avoid delays in the completion of the project or incurring unnecessary repair costs. So, it is best to do a thorough job when installing the subfloor for a higher chance of successful floor installation. 

Here are some traits to help you to identify an ideal subfloor.

Clean

Dirt compromises the ability of the underlayment or floor finish to adhere to the subfloor. So, ensure that the subfloor is free of contaminants such as dirt, dust, curing agents, paint, and old adhesives.

Male hand applying wood care products and cleaners on hardwood floor surface.

How To Clean A Subfloor

Since most subfloors are made from wood, avoid soaking them in water as it can cause rotting. Work in small sections as you follow the below steps, regularly drying the areas you clean on the go.

  1. Sweep thoroughly or vacuum to eliminate the loose dirt and dust.
  2. Scrap off paint, glue, or grease using a floor scrapper or putty knife. You can also use a non-flammable paint stripper to eliminate any stubborn stains.
  3. Make a 1:1 water and vinegar solution or a 1:3 bleach and water mixture.
  4. Dip the tips of a stiff broom into the solution and scrub the subfloor.
  5. Use a dry mop or old towels to dry the cleaned section.
  6. Repeat steps 4 and 5 to clean the entire subfloor.

See this scraper on Amazon.

You do not need to dry the concrete slab subfloor when cleaning it. Instead, use a rotary floor scrubber or a wire brush to scrub, then rinse it thoroughly with clean water.

Next, use an industrial wet vacuum to vacuum - it helps the concrete dry faster and ensures you've done a thorough cleaning job.

Dry

Modern living room with parquet flooring and stylish furniture

The subfloor should be free from moisture before you can proceed to install the underlayment or floor finishing.

Otherwise, the excessive moisture trapped in the subfloor can wreak havoc on your structure. In addition, it creates a breeding ground for mold and mildew, which may result in negative health implications for you and those in your household.

How To Test The Moisture Content In Plywood And OSB Subfloors

Before proceeding with floor installation, test the subfloor for moisture content. You can use a moisture meter to test the moisture content in plywood or OSB subfloors.

Take at least 20 readings per 1000 square feet of subfloor and accurately document them. You can also include the date you capture the moisture content readings in your notes.

These records may come in handy as proof that you followed the correct procedure in case a problem arises later.

If you intend to install solid strip flooring, ensure that the difference in moisture content between the flooring and the subfloor does not exceed 4%. But if you are using solid plank floorings that are thicker than 3 inches, ensure that the difference in the moisture content does not exceed 2%.

You can use this moisture meter on hardwood, softwood, drywall, and masonry. Find it on Amazon.

How To Test The Moisture Content In Concrete Slab Subfloors

Concrete subfloors are prone to moisture problems that may arise from the soil beneath them or a problematic concrete mix. So, it is prudent to put in place measures that can mitigate the impact of moisture vapor transmission on your flooring.

If you intend to install solid wood flooring that measures 3/4 inches over a concrete slab subfloor, ensure that you include a vapor retardant during installation.

Also, carefully choose the floor finish to install on concrete slabs to ensure that it has the qualities and characteristics that guarantee good performance. 

Allow the concrete slab to cure for at least 30 days, then test for moisture. Some ways to test the slab's moisture content include calcium chloride (salt) or relative humidity tests.

Calcium Chloride Probe

The calcium chloride test can help you determine how much moisture a 1000-square-foot concrete slab emits. It entails placing a small dish of calcium chloride on a clean concrete slab, covering it with a plastic dome, and leaving it for about 60 to 72 hours.

You then measure the calcium chloride's weight gain and use that to calculate the moisture vapor emission rate (MVER).

Relative Humidity Test

The relative humidity in-situ method can determine a concrete slab's moisture content. When using this method, drill test holes that extend to about 40% of the slab's thickness, place small probes into the holes, and leave them for some time, then check the readings.  

Because the calcium chloride test only determines the slab surface conditions, it is not the most reliable. It is better to use the relative humidity test as it tests the moisture content within the slab.

Also, since different slab sections can dry unevenly, it is recommended to test different spots to get relatively reliable readings. 

If the slab's moisture content measures 4.5% or higher, it is not ready for flooring installation. You may need to wait longer for the slab to dry further and reach desirable levels that will not result in unwanted moisture problems.

Flat

man worker hands installing timber laminate floor. Precise finishing using spirit level

The subfloor should be flat to avoid the chances of your floor splitting or cracking in the future. Please use a spirit level to determine whether the subfloor is level, then apply necessary corrective measures, if needed, before installing the rest of the flooring.

See this Amazon Basics Torpedo level and ruler on Amazon.

Wrapping Up

Skilled worker preparing the subfloor base using a hand tool

We hope that this post has provided you with a basic understanding of subfloors and that you are now better placed to choose the ideal subfloor thickness for your home.

But supposing you are not confident proceeding with the new installation or floor replacement. In that case, it is best to consult an experienced contractor who can walk with you toward ensuring a successful installation.

Before you go, consider reading our other posts:

Does A Hardwood Floor Need Underlayment Or Subfloor?

How To Find Floor Joists Under Hardwood [Various Methods To Try]