Everything Subfloor: What You Need To Know Before DIY A Sub Floor

The subfloor is the foundation of a floor and provides support to the materials that are installed on top. It should be prepared with care, made out of durable material, and properly installed before any other flooring is added. If this isn’t done correctly, the homeowner will have issues like squeaky floors or rooms where water pools after it rains. This blog post will discuss everything you need to know about your subfloor!

The proper preparation and installation of your subfloors will ensure the longevity and functionality of your new flooring.

In most cases, homeowners do not realize how important a good subfloor is and the extensive preparation required to determine its integrity.

To ensure the overall performance of your new floors, it is crucial to ensure the subfloor is properly prepared.

Subfloor preparation is critical to the success of any home improvement project. The following are some factors that are important to prepare a subfloor. But first, let’s define what exactly a subfloor is.

 What Is A Sub Floor?

In a building, a subfloor is the foundation floor. 

Concrete and plywood are both common types of subflooring. The subfloor of basements is typically made of cement, while the subfloor of aboveground construction is typically made of plywood or structural grade particleboard.

In above-ground level floors, a subfloor is generally plywood or structural particleboard (in Australia & New Zealand). It provides an even, stable surface for your new flooring to be installed on top of. It also creates a space for insulation between the joists or rafters below which can help keep any air from getting trapped in there.

If someone is installing new flooring directly on top of old flooring, without ripping up the old flooring, the old flooring can also be referred to as a subfloor.  

If you’re getting new flooring, you will want to know what type of subfloor is under your existing floor. If it’s plywood or particleboard, they’ll need that information so they can plan how much material and time are needed for installation.

There are a variety of factors to consider before installing new floors directly over old ones though. Putting a new floor over an old one can cause a variety of problems, including the appearance of the flooring, appliances that may not fit, and doors that cannot swing freely.

Also, if someone has old hardwood floors on an uneven subfloor, it may be a good idea to rip up the old flooring and install new plywood or particleboard subflooring.

Subflooring: Why Is It Important?

How important is subflooring? Answer: Because a floor can only be as good as its subfloor, it is very important. In direct relation to the flooring, the subfloor is a critical component, so if the subfloor is defective, the flooring will become underperforming or failing.

Not having a properly flat and dry subfloor, not being properly cleaned, not being structurally sound or otherwise faulty in any way will lead to a host of unwanted consequences, ranging from:

• High spots are excessively worn out
• Spots that are uneven
• Lifespan is reduced
• Surface irregularities that are unsightly
• There is squeaking within the flooring system
• There is sagging in some areas of the flooring system
• Excessive shifting and movement
• A decrease in comfort when walking on the floor
• Damage to the flooring system
• The separation between the subfloor and top flooring material

If left unattended, these issues can worsen over time and, at worst, lead to the need for flooring repair or replacement.

There are a number of ways to avoid these problems, but the most effective way is through comprehensive subfloor preparation. It should be noted that this can take up to two days and will require some tools such as: chisels, crowbars or hammers (to remove old flooring), drills (for preparing holes for new screws), and a saw (for cutting the subfloor).

Subfloor Preparation Requirements

The subfloor will need some degree of preparation, regardless of the type of flooring you select. Depending on the installation, there will be specific preparations to make.

Before a subfloor can be used as a base, several considerations must be addressed:

• Thorough cleaning and removal of all dust and debris is performed
• Flat (level within 5 millimeters for every 3 meters)
• Good structural integrity
• In order to maintain an even surface, grout lines must be filled in if tile is used as the subfloor
• It is imperative to sand or repair plywood subfloors with cement compound in order to ensure a level surface
• Minor irregularities can be covered with underlayment, but it is not a fix for major problems
• Make sure that no nails or screws are loose
• The subfloor is completely dry

Questions To Ask Before Installing New Floors

Several questions need to be addressed before installation:

• What kind of flooring will be installed?
• Is it a remodel or a new construction?
• Has the pH and moisture of the subfloor been tested?
• Are you working on a commercial project or a residential one?
• Do you have a level, dry, smooth, and clean subfloor?
• Which subfloor does the current floor rest on?
• How was the compound or sealer applied?
• Has there been a crawl space built?

The following is merely a general overview of some of the most important aspects of subfloor preparation.

Other Considerations

It is also important to take into account “grade levels” when evaluating the integrity of a subfloor. A home is divided into several levels:

• Floors above ground level, such as floors two and three
• Ground floor – ground level of the building
• A basement is a floor beneath the ground level (below grade).

Different aspects have different effects on each of these levels. An on-grade installation, for example, should take into account the ventilation in the basement or crawlspace.

For a ground-level subfloor, the ground floor is the surface on which the subfloor will rest. A concrete slab or wood flooring should be considered for this area, but it must be level and solid before installing a new piece of plywood or carpet.

If you are building a crawl space beneath your home, you need to consider adding ventilation in order to prevent any problems with humidity.

The same is true for a basement, but you must also consider drainage and how to prevent water from pooling in the area where your subfloor will be installed.

Below-grade installations are plagued by moisture problems.

Ground-level floors are less prone to moisture damage, so an on-grade installation should work well for this type of surface.

For above-ground installations, the subfloor must be able to bear the weight of the flooring before an installation can be done.

 Can An Owner Handle Subfloor Prep?

Can I do subfloor prep myself? The answer depends on the specific project, the type of flooring, how much experience an owner has, and many other factors. For example, for some simple kind of subfloor installations, an owner might be able to prepare a subfloor by laying down a vapor barrier and adding insulation etc. But for many installations, an owner will need to hire someone who has the proper tools and experience to do it correctly.

If you have any questions about subfloor preparation, you should consult a professional. A growing trend is for homeowners to install their own flooring, but it is important to get expert assistance and knowledge before embarking on the DIY method.

What are the benefits of hiring a professional? Hiring a qualified subfloor installer can help to ensure that your floors will last longer. For example, an experienced contractor may be able to treat wood with chemicals or sealants in order to protect it from moisture and pests. They might also install underlayment for carpeting installation, which provides a barrier between the carpet and wood that prevents stains.

Removing Old Subflooring

If you are going to DIY, or at least do some of it yourself, you are going to need to start by removing all of the old flooring.

This can be done by prying up any nails with a crowbar or hammer, using an electric screwdriver for screws, and pulling out staples with pliers or other tools. When this has been completed, it’s time to get to work on the subfloor.

 First, you will need a good pry bar and heavy hammer for wood floors or a crowbar for vinyl flooring that needs to be removed. Make sure your method is appropriate before proceeding! Next, break up any boards with nails in them by using the screwdriver and pry bar. Then, use the crowbar to remove any nails in solid flooring or pull up boards with staples by first using a flathead screwdriver to get under it.

 Finally, sweep up all of the debris from your workspace and vacuum thoroughly before applying new subflooring.

The cost for this process varies depending on the type of flooring you are preparing for, but getting rid of the old subflooring yourself is at least something in reach of the average do-it-yourself person, before calling in the professionals to proceed with putting in the new subfloor.

Conclusion

Skipping subfloor preparation can lead to costly and frustrating problems in the future if you don’t pay attention to it.

When prepping your subfloor, there are many factors to take into consideration, and that’s where professional experience comes in. It takes years of experience, industry-specific knowledge, and a thorough understanding of subflooring to prepare a subfloor correctly, so proceed with caution if you want to do it yourself.

FAQ

Q: What’s the best way to build a subfloor on concrete slab with vapor issues?

A: For concrete slabs with moisture problems, start by installing a proper vapor barrier directly on the clean concrete. Then create a sleeper system using pressure treated 2×4’s laid flat and spaced 16 inches on center. Secure these to the concrete using concrete fasteners. Install rigid foam board insulation between the sleepers, and top with 3/4-inch tongue-and-groove plywood or OSB, ensuring you stagger the joints and leave a 1/8-inch gap at the perimeter for expansion. Use subfloor adhesive between the sleepers and plywood for added stability. This approach creates a moisture-resistant subfloor system that will protect your finish floor from potential moisture damage.

Q: How do I fix a squeaky plywood subflooring?

A: To fix squeaky plywood subflooring, first identify where the squeak is coming from. Typically, squeaks occur when the subfloor has separated from the floor joists beneath. The best approach is to reattach the subfloor to the joists using screws (not nails) and construction adhesive. If you have access from below, apply adhesive to the gap between the joist and subfloor, then drive screws up through the joist into the subfloor. If you can only work from above, use special squeak-elimination screws designed to break off below the surface after installation. For particularly troublesome areas, you might need to remove the finish floor to properly secure the subfloor to the joists. Address any squeaks prior to installation of new flooring for best results.

Q: What subfloor material should I use over floor joists for different finish floor types?

A: The best subfloor material depends on your finish floor type. For most applications, 3/4-inch tongue-and-groove plywood or OSB (oriented strand board) is the standard choice. High-performance engineered wood products like Advantech provide superior moisture resistance and stability. For tile installations, use 3/4-inch plywood with a 1/4-inch cement backer board on top. For hardwood floors, 3/4-inch tongue-and-groove plywood is preferable. For vinyl, LVT, or laminate, ensure your subfloor is perfectly smooth; you may need to add a layer of 1/4-inch plywood to create a suitable surface. The subfloor must always be flat, stable, and appropriate thickness to support your specific finish floor. As a best practice, check your finish floor manufacturer’s recommendations for specific subfloor requirements.

Q: How do I install a proper vapor barrier under my subfloor?

A: Installing a proper vapor barrier begins with choosing the right material—typically 6-mil polyethylene sheeting for most applications. On concrete slabs, clean the surface thoroughly and repair any cracks prior to installation. Roll out the vapor barrier with seams overlapped by at least 6 inches and sealed with appropriate tape. The barrier should extend up the walls about 2 inches. For wood joist systems, the vapor barrier should be installed on the warm side of the insulation (typically the top). When using foam board insulation, remember that closed-cell foam acts as its own vapor barrier. Always ensure the entire floor area is covered without gaps, and take care not to puncture the barrier during installation. Some systems may require a specialized vapor barrier like a dimpled membrane or products with built-in vapor protection, especially in areas with severe moisture issues.

Q: What’s the proper way to fasten plywood subflooring to floor joists?

A: To properly fasten plywood subflooring to floor joists, start by applying a continuous bead of subfloor adhesive along the top of each joist where the plywood will make contact. Use exterior-grade construction adhesive for this purpose. For the fasteners, 2-inch to 2½-inch screws are preferable to nails as they resist pulling out. Space fasteners 8 inches apart along the edges of the sheets and 12 inches apart in the field (middle areas). The plywood sheets should be installed perpendicular to the joists with their edges centered on the joists, and joints staggered between rows. Leave a 1/8-inch gap between panels and at the perimeter to allow for expansion. For enhanced stability, some professionals use a combination of screws and ring-shank nails. Always check local building codes for specific requirements in your area.

Q: How do I level a concrete slab before installing a wood subfloor?

A: Leveling a concrete slab before installing a wood subfloor is crucial for a successful installation. Start by checking the slab with a long straight edge or string line to identify low spots and high areas. For minor unevenness (less than 1/8 inch), a self-leveling compound can be applied after properly cleaning and priming the concrete. For more significant low spots, use a portland cement-based patching compound. If there are high spots, they’ll need to be ground down with a concrete grinder. Once the slab is level within 1/8 inch over 10 feet, let it cure completely before proceeding. For significant variations, consider using adjustable floor joist supports or a system of shims under your sleepers to create a level base. Proper leveling prior to subfloor installation will prevent future issues with your finish floor and eliminate potential squeaks or bounce.

Q: What’s the best approach for installing a subfloor on plywood that’s already there?

A: When installing a new subfloor over existing plywood, first assess the condition of the current surface. Remove any damaged sections and ensure the existing plywood is securely fastened to the joists below. For the new layer, use minimum 3/8-inch plywood, though 1/2-inch is preferable to match the height with the rest of the floor. Install the new plywood perpendicular to the existing sheets, staggering all joints so they don’t align with the seams below. Leave a 1/8-inch gap between sheets and at the perimeter for expansion. Use construction adhesive between the layers and screw the new plywood through to the joists where possible. When not over joists, fasten every 8 inches in the field with screws long enough to penetrate but not go through the bottom of the existing subfloor. This approach creates a stronger, more stable base for your new finish floor while addressing any deficiencies in the original subfloor.

Q: How thick should my plywood subflooring be for different applications?

A: The required thickness of plywood subflooring varies by application and joist spacing. For joists spaced 16 inches on center, 5/8-inch plywood is the minimum for most residential applications, though 3/4-inch tongue-and-groove is preferable for greater stability. For joists at 19.2 inches on center, use minimum 3/4-inch plywood. For 24-inch joist spacing, you’ll need 7/8-inch or thicker plywood to prevent flex. For tile installations, the industry standard is a minimum 1-1/4 inch total thickness (subfloor plus underlayment) to prevent cracking, often achieved with 3/4-inch plywood plus cement backer board. For hardwood floors, 3/4-inch T&G plywood is standard. When installing over concrete, 3/4-inch plywood provides adequate elevation and strength. Always check your finish floor manufacturer’s recommendations and local building codes, as they may specify different requirements based on your specific situation and materials.