A high-quality subfloor is the crucial foundation upon which beautiful, long-lasting flooring is installed in any home or building. Just as a sturdy foundation is imperative for constructing a reliable house, a properly prepared subfloor is key to achieving success with any new flooring project. In this comprehensive guide, we will explore all the critical aspects of subflooring and provide expert tips for ensuring your subfloor provides the ideal base for your new flooring.
What is a Subfloor?
The subfloor is the bottom-most layer of flooring in a building, installed directly on top of the joists or concrete slab. It provides a sturdy, smooth, and level surface on which the finished flooring, such as hardwood, tile, or carpeting is installed. The subfloor takes any irregularities in the substrate and ensures the floor above will be even. It also provides thermal insulation, sound deadening, and structural rigidity.
In general, there are two main types of subfloor systems:
Wood Subfloors
- Constructed from plywood, OSB (oriented strand board), or boards.
- Installed over wood joists in framed construction.
- Most common in residential buildings.
Concrete Subfloors
- Poured concrete slab or pre-cast concrete panels.
- Laid directly on the ground or over a substrate.
- Most common in commercial buildings and basements.
No matter the specific type, a properly installed subfloor is essential for preventing cracks, squeaks, and other issues with the finished flooring down the road.
Subfloor Preparation – Key Steps
Preparing the subfloor correctly is a crucial first step for any new flooring installation. Rushing this process can lead to major problems after the new flooring is in place. Here are the key steps involved in subfloor prep:
Step 1: Inspect and Repair
Carefully examine the existing subfloor for any issues. Look for:
- Rot, cracks, or delamination.
- Nails popping up.
- Major uneven spots or voids.
- Moisture damage or water stains.
Repair any damaged, delaminated, or warped areas. Replace severely compromised sections.
Step 2: Clean Thoroughly
Sweep away debris then mop using a heavy-duty cleaner. For concrete, etch and clean with an acidic wash. Thorough cleaning removes dirt, grease, wax, and other contaminants.
Step 3: Level Uneven Areas
Use a long straightedge to find any low or high spots. Grind down high points or fill low areas with a self-leveling compound. The subfloor must be flat within 1/8 inch over 6 feet.
Step 4: Calculate Moisture Levels
Excess moisture is hugely detrimental to flooring adhesives and finishes. Test moisture levels in both wood and concrete. For wood, the maximum acceptable moisture content is 12%. For concrete, conduct calcium chloride and plastic sheet tests. Address any moisture issues through sealing and dehumidification before proceeding.
Step 5: Prime and Seal
Apply primer and sealants to wood subfloors to minimize dust and prepare for adhesives. For concrete, sealers reduce moisture vapor emissions and alkalinity, improving adhesion.
With the subfloor fully prepped, you can proceed confidently with the new flooring installation, knowing the foundation beneath is sound.
Choosing the Best Subfloor Materials
Selecting suitable subflooring materials is an important decision which impacts flooring performance down the road. Key factors to consider when choosing subfloor materials include:
Structural Strength and Stability
The subfloor must be rigid enough to stay flat and not flex significantly under load. It should also be dimensionally stable and not easily affected by changes in temperature or humidity.
Moisture Resistance
Excess moisture from below can damage flooring adhesives, warp boards, cause mold, and lead to other major issues. Effective subfloors provide a moisture barrier.
Noise Reduction
Quality subflooring dampens impact noise and enhances soundproofing between building levels. Materials with greater mass and density perform better.
Thermal Resistance
In addition to their structural role, subfloors also provide a degree of insulation and improve energy efficiency in the building.
Cost Effectiveness
The monetary expense of subfloor materials should fit within overall project budget constraints. Labor costs for installation are also a consideration.
With these criteria in mind, let’s examine popular subflooring options:
Plywood
Plywood offers outstanding strength and stiffness. It resists warping from moisture better than plain OSB. The layered construction hinders noise transmission. Plywood panels are easy to cut and install. However, avoid low-grade, thin panels prone to deterioration.
Oriented Strand Board (OSB)
OSB is composed of wood chips and adhesives pressed into sheets. OSB is structurally sound but more prone to swelling and degrading from moisture than plywood. OSB tends to be more affordable than plywood. Use enhanced, moisture-resistant OSB for best performance.
Concrete
Poured concrete slabs make an incredibly strong, durable, and moisture-resistant subfloor. Concrete lasts for decades with little maintenance required. However, it offers no insulation value nor sound dampening. Cracks and uneven surfaces must also be addressed.
Cork
Cork is a highly moisture-resistant natural material that also provides cushioning and noise reduction. However, cork lacks the structural strength of wood or concrete. It is often used as an underlayment supplement over other subfloors.
Fiberboard
Fiberboard made from pressed wood fibers is a low-cost subfloor option. However, fiberboard is prone to crumbling and swells easily with any moisture. Most experts recommend avoiding fiberboard subfloors as they can undermine flooring.
Installing Subfloors Over Concrete
Concrete makes an excellent subfloor for tile, stone, and many other hard surface flooring types. When installing a subfloor over a concrete slab, follow these best practices:
Ensure Concrete is Fully Cured
New concrete requires at least 60 days to fully cure and dry. Do not install subfloors until cured to prevent moisture issues. Test moisture levels in several areas.
Clean and Profile the Concrete Surface
Acid wash or etch concrete to remove dirt, oils, and residue. Profiling opens pores for better adhesive bonding. Rinse thoroughly after cleaning.
Apply Primer
Priming before subfloor installation improves adhesion dramatically while reducing moisture vapor emissions. Use the subfloor manufacturer’s recommended primer.
Choose Appropriate Materials
On concrete, use rigid panels like plywood, OSB, fiber-cement board, magnesium oxide board, or cement board. Avoid soft materials like cork.
Follow Expansion Gap Guidelines
Leave 1/4″ expansion gaps around the perimeter and any obstructions to accommodate natural movement. Do not span gaps with fasteners.
Use Correct Fasteners and Glue
For panels, use screws or hammer nails designed for concrete application. Also apply construction adhesive between the panels and concrete.
Proper subfloor installation and prep helps ensure the concrete beneath contributes to, rather than detracts from, flooring performance.
Installing Subfloors Over Wood
Wood framed construction presents a different set of considerations for subfloor installation. Here are best practices when installing over wood joists:
Check Joist Spacing and Orientation
Confirm joists are spaced 16 inches on center or less. Subfloor panels should run perpendicular to joists. Properly spaced joists prevent bounce and cracking.
Stagger Panel Layout
Break panel joints over different joists to disperse forces evenly. Stagger end joints between rows. Use full sheets when possible to minimize seams.
Allow Proper Gap at Walls
Leave a 1/8” gap between panel edges and walls. This allows for expansion and contraction of materials. Caulk gaps after flooring installation.
Screw and Glue Panels
Screw panels into each joist, spaced 6 to 8 inches apart. Also apply construction adhesive between panels and joists for maximum stability and squeak prevention.
Install Backer Board at Edges
Use backer board made of plywood or cement board around the perimeter and doorways. This prevents edge fasteners from squeaking through finished flooring.
With careful installation techniques, the wood subfloor will provide the essential flat, stable foundation needed to support floors for the life of the building.
Signs of Subfloor Trouble
Detecting subfloor problems early is critical to avoid major headaches down the road. Watch for these warning signs of subfloor trouble after flooring installation:
- Squeaking or popping sounds when walking across floors
- Bumps, ridges, or uneven spots in flooring
- Creaking noises from subfloor shifting
- Gapping between floorboards or tiles
- Flooring becoming loose or cupping
- Mold or moisture discoloration of flooring
- Flooring materials deteriorating prematurely
Catching subfloor deficiencies quickly limits damage to the finished floor. Address any issues with the subfloor itself before simply reinstalling replacement flooring materials. Otherwise the same problems are likely to reappear.
How to Reinforce a Subfloor for Tile
Tile flooring requires an exceptionally rigid, stable subfloor to prevent cracking grout and other issues. Here are great ways to reinforce a subfloor for tile installation:
Add a Second Layer of Plywood
Glue and screw a second plywood layer at 45° angles to the first. This greatly stiffens the floor and spans any weak points.
Use Cement Board
Cement board made of concrete and fiberglass mesh does not flex or compress. Setting tile in a mortar bed over cement board creates a rock-solid subfloor.
Apply Fiberglass Mesh
Embedding fiberglass mesh into thinset mortar helps reinforce the subfloor and reduce cracks transmitting upward.
Install Thicker Tile
Natural stone tiles, porcelain tiles, and larger format tiles are less prone to cracking than small, brittle ceramic tiles.
Check Subfloor Deflection
Bounce a heavy object like a refrigerator on the subfloor. If deflection exceeds 1/360th of the span, reinforce the subfloor before tiling.
With proper reinforcement, your tile subfloor will provide a perfectly flat and rigid surface free of annoying cracks or loose tiles.
Signs You Need New Subflooring
In older homes or buildings, the subflooring itself may need replacement to restore proper floor function. Consider new subflooring if you see these red flags:
Major Unevenness in Flooring
A subfloor losing structural integrity can cause the floor above to sink or bow badly in areas. This may require reinstalling subfloor sections.
Severe Water Damage or Rotting
Water leakage causes irreparable damage to subfloor integrity. Rotten or delaminated subfloor panels must be removed and replaced.
Insufficient Thickness
Subfloor panels less than 5/8” thick may lack sufficient stiffness, leading to excessive bounce and flexing under load.
Large Gaps Present
Big gaps between subfloor panels or loose boards will telegraph through to the finished floor, causing cracks and instability.
Asbestos Present
Asbestos fibers in old vinyl sheet flooring or adhesive mastics require hazardous removal by professionals.
Extensive Previous Patch Jobs
Sections with many seams, patches, and repairs have compromised strength. At some point, it is better to start fresh with new subflooring.
Replacing problem subflooring provides the healthiest base for new floors to last many years.
DIY Subfloor Repairs
Homeowners can make minor repairs and fixes to damaged or unstable subflooring themselves in many cases:
Re-nail Loose Boards
Use a hammer and punch to sink protruding nails then drive new nails into joists to re-adhere boards. Fill old holes with wood putty.
Reinforce Squeaky Areas
Squirt construction adhesive onto squeaky zones and screw boards tighter to joists. For added strength, also screw through a backer board underneath.
Fill Gaps
Use wood putty or plastic wood filler to patch subfloor gaps up to 1 inch wide. Wider gaps require shimming and new board inserts.
Replace Damaged Patches
Cut out weakened sections of plywood or OSB. Cut replacement patches to fit and glue and screw into place. Match subfloor thickness.
Level Uneven Spots
For dips less than 1/2 inch deep, use an underlayment patch. Grind down any high points.
With simple tools and attention to detail, homeowners can tackle many subfloor fixes themselves without the expense of calling in contractors.
When to Hire Subflooring Professionals
Extensive subfloor repairs or full replacements often benefit from hiring experienced contractors. Consider calling in the pros for:
Concrete Slab Resurfacing
Heavily deteriorated concrete may require diamond grinding, scarifying, overlayment, or replacement which are complex jobs.
Crawl Space Access Issues
Accessing subfloor damage below may require lifting floor joists which is dangerous without proper equipment and training.
Asbestos Abatement
Removing asbestos requires certified professionals to contain and dispose of hazardous materials safely. Do not attempt this yourself.
Mold Remediation
Contractors with specialty expertise must contain, clean, and eradicate harmful mold growth if present in subflooring.
Large-Scale Replacement
Tearing out and replacing large sections or entire subfloors demands manpower, debris removal, and possible structural work.
While minor subfloor repairs are doable for DIYers, extensive issues often merit getting professional assistance.
Frequently Asked Questions About Subflooring
Many homeowners have additional questions about subflooring as they take on flooring projects. Here we answer some of the most commonly asked queries:
How thick should a plywood subfloor be?
For most homes, 5/8 inch thick plywood or OSB panels suffice for the subfloor. Go with 3/4 inch panels for a tile subfloor or if joist spacing exceeds 16 inches.
What is the best plywood for a subfloor?
Plywood rated Exposure 1 or Exterior grade works best. Look for plywood with few knots and a solid A-grade veneer face. Avoid thin, economy grade panels.
Do I need underlayment over subfloor?
Underlayments or backer boards provide extra moisture protection and noise insulation. Use cork, foam, or felt underlayments under wood, laminate, and vinyl floors. Backer board is recommended at transitions.
How flat should a subfloor be for vinyl plank flooring?
Vinyl plank flooring requires very flat subfloors within 3/16 inch over 10 feet. Unevenness leads to gaps, locking mechanism failure, and planks popping loose.
Can I install hardwood flooring over particle board subfloor?
Particle board is highly prone to expansion and deterioration from moisture. Do not use as a subfloor for solid or engineered hardwood. Replace it with plywood.
How do I soundproof an existing subfloor?
To reduce noise transmission through a subfloor, install an acoustic sound mat, seal perimeter cracks, cover walls with mass loaded vinyl, and use thick underlayment.
Conclusion
A subfloor in top condition provides the critical foundation for successfully installing beautiful, long-lasting flooring. Paying close attention to subfloor preparation, using quality materials, and making any necessary repairs ensures your new flooring gets off on the right foot. With the robust support of a flat, level, and moisture-protected subfloor beneath, you can enjoy flawless flooring performance for years to come.