When constructing or remodeling a home, understanding the differences between subflooring and underlayment is key to ensuring a stable and comfortable finished floor. Both subfloor and underlayment play important roles in providing a smooth surface for the final floor covering, but serve different purposes.
What is Subflooring?
Subflooring is the first layer of material installed over the floor joists or concrete slab foundation. The subfloor provides a sturdy, flat surface on which to install the finished floor.
The main purposes of subflooring are:
- Create a stable base: Subfloors provide structural support and stiffness to prevent squeaks and bounce. They distribute the weight placed on the floor over the joists.
- Provide a smooth surface: While not perfectly smooth, subfloors create a relatively even surface on which to adhere the underlayment and finished flooring.
- Allow drainage: Subfloors include gaps between panels or boards to allow moisture that accumulates below to evaporate.
Subflooring is typically made from plywood, oriented strand board (OSB) or boards. It may be glued and screwed to the floor joists or float over them unattached. For concrete slabs, sheets of plywood or OSB are laid down first before pouring the concrete.
The thickness of subfloor panels is generally either 3/4″ or 5/8″ for most residential flooring needs. Multiple layers may be used to increase rigidity.
What is Underlayment?
Underlayment, as its name implies, is the layer installed underneath the final floor covering. It goes on top of the subfloor to create an ultra-smooth, uniform surface for the flooring.
The main purposes of underlayment are:
- Smoothen surface irregularities: It fills in gaps, cracks, and voids in the subfloor for a flat look.
- Correct imperfections: Minor subfloor defects like crowns, bowing, or cupping can be remedied with underlayment.
- Reduce noise: Underlayments made of cork, foam, or felts deafen impact sound transmission through the floor.
- Cushion floors: Materials like felt cushion resilient flooring and absorb shock from walking.
- Allow flooring to float: Non-adhesive underlayments permit floating engineered wood and laminates to expand and contract.
Common types of underlayment include:
- Plywood – Provides a smooth surface for resilient flooring or carpets.
- Cement board – Prevents cracks transmitting through ceramic tile floors.
- Cork – Offers comfort underfoot and sound insulation.
- Foam – Excels at noise reduction and shock absorption.
- Felt – Helps stop reflected sound vibrations.
Underlayment thickness ranges from 1/4″ for materials like cork to 1/2″ for plywood. Thicker underlayments may be used to flatten uneven subfloors or raise floor height.
Key Differences Between Subfloor vs. Underlayment
While subfloor and underlayment work together to build up the floor assembly, there are several major ways they differ:
Purpose
- Subfloor: Provides structural support and stiffness for the floor.
- Underlayment: Creates an ultra-smooth surface for installing the finish flooring.
Position
- Subfloor: First layer installed directly over the joists/foundation.
- Underlayment: Second layer installed over the subfloor.
Flatness
- Subfloor: Relatively uneven with gaps between panels.
- Underlayment: Flattens surface irregularities for a smooth finish.
Fastening
- Subfloor: Glued/screwed to joists or floated.
- Underlayment: Glued, stapled, or floated over subfloor.
Materials
- Subfloor: Plywood, OSB, boards.
- Underlayment: Plywood, cement, cork, foam, felt.
Thickness
- Subfloor: Typically 3/4″ or 5/8″.
- Underlayment: 1/4″ up to 1/2″ thick.
Do You Need Both Subfloor and Underlayment?
Installing both a subfloor and underlayment is best practice for most flooring installations. Relying on just one layer can cause problems:
- Subfloor only – The rougher, uneven surface of only a subfloor may telegraph through and damage smooth flooring like vinyl and laminate. The joints can also crack tiles or wood flooring.
- Underlayment only – A single layer of underlayment does not provide enough structural support on its own and can result in bouncy, unstable floors.
However, there are a few cases where just a subfloor or underlayment will suffice:
- Concrete slab subfloor – Installing ceramic, stone, or porcelain tiles directly over a cured, smoothed concrete slab without underlayment is acceptable.
- Solid wood floor subfloor – Tongue and groove hardwood planks can be nailed directly to a 3/4” plywood subfloor.
- Double layer subfloor – Some floating wood or vinyl floors can be installed over two layers of sturdy subflooring like 5/8” OSB without underlayment.
Choosing Appropriate Subflooring Materials
Selecting suitable subflooring materials is important to properly support the flooring system. Common types include:
Plywood
Plywood provides excellent strength thanks to its cross-layered construction. It resists expansion and contraction better than OSB. CDX and exposure 1 plywood rated for structural use make sturdy subfloor choices.
Oriented Strand Board (OSB)
OSB offers similar strength to plywood but at a lower cost. Look for OSB panels rated as subflooring or sheathing with a thickness of 23/32” or 3/4″. Sturd-I-Floor is a popular OSB subfloor brand.
Boards
1” x 6” boards were traditionally used as subfloors. Modern dimensional lumber options include 2×6 and 2×8 boards or engineered I-joists. Boards allow access to wiring and plumbing below.
Concrete
Poured concrete slabs with a vapor barrier underneath make a durable subfloor. They need proper curing time and may require a floated plywood layer for some flooring.
Selecting the Right Underlayment
Key factors to consider when selecting underlayment include:
Intended Flooring
Match the underlayment to the flooring type. For carpets, felt and foam underlays work well. Cork boards suit hardwood. Cement backer boards are recommended under tile.
Noise Reduction
If sound insulation is important, choose a sound-absorbing underlay like cork or thick foam. Rubber, fiber, and felt underlays also help dampen noise.
Moisture Protection
For concrete subfloors, vinyl or rubber underlayments create a moisture barrier to prevent water damage to flooring.
Floor Leveling
Thicker underlayments like 1/4″ plywood can help smooth out minor subfloor imperfections. Foam and rubber are compressible options.
Cost
Less expensive underlayment options include rosin or building paper. Higher end choices like cork and rubber provide more benefits but at a greater cost.
Subfloor and Underlayment Installation Tips
Proper installation of subflooring and underlayment is crucial for optimal floor performance:
- Allow subfloors over joists to acclimate to room humidity levels before installation.
- Make sure subfloor panels are level across joists and screws/glue reaches joists.
- Stagger subfloor seams in a bricklike pattern and space panels 1/8” apart.
- Fill subfloor joints with elastomeric caulk rather than wood putty for flexibility.
- Check underlayment thickness is uniform and combines to 1-1/4” total for floor rigidity.
- Underlayment seams should not coincide with subfloor seams. Offset by several inches.
- Always use recommended adhesives and follow manufacturer’s instructions for underlayment application.
- Tape underlayment joints to prevent air gaps. For floating floors, do not tape seams.
Proper prep and installation of subfloors and underlayments as recommended ensures the flooring performs as intended.
Subfloor vs. Underlayment: Which Goes First?
The order of installation is clear – subfloor materials always go down first before underlayment.
Subflooring creates the structural base on which underlayments and finished floors are built upon. Underlayments cannot fulfill this role and need the strength of plywood, OSB or concrete beneath them.
If underlayment was installed before subflooring, the finished floor would lack sufficient stiffness. It could flex, squeak and become damaged without the rigid underlayer of subfloor.
Following proper assembly order is key:
- Install subfloor over joists/foundation
- Fasten subfloor securely with screws/adhesive
- Inspect subfloor flatness, repair imperfections
- Apply suitable underlayment over subfloor
- Glue/staple/float underlayment as recommended
- Install finished flooring over smooth underlayment
The Combined Thickness of Subfloors and Underlayments
For optimal flooring performance, the combined thickness of the subfloor-underlayment assembly is an important specification.
Insufficient total thickness can lead to inadequate stiffness, while excessive thickness may create transition problems between rooms.
Here are some general guidelines on combined thickness:
- Hardwood Floors: Recommended total thickness is 1-1/8” to 1-1/4”
- Engineered Wood Floors: Minimum thickness is 1”
- Laminate Floors: Total should be 1 – 1-1/8” thick
- Resilient Flooring: No less than 1-1/8” thickness
- Ceramic Tile Floors: Total needs to be 1-1/4″ thick minimum
The most common combination is 3/4” tongue and groove subflooring with 1/4″ underlayment on top to equal 1” thickness.
Two layers of 5/8” subfloor may be used instead to avoid excess height. Thicker underlayments can help level or enhance cushioning.
Proper assembly from bottom to top ensures every layer provides the right benefits for a durable, comfortable floor.
Common Problems from Incorrect Subfloor and Underlayment Installation
Installing subfloor and underlayment improperly can cause issues once flooring is applied:
- Uneven surfaces: Gaps or inconsistent underlayment thickness creates bumps and dips visible through flooring.
- Excessive deflection: Insufficient subfloor rigidity allows floors to sag, crack, and detach under load.
- Rot and mold: Lack of subfloor gaps causes moisture accumulation and water damage.
- Squeaking and popping: When subfloor panels move at seams, they make noise under the flooring.
- Loose flooring: Inadequate gluing/fastening of underlayment prevents flooring bonding securely.
- Cracking grout and tiles: Discontinuous underlayment surfaces transmit subfloor flaws through to tilework.
- Buckling: Flooring not able to move due to underlayment or subfloor being glued/fixed incorrectly.
- Excess height: Too-thick subfloor assembly impedes transitions between floor levels.
By selecting suitable materials, allowing proper acclimation, and taking care in fastening every layer, these common problems can be avoided.
Frequently Asked Questions
What are the best subfloor materials?
Plywood, OSB, and dimensional lumber boards make excellent subfloor choices. For concrete slabs, an additional plywood layer is often placed overtop.
Can you install hardwood floors directly over subfloor?
Yes, provided it is a solid 3/4” tongue and groove subfloor rated for use under hardwood. Engineered floating floors still require underlayment.
Should subfloor and underlayment seams be aligned?
No, subfloor and underlayment seams should be staggered at least several inches. This prevents weak points forming in the overall floor assembly.
What subfloor thickness is needed for 12” tile?
A minimum 5/8” subfloor is recommended under 12” tiles, but 3/4″ is better. Combined with 1/4″ underlayment gives the 1-1/4″ total thickness needed.
Can laminate floors be installed over concrete?
Yes, provided a foam underlayment and 6 mil polyethylene sheet are laid first. This prevents moisture damage to the laminate flooring.
Conclusion
Understanding subfloor vs. underlayment roles ensures floors have the right structural base and finish surface. While subfloors support the entire system, underlayments prepare and enhance it for the final flooring. Using suitable materials, proper assembly order, and recommended thicknesses helps build an optimal floor that remains smooth, level, and blemish-free for the long term.