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What Is the Difference Between Underlayment, Subfloor & Joists?

Difference Between Underlayment, Subfloor & Joists?

Do you know what goes under your flooring? Is there more to your flooring than what you see at the top?

The answer is yes. 

The flooring system consists of multiple layers, known as flooring layers, that all come together to create a sturdy foundation for the topmost flooring. Typical flooring layer includes subfloor, underlayment, joists, and floor covering. So, what is the difference between underlayment, subfloor, and joists? 

The main differences between underlayment, subfloor, and joists are their purposes and placement. The topmost flooring layer is the flooring covering or finished floor. The underlayment goes between flooring covering and subfloor, while joists lie beneath to support the layers above it. 

Whether building your flooring from scratch or remodelling, a thorough understanding of the differences between underlayment, subfloor, and joists is crucial for an excellent finish that you can enjoy for a long time.

What is Underlayment? 

Underlayment is the flooring layer that lies immediately under the visible flooring. It is usually ¼ inch or ½ inch thick, which can be made of many different materials that depend on the floor covering. You might not always find this flooring layer as many flooring materials do not require them. The primary function of underlayment is to provide a smooth surface for installing the floor covering. 

The other purposes of underlayment are –

  • Protecting the floor covering from moisture
  • Absorbs pressure and provides compression resistance 
  • Reduces sound and dampens noise
  • Provides additional insulation to tackle seasonal temperature changes

What is Subfloor? 

The subfloor is the bottom-most layer of flooring that rests on the joist and supports all the other layers. It is a structural part of the floor.

Subfloors are typically very thick, with the thickness ranging from 19/32 inch to 1 1/8 inch. It is usually made of plywood, particleboard, or OSB. For the concrete slab floor, the slab is considered to be the subfloor. 

The primary purposes of subfloor are – 

  • It is a structural part of the floor
  • Connects the other flooring layers to the joists
  • Provides strength and rigidity 
  • Supports finished floor and underlayment

What are Joists?

Joists are a horizontal structural part of the flooring that spans an open space, typically between beams, which transfer the load to vertical structural components of the house. They carry the weight of everything inside a room, including walls, furniture, appliances, and even people. 

Floor joists are usually made of engineered, laminated wood, steel, or dimensional lumber. Engineered wood joists may have a cross-section resembling the Roman capital letter “I,” known as I-joists. Steel joists can be of different shapes, resembling the Roman capital letters “C,” “I,” “L,” and “S.” In general, joists spacing is 16 inches apart on centre. However, the spacing might vary depending on the building codes or other structural requirements of the house. Concrete slab flooring does not need joists. 

The purposes of floor joists –

  • Provides structural support for the entire floor
  • Transfer loads to the beams or vertical structural components
  • Provides stiffness to the subfloor sheath 
  • Functions as the horizontal diaphragm of the floor

What is Floor Covering or Finished Floor? 

Floor covering or the finished floor is the part of the flooring layers you can see or walk on. There are many different flooring options for you to choose from for your home. Ceramic tile, hardwood, engineered wood, laminate, luxury vinyl, bamboo, cork, and carpets are the most popular flooring options.

The floor covering is the topmost flooring layer, and it is not a structural part of the floor. It provides a smooth surface for you to walk on, place furniture, and carry on your daily activities. Another purpose of the finished floor is to enhance the aesthetic of the area. It provides a solid and durable base that is easy to clean and maintain. 

Do You Need Both Underlayment and Subfloor? 

How many layers of flooring you need depends on the material of the finished floor. If you use a stained concrete floor, you will not require underlayment or subfloor. Additionally, you can install the flooring directly over the subfloor if it is smooth and solid. However, an essential factor to consider is if the flooring material is water-resistant. Installing hardwood floors directly on concrete subflooring can cause moisture-induced damages. 

As subflooring is a structural part of the floor, any other floor than concrete slabs (in which case the slab acts as subfloor) requires it. Whether you need underlayment or not depends on your subfloor and flooring. For some flooring materials, you need both underlayment and subfloor. If the subfloor is not smooth enough to install floor covering, you need to put underlayment before the floor covering. In some houses, you might find more than three flooring layers. However, there is no purpose for having more than three flooring layers. In most cases, the additional layers are put on top of old layers during home renovations to cut off costs. 

What Type of Flooring Does Not Need Underlayment?

  • Luxury vinyl
  • Hardwood (optional)
  • Cork
  • Engineered wood (optional)

A big selling point for luxury vinyl flooring is, it does not require underlayment as it is softer. Adding underlayment to luxury vinyl will only add excess cushioning to the floor. Underlayment is not an absolute necessity for hardwood flooring, but it can add extra stability to the floor and create a moisture barrier between the subfloor and hardwood. 

Cork flooring also does not require underlayment. It only requires a vapour barrier in plastic sheeting when installed over concrete. Plywood or wood-based substrates should not have a vapour barrier used when installing flooring over the top. Similar to hardwood, engineered wood does not require an underlayment but certainly can benefit from adding it.  

Which Flooring Materials Require Underlayment? 

  • Carpet
  • Hardwood (optional)
  • Vinyl or linoleum
  • Tile
  • Laminate
  • Bamboo

Underlayment is a must for carpet flooring regardless of the subfloor condition. The underlayment makes the carpet installation smoother, provides thermal and sound insulation, and increases durability. Hardwood flooring can also benefit from felt underlayment as it offers a moisture barrier for the wood.  

For vinyl and linoleum flooring, the underlayment is optional. You can install vinyl and linoleum flooring directly on the subfloor if it is plank or OSB and has a good condition. The most common underlay for vinyl floors is plywood. Additionally, tiles flooring requires underlay to improve its functionality. A tile flooring underlayment should accommodate humidity and temperature changes. It should also have the flexibility to absorb pressure from movements on the floor.

Underlayment is a must for laminate flooring since it is a floating floor. The laminate needs to be evenly distributed across your subfloor to spread out the load. Underlayment allows the floor to float, gives it stability, reduces noise, and supports the locking systems between planks to provide you with the sturdiest foundation. 

How do I find Joists Under Subfloor?

You will need a hammer to find the joist under the sub-floor from the floor covering like carpet, vinyl or hardwood floor. Lightly tap the floor with the hammer from an edge or a corner and listen for a thud sound. Generally, most areas will have a hollow sound, and a thud sound signifies the joist’s presence. Continue until you determine the location of the next joist. If they are installed the same distance apart, you can quicken the process by measuring the distance of the identified joist to locate others.

How to Find Out How Many Flooring Layers I have? 

It is pretty easy to determine the number of flooring layers you have in your home. You can do it simply by removing an HVAC vent, giving you a clear visual of the flooring layers. You can also check your flooring layers at the top of the stairs if it has an unfinished opening that gives a cross-section view of the floor. Even if the stairs are fully finished with no opening, you can get an approximate reading of the stairs’ thickness by measuring the distance from the top of the riser to the top of the finish flooring.

How many flooring layers does your floor have? Comment below and let us know! 

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