Big Bob's Flooring Blog

What is a Sub-Floor and Why Does it Matter So Much?

Jul 15, 2019 10:30:00 AM / by Rob Lewis

floor plan home


Let us begin with one of the biggest disclaimers for any remodeling project:


We cannot plan for what we cannot see.


I had a conversation with a store manager in my area about unexpected remodeling projects. He gave some insight to put the importance of taking sub-floors into consideration:

Sub-floor or structural issues are in a good portion of projects - I'd say 15-20% of projects or higher depending on how old the home is in which you’re working.”- Jake L, Manager at Floors & Kitchens Today.


With this in mind, it is important that we get familiar with these areas of your home so that we can be more familiar with the remodeling process.



In this article we will be discussing:


What is a Sub-floor?

The sub-floor of your home can be considered anything under the floor covering which consists of the topmost layer as well as any underlayment.

Underlayment is underneath the flooring but is typically only used to protect the top material from shifting and keeps moisture from seeping to wooden parts of the sub-floor.


*Notice how the subfloor doesn’t start until after the underlayment!


Here are the most common types of subfloors you could find:


Two basic types, lots of subtypes (Wood or Concrete, then breakdown)



  • Wood Planks - You can use any type of floor covering on top of wood planks. If you decide to use glue down products, you’ll need more preparation in order to make sure the floor is level (under solid hardwood, you cannot use leveling compounds). However, be aware that this subfloor squeaks! Any work to prevent squeaking can’t be guaranteed.
  • Plywood
  • Fiber Rock or Cement board
  • Concrete
  • Strandboard (OSB, particle board) - You can use any covering with the exception of nailed down hardwood flooring and some glued down products as well. Nails and staples won’t cling properly and the work can’t be guaranteed.


The wood products here will provide similar results but allow for different applications for floor covering. If you are installing porcelain and tile products while using these wooden subfloor options, you’ll need additional materials like an underlayment.

The wood will absorb the moisture from the mortar used to lay the tiles which can lead to cracking over time.



Understanding Underlayment



Image courtesy of

This orange material is known as Ditra, an underlayment commonly used for tile. It doesn’t provide any structural support in the way of holding anything up or keeping people from falling through, but it does help isolate the material and reduce the likelihood of cracking by isolating the product from other subfloors. It is often used with Natural Stone Installations or larger format tiles 18 x 18 or larger.



There are different underlayments for different types of floors.




For tile, you can expect the material shown in this image, Ditra, or Strata Mat can be used as the underlayment, plywood or concrete.


For sheet vinyl, it would be most likely birch plywood if it isn’t going directly onto a concrete floor.

For vinyl tile, you’ll most likely use plywood.


If you are installing laminates, floating engineered wood, or carpet, you will need another type of underlayment: foam.


image courtesy of ebay

Foam is required for many different reasons:


  1. It’s required for the warranty to be valid.
  2. Reduces abrasion and friction from floor covering and its underlayment.
  3. Provides a barrier to moisture.
  4. Relieves stress on the locking joints.
  5. Increases softness and makes the floor comfortable to stand on for extended periods of time.
  6. Dampens sounds, making the floor quiet to walk on.
  7. Increases the “R Value” of the floor which keeps the heat from draining out of the room.


Solid hardwood requires a wood or plywood subfloor with a vapor barrier on top of the underlayment which is similar to a felt paper. It moderates and keeps the moisture under control.


Glue down products like vinyl products, linoleum, and engineered wood need a surface that they can be adhered to which will also keep them level. Anything from plywood, luan or, even in some cases, existing vinyl floors so long as they are in good condition.



All About Joists



image courtesy of Wikipedia


Everything we have discussed so far is resting on joists! They are our number one support system for floors.


While there are different types and sizes, joists are the main structural support of floors and determine how much weight the floor can hold over time.


Regarding joists, we need to talk about deflection.


Horizontal Deflection - Most often caused by the natural expansion of the flooring material and subfloor. This expansion and contraction will cause separation and make the floor unlevel.


Vertical Deflection - This type of deflection is most often caused by too much weight or too little weight on the floor. The amount of deflection is brought by the weight of the floor and the resistance by the subfloor.


Materials such as ceramics or natural stone tiles are heavy and their weight might cause some deflection. If you are planning to use these types of materials for your floor, it would be wise to find the dimensions of your joists and plug them into a Deflect-O-Meter to see if your support system can handle the heavier products.


If anything, the discussion of what goes on underneath your brand new hardwood floors shows how involved and unpredictable subfloors can be.

The subfloor, underlayment, and joists are the bones of every floor and need to be in tip-top shape for the remodel to go smoothly and last a lifetime.


More Resources!




Topics: cost, diy, installation, maintenance, time

Rob Lewis

Written by Rob Lewis