# Do I need 3/4 inch plywood subfloor?

I'm going to install tiger wood 3/4 inch hardwood floors in my house. The manual says you need 3/4 inch CDX plywood subfloor. I have 1/2 CDX plywood subfloor. Is the extra 1/4 inch really needed? Some people have told me it is not needed, but I want more opinions.

• Engineered or solid? Staple install? Feb 8, 2015 at 1:25

If you are interested in the science behind your question, the answer lies hidden in your question. You mentioned "is the extra 1/4 inch really needed".

As do most people, you translated this in your mind to "I know how little stiffness there is in a wimpy piece of 1/4 inch plywood, so how could it really make any difference".

The stiffness of a piece of material is only determined by simple addition in a case where the two layers are free to slide against each other. If just laid one on top of the other with no connection between them, two 1/4 inch thick pieces of plywood form a combination that is merely twice as stiff.

In the more common case of well attached or glued layers (or in this case just a thicker single sheet of plywood), doubling the thickness of a sheet MUCH more than doubles its stiffness. The 3/4 plywood is more than twice as stiff as 1/2" plywood.

• The "much" more quantity is the cube of the thickness increase. For area moment of inertia of a rectangular cross section, I = w*t^3/12. So a 3/4" sheet of plywood will deflect 3.4X less than a 1/2" sheet of plywood, and 3X less than stacked 1/2" + 1/4" sheets. Mar 11, 2015 at 22:49
• Agreed Doresoom. I'm an engineer who deals with "normal" people all day long at work. Only a very small percentage of the people on the planet understand there is no such thing as 110% of something...and even fewer understand what cube means. I "dumbed it down" for a "normal" audience. Mar 12, 2015 at 23:12
• I 110% agree with you. :D Mar 13, 2015 at 13:32

If you are installing the new flooring perpendicular to the joists you will be ok with 1/2" subfloor but if you are running in the same direction the joists you will need to replace the subfloor with 3/4" t&g plywood