Q&A / 

Gypsum Fiber Underlayment

I can't begin to tell you how many pieces of wood underlayment I have installed over the years. It has to be in the thousands. I have it in my own home. You name it, I have used it: Masonite™, particle board, plywood, luan plywood, etc. Fortunately, I have only had problems on one occasion.


I'll never forget the first big house I built. It was an ultra custom contemporary home that had marble and tile floors. I used 3/8 inch plywood underlayment beneath the marble and tile. The tile was set in thinset - cement based product that bonds tile to wood or to concrete.

It took me two days to install the tile floor in the kitchen. The morning of the second day, I arrived to work and saw that an area of the tile had a hump in it that I didn't remember from the day before. Three or four tiles had popped up. I walked over and stepped on them and the area of the floor was spongy. The thinset bond between the tile and the plywood was broken and I lifted up the tiles to discover the plywood had bubbled up. The water from the thinset caused the plywood to swell creating this large bubble.

Using long drywall screws, I was able to get the plywood to flatten out. But I had learned my lesson. Water and wood underlayment products are a recipe for disaster.

Cement Board

Someone else had the same problem as I did. They decided to invent an underlayment that was bulletproof against water. All they did was modify time tested technology that tile setters had used for years. Any old tile setter will tell you that they installed their tile over wood floors but that they separated the tile from the wood with a layer of cement mortar. It was a very labor intensive job to pour and level the mortar.

The wise inventor simply decided to make cement underlayment in one half inch thick sheets! You simply nail these over newer wood flooring systems. The cement board is totally waterproof, but it is heavy, tough to cut and seams are hard to smooth. Also, pesky grit is produced as you install and work with it. This grit must be removed before you install flooring materials.

A Compromise

The cement board product was and is a fantastic product for certain applications. But there was still a need for an underlayment that was easy to work with and highly water resistant. This opportunity was filled with the new gypsum fiber underlayment.

Easy to Work

The gypsum fiber underlayment is so simple to work with. I have cut some of it with a standard razor knife. It snaps just as a regular piece of drywall snaps. There was virtually no dust when I scored and snapped it. Installing fasteners is a breeze. The product is virtually unaffected by water. If and when water does harm the underlayment, I am quite sure you might have more severe problems with the actual finished flooring, the wood sub-flooring system and/or adjacent walls. In other words, the fact that it is highly water resistant is probably all most of us need.


The gypsum fiber has great distribution. It also comes in different thicknesses and panel sizes. You should be able to find it at most home centers and large scale drywall distributors. Be sure that the panels come with literature that speaks to the water issue. You should be able to verify that the product is indeed water resistant.

I urge you to also obtain and READ a copy of the installation instructions. This is very important. Don't forget to do this.


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