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Gypsum Underlayment Installation Tips

Installation Tips

Tools and Storage:

Use hand saws, razor knives, power saws and holes saws to cut and shape the gypsum fiber underlayment. It is a score and snap product just like drywall. If you desire to cut it with a saw you can, you will simply create extra dust.

Store the product flat and up off the ground. Lay down four or five 2x4 pieces if you want to stack them flat. If you stand them up on edge, it is wise to place them on short pieces of 2x4 as well. But be careful! It is dangerous to stack them upright. People (children) have been crushed when a stack falls over.

You need to bring the gypsum fiber panels indoors and let them acclimate to the jobsite conditions. Store them inside for at least 48 hours before you start to install the product.

Crawl Spaces:

If you install gypsum fiber panels over a crawl space, a vapor barrier must be in place over the soil in the crawl space area.

Floors Directly Over Furnaces:

Install 2 inches of fiberglass batts on the underside of subflooring that is directly over or near furnaces.

Existing Vinyl Floors:

Do NOT install gypsum fiber panels directly over heavily or thick cushioned sheet vinyl flooring. You must remove the sheet vinyl and its foam backing first.

Replace or fill in any missing solid vinyl tiles. You want the floor beneath the new underlayment to be solid and free of cracks and voids.

Wood Subfloors:

The wood subfloor needs to be secured to the joists. For best long term results, screw the wood subfloor to the joists before you install the underlayment. Sand any high spots where the subfloor seams are uneven. Fill in any depressions so that the subfloor is in the same plane. Vacuum all debris and be sure the subfloor has a moisture content of 15% or less. Use a moisture meter if you think the subflooring is damp.

Cutting Panels:

As said earlier, you can use a razor knife to score and snap panels. But if you have to cut off a corner or make a notch, you will need to saw at least one or two lines and then snap the final marked line.

It is always best to place cut edges against a wall. Always leave a minimum 1/4 inch space between panel and the wall. This is for expansion.

Lightly butt panel edges together. Do not force panels together. In a large room, offset the butt joints on panel edges. You are trying to avoid long seam lines. It can't be avoided every four feet as you proceed installing rows of the panels, but a long seam can be avoided as long as you start with different sized pieces as you come off one of the walls.


Fasteners are very important. The heads of the fasteners must be flush with the top of the panels once driven. You can use ring shanked nails, staples or screws. Avoid rosin or cement coated fasteners as they can stain vinyl flooring.

Keep fasteners 1/2 inch away from the edge. Install nails every two inches along the outer edge. All nails within the field should be on 4 inch centers.

Final Touches:

Be sure seams of underlayment are smooth. You can sand FIBEROCK™ and even treat the seams as you would drywall. If you apply a compound to the FIBEROCK™ be sure it is approved by the finished flooring manufacturer.

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