How to Install Plywood Underlayment for Resilient Flooring

Why this matters

Sheet vinyl and glue-down vinyl tile are thin and soft enough to print whatever sits under them. A staple crown standing up a thirty-second of an inch, an unfilled seam, a void in a cheap panel's core or a pen mark on the face can all show through as a bump, a line, a dent under a chair leg or a stain weeks after install. Most resilient makers will not warrant their floor over a subfloor panel, so the job is a layer of underlayment-grade plywood whose only purpose is to be smooth, solid and stable, and the quality of that layer is the quality of the floor.

This card is installing that layer over a sound wood subfloor. Whether the subfloor is sound enough to receive it is 'Subfloor Assessment + Repair Reference', and squeaks get fixed first with 'How to Stop Subfloor Squeaks Before New Flooring', because underlayment fastened over a moving panel moves with it.

Choose a panel the resilient maker accepts

The resilient flooring maker's installation instructions name the underlayment it accepts, and that list is the decision. What makes a panel suitable:

• A smooth, sanded face with no open knots or splits, so nothing has to be filled except seams and fasteners.
• Solid inner plies, so a high heel or a chair leg cannot punch through the face into a void.
• Dimensional stability, so the panel does not swell at its edges and print its seams.
• No face treatment or ink that migrates into vinyl and stains it.

Many resilient makers exclude lauan, particleboard, OSB and hardboard as underlayment, or allow them only under conditions; read the list rather than assuming. Panels sold specifically as underlayment for resilient flooring often carry their own warranty that names the flooring types they support.

Store and acclimate the panels in the room with the HVAC running, flat and off the floor, as 'How to Acclimate Flooring and Confirm It Is Ready' lays out. A panel installed damp shrinks afterward and opens its seams under the floor.

Before you start

• The subfloor is fastened, flat to the flooring maker's target, squeak-free, and dry enough for wood flooring work.
• The old floor is off, or the decision to go over it is made. Any existing resilient floor or black adhesive of unknown age is presumed to contain asbestos until tested; it is not sanded, scraped or torn up to make the underlayment fit. See 'Asbestos in Old Resilient Floors and Adhesive'.
• Door jambs are undercut to the height of the underlayment plus the finished floor ('How to Undercut Door Jambs and Casing').
• Fasteners are the type and length the panel or flooring maker specifies, long enough to grip the subfloor and short enough not to come through its underside.

The layout: offset from below and from each other

      0'  2'  4'  6'  8'  10' 12'  (1 char = 6 in)
SUBFLOOR 3/4 ply (screwed down first)
      +-----------------------+
      |               |       |  A 0-4 ft, joint at 8
      +-------+-------+-------+
      |       |               |  B 4-8 ft, joint at 4
      +-------+---------------+
UNDERLAYMENT 1/4 ply, laid over it
      +-----------------------+
      |           |           |  U1 0-2 ft, joint at 6
      +---+-------+-------+---+
      |   |               |   |  U2 2-6 ft, joints 2, 10
      +---+-------+-------+---+
      |           |           |  U3 6-8 ft, joint at 6
      +-----------+-----------+

The rule the drawing shows: no underlayment joint lands on a subfloor joint, and no four underlayment corners meet. A seam stacked on a seam is a hinge, and the floor flexes and prints there. Here every underlayment seam sits 2 ft from the subfloor seam below it, and the end joints in neighboring underlayment rows are 4 ft apart.

Step 1: Clean and plan the sheets

Vacuum the subfloor and scrape off any lump of old adhesive, drywall mud or paint; anything left under the panel becomes a bump on top of it. Lay out the seams on paper or with chalk before cutting, so the offsets in the drawing are planned rather than discovered. What breaks if you skip it: cutting as you go puts a seam over a subfloor seam somewhere in the room, usually in the doorway, where the traffic is.

Step 2: Set the panels with a small gap at every joint

Lay each panel smooth face up and leave the small gap at panel ends and edges that the panel maker specifies, commonly about 1/32 inch. That space lets the panels take up a small moisture change without their edges pushing against each other and peaking. Leave the space at walls and fixed objects the panel or flooring maker calls for. Never write on the face with a felt-tip marker or ballpoint: those inks are the ones that migrate into vinyl. Use a pencil, and only where it will be covered by patch.

Step 3: Fasten on the maker's schedule, from the middle out

Fasten on the schedule the panel or flooring maker states. A common one for 1/4 inch underlayment is about every 3 inches along the edges, about 3/8 inch in from the edge, and about every 6 inches each way in the field, and some resilient makers ask for closer spacing. Start fastening at one end or the center and work toward the free edges, pressing the panel down ahead of the fastener, so any bow is pushed out to the edge rather than trapped as a bubble in the middle. Some panel and resilient makers say not to drive underlayment fasteners into the joists, so joist movement cannot push them up; follow that where it is stated.

Every fastener head sits flush or very slightly below the surface. A pneumatic stapler set for the right depth does this consistently; adjust it on a scrap first. Wear eye protection, keep the free hand clear of the nose, and disconnect the air before clearing a jam. What breaks if you skip it: a proud staple crown prints through thin vinyl as a dot, and a staple sunk deep breaks the face and makes a dimple that needs filling.

Step 4: Fill seams and fastener heads, then smooth them

Fill seams, fastener heads, and any surface dent with the patch the resilient maker accepts for wood underlayment, troweled flat and feathered so there is no ridge at the seam. Once it is set per its data sheet, knock down any ridge with a scraper. Sanding cured cement-based patch makes silica dust; sand only with a sander on a HEPA extractor, and HEPA-vacuum after (see 'Silica Dust in Flooring Work'). Mixing the patch is skin contact with an alkaline material: waterproof gloves and eye protection, and wash skin it touches.

Some resilient makers do not want seams filled on certain underlayment panels; where they say so, their instruction wins.

Step 5: Check for show-through before the floor goes down

Put a work light on the floor and look across the underlayment from low, then run a flat hand over every seam and every row of fasteners. Anything you can see in raking light or feel with your palm will show through sheet vinyl.

Worked example: a 12 by 8 foot kitchen for sheet vinyl

The kitchen in the drawing: 12 ft by 8 ft, 96 sq ft, 3/4 inch plywood subfloor already screwed down, sheet vinyl going down with a paper template. The vinyl maker lists a 1/4 inch sanded underlayment plywood made for resilient flooring; the panel maker's schedule, read for this product, is 3 inches on edges and 6 inches each way in the field, 3/8 inch in from edges, with 7/8 inch staples.

Sheets. 96 sq ft at 32 sq ft a sheet is 3 sheets of area exactly. The offset layout nests into 3: one full sheet for the middle of U2, one ripped into the two 8 ft lengths of U1 and U3, and one cut into the four 2 by 4 ft end pieces. That leaves nothing for a miscut or a damaged corner, so the crew brings 4 sheets.

Fastener check. 7/8 inch staples through 1/4 inch underlayment leave 7/8 - 1/4 = 5/8 inch in a 3/4 inch subfloor, 1/8 inch short of its underside. That grips without poking through into the basement.

Fastener count for one full 4 by 8 sheet. Along each 96 inch edge at 3 inches, 96 / 3 = 32 spaces, 33 staples; along each 48 inch edge, 48 / 3 = 16 spaces, 17 staples. Four edges with the four corners counted once: 33 + 33 + 17 + 17 - 4 = 96 staples. In the field at 6 inches each way, inside the edges: 15 positions along the length (6 to 90 inches) and 7 across (6 to 42 inches), 15 x 7 = 105 staples. About 201 per full sheet. A crew that counts roughly 200 per sheet knows a sheet with 80 staples in it was not done on schedule.

Gap. Each seam gets the 1/32 inch the panel maker calls for, set with a feeler shim the crew carries for it.

Fill and finish. Seams and staple heads filled with the vinyl maker's listed patch, scraped smooth once set. Under the raking light, two staples in U2 near the sink base stand proud; the crew sets them and re-fills. Hand and light check clean.

What would change it. If this were glue-down luxury vinyl tile with a maker who accepts a thicker underlayment and a different schedule, that maker's numbers replace these. And if the subfloor had been 5/8 inch rather than 3/4 inch, 7/8 inch staples through 1/4 inch underlayment would leave 5/8 inch in the subfloor, exactly its full thickness, putting the points at the underside; the crew would step down to a shorter staple the panel maker lists.

Finish test

A smooth, solid surface with no raised fasteners: no fastener head or seam you can feel with a flat palm, nothing visible in a light laid across the floor, no hollow sound when a seam is tapped, no ink on the face, and the patch cured per its data sheet before the vinyl or adhesive goes down.

References

• See related: How to Stop Subfloor Squeaks Before New Flooring
• See related: Subfloor Assessment + Repair Reference
• See related: Silica Dust in Flooring Work
• Resilient flooring maker's installation instructions for accepted underlayments, patch and seam treatment; underlayment panel maker's installation and fastening schedule.