A floating floor does not need a level subfloor. It needs a flat one. A dip lets planks flex over the low spot, and that is where joints click, open or break later. Around King and Snohomish counties the dips come from familiar places: a settled corner of a daylight-basement slab, a plywood seam near an old bathroom leak, a patch where someone once moved a wall. Self-leveling underlayment fixes some of these with a bag, a bucket and a careful hour. Others it makes worse.
Flat is what you are after
Check your flooring's installation instructions first. A common tolerance for floating floors is 3/16 inch in 10 feet, and some products add a shorter span such as 1/8 inch in 6 feet. Lay an 8- or 10-foot straightedge across the floor in several directions, mark the gaps with a pencil and measure the deepest one. That number tells you whether you need to do anything at all.
High spots are a different job; they get ground or sanded down rather than filled. A floor that runs downhill across a whole room is a third question, about framing or the foundation, and a bag of compound will not answer it.
When a DIY patch works
It is a reasonable job when all of these are true:
- The area is small, about what one or two bags can cover. On two common products' data sheets, a 50-pound bag covers about 47 square feet at 1/8 inch and about 23 square feet at 1/4 inch.
- The depth fits the product. Many self-levelers have a minimum pour, such as 1/8 or 3/16 inch, and are not meant to fade out to nothing. A shallow dip with fading edges is often better done with a trowel-on patching compound.
- The base is sound, clean and dry: concrete, or well-fastened exterior-grade plywood or OSB prepared the way the product sheet requires.
- The floor does not move. Self-leveler is not structural. It will not stiffen a bouncy floor, and it cracks over moving seams.
When it does not
Stop and rethink in any of these cases:
- Particle board. Some 1960s to 1980s houses in the region have it as underlayment. Product sheets list it as unacceptable, and a water-based compound can swell it.
- Lauan, hardboard or old strip flooring, which product sheets also tend to rule out as a direct base.
- Old vinyl, vinyl tile or black mastic in a house from before about 1980. Have it tested; do not scrape or sand it. Puget Sound Clean Air Agency rules cover asbestos surveys before renovation, and an owner-occupant doing their own survey still has to get it right.
- Tacky or pressure-sensitive adhesive. New material will not bond well over it.
- A damp slab or a wet crawl space. Self-levelers are not moisture barriers. Fix the source first.
- Moving cracks and joints. Product sheets say not to cover them.
- Radiant heat. Heating wire and tubing come with their own minimum cover and rules, so follow both makers' instructions or hire it out.
Step by step: pouring a small patch
- Buy by the numbers. Work out the volume from your marks and buy at least one more bag than you think you need. Read the whole sheet before you open anything.
- Fix what is underneath. Screw down loose panels and sand high seams.
- Clean to a bare, sound surface. Scrape off loose material, paint, wax and glue spots, then vacuum. Do not use chemical adhesive removers; some product sheets forbid them.
- Warm the job. The two products we checked call for air between 50 and 90 degrees F, and one says to keep the bags in that range for 24 hours first. In a Puget Sound winter that means turning the heat on a day ahead, especially in a lower level or a garage conversion.
- Dam and seal. The mix flows like pancake batter and finds every gap. Set foam or weatherstrip at the edges of the patch, at doorways and around floor registers, and seal gaps at walls and pipes so it does not drip into the crawl space or onto the ceiling below.
- Prime. Use the primer the product names and let it dry as instructed.
- Add lath on wood if required. The product sheets we checked call for metal or plastic lath on single-layer plywood or OSB, fastened every 6 to 8 inches.
- Mix it exactly. Wear a respirator, gloves and eye protection, because the dry powder is dusty and the wet mix can burn skin. Measure the water, add the powder and mix with a drill and paddle for the time the sheet gives. Extra water weakens it. Never add water to loosen a batch that is thickening.
- Pour at once. Start at the deepest point and guide the mix with a gauge rake or smoother. Working time can be short: one sheet lists 25 to 35 minutes at 70 degrees F, another a pot life of more than 15 minutes. Have everything ready first.
- Leave it alone. Keep people off until the sheet says otherwise, and keep the room in the temperature range.
- Check and wait. Put the straightedge back on and lightly sand any ridges. Many products can be walked on within hours, but moisture-sensitive flooring can take a day or more, longer in a cool, damp house. Follow the flooring's moisture limit as well as the bag.
What it costs
These are typical Puget Sound planning ranges, and they vary by product, county and the condition of the subfloor. The written quote after an in-home measure is the real number.
| Item | Typical range | Notes |
|---|---|---|
| Self-leveling underlayment | About $35 to $60 per 50-lb bag | Coverage falls fast with depth |
| Primer | About $30 to $60 per gallon | Use the one the product names |
| Trowel-on patching compound | About $20 to $40 per bag or pail | For shallow dips |
| Lath, foam, drill paddle, bucket, gauge rake | About $40 to $120 | Lath adds up on larger patches |
| 8- to 10-foot straightedge | About $20 to $60 | A straight board works if you check it first |
| Leveling and repair by an installer | Commonly about $1 to $3 per sq ft where needed | Priced as measured |
The Puget Sound flooring cost guide shows what else goes on a quote, so you can compare a patch you pour yourself with the whole job.
When to call a professional
Hand it over when:
- The patch is more than a bag or two, or the dip covers most of a room.
- You find particle board, soft spots or past water damage. Subfloor repair and leveling covers what an installer does with them.
- Old vinyl or black adhesive is in the way, or you are not sure what a residue is.
- The slab tests damp, or the crawl space below is wet.
- The floor has a slope, a bounce or cracks that move. That is a framing or foundation question first.
- The job is a whole floor, and other rooms depend on your result. Whole-house flooring explains how that is staged.
Floating floors and expansion gaps explains why flatness matters so much to this kind of floor, and DIY vs professional installation helps you draw the line. Installers work across King and Snohomish counties; call (425) 842-3483 to book a measure.
