The Floating Floor That Buckled in the Sunroom
Why this matters
A rigid-core plank floor in a room full of glass lives a different life from the same floor in a bedroom. Direct sun heats the planks far above the room's air temperature, the core grows with that heat, and a floor that has been pinned at its edges has nowhere to put the growth except up. A buckled floor is also a trip hazard the minute it rises, in a room people walk through barefoot. This case starts with getting the room safe the afternoon it was reported, then traces why the floor was able to buckle at all.
The afternoon call
In late June a homeowner called at about 3 p.m.: the new floor in her sunroom had "humped up" near the glass, high enough that her son had caught his foot on it. The shop had installed rigid-core vinyl plank, a floating floor with a stone-polymer core, four weeks earlier through the sunroom and on into the adjoining family room. She added that the hump had been there the previous afternoon too, smaller, and was gone by morning.
That detail, up in the afternoon and down overnight, said heat before anyone arrived. The first job was still the room, not the diagnosis.
Containment before diagnosis
The tech on call had a crew lead on site within the hour and gave the homeowner three instructions over the phone while he drove:
- Keep people off the raised area. A tented plank can snap its locking joint if someone steps on it, and the edge of the hump is what catches a foot. A dining chair was set on each side of it as a marker.
- Close the blinds or cover the glass. The room had a wall of south-facing glass with blinds that were open. Closing them takes the heat load off the floor and lets it start settling.
- Do not try to stamp it flat. Weight on a buckled floating floor drives the problem into the joints.
On arrival the crew lead found a tent along the glass wall about 9 feet long, standing about 3/4 inch at its peak, roughly 2 feet in from the window sill. He took the next steps in order of what would relieve the floor fastest:
Relieve the edge first. Along the glass wall, quarter round had been run along the base of the knee wall below the windows. A thin putty knife would not slide under it anywhere. He pulled the quarter round with a flat bar and a wood block against the wall, keeping the bar low and away from the glass above, because a bar that slips upward at a window wall finds the glass. Under the trim he found finish nails driven at an angle down through the edge of the planks. With the trim off and nails pulled, the tent dropped noticeably within about twenty minutes, which told him the perimeter pin was a large part of the force.
Keep the sun off. The blinds stayed closed for the rest of the day and the homeowner agreed to keep them closed on afternoons until the repair.
The floor was flat enough to walk by evening. Diagnosis waited for a cool morning, because the planks needed to be at room temperature for any measurement of gaps and joints to mean anything.
Measuring the heat
Before leaving, the crew lead took infrared thermometer readings, aiming at the plank surface:
- planks in direct sun 2 feet from the glass: 124 F
- planks in the shade of a chair 6 feet in: 84 F
- room air at waist height: 79 F
The floor in the sun was running 40 degrees above the shaded floor a few feet away and about 45 above the room air. The thermostat reading was irrelevant to what the planks were doing. That surface temperature is the number the plank maker's heat guidance is written against, and why many makers' instructions call for window treatments in rooms with large areas of direct sun. Read your product's limits; some makers also exclude unconditioned three-season rooms from warranty altogether. This sunroom was on the house's HVAC year-round, so the product was allowed in the room; the question was how it had been installed.
The cool-morning inspection
The next morning at 8 a.m., planks at 74 F in the shade and 76 F near the glass, the crew lead walked the whole installation.
The glass wall. With the quarter round off, the gap between the planks and the knee wall measured 1/4 inch along most of its length, the low end of what the maker required for this floor, and 1/8 inch for about 4 feet where the knee wall bowed in. The finish nails had pinned that whole edge to the subfloor.
The opposite wall. Baseboard with shoe molding. The shoe had also been nailed down through the planks, every 16 inches or so along the run. So both long edges of the sunroom floor were pinned.
The doorway. The floor ran straight through a 6-foot cased opening into the family room with no transition, and continued 20 feet to the far family-room wall. The sunroom is 14 feet deep, so the continuous run was 34 feet. The 34-foot run was inside this product's maximum length, so length alone was not the issue: what mattered is that a room with a heat load very different from its neighbor was tied to that neighbor, so the family room's cool floor held the sunroom floor's movement in check while the sun drove it to grow. The maker's instructions required a break at doorways between rooms and anywhere the floor passes through a narrow opening between larger areas.
Moisture from below, ruled out. The sunroom sat on a slab, and a floating floor over a damp slab can also lift, so the crew lead pulled a plank at the tent and checked under it. The underlayment's attached vapor retarder was intact with its seams taped, the slab under it was dry to the hand with no darkening, and the underside of the plank was dry. A moisture-driven lift also does not go up in the afternoon and down overnight; it stays up and grows over weeks. Had he found condensation under the retarder or a darkened slab, the next step would have been a slab test per 'Slab Moisture Testing for Flooring' before any repair.
The planks. In the tented section, five planks had locking joints that were cracked or had pulled apart at the short end and would not seat flat. The rest had relaxed back down intact.
Why it buckled
Every material involved behaved as designed. A rigid-core plank grows with temperature; its maker allows for that with expansion space at the walls and a break between rooms, and expects the floor free to slide. Three installation choices removed that freedom:
- Nails through the shoe and quarter round into the planks turned both long edges into fixed points. Expansion space is per the maker, commonly about 1/4 to 1/2 inch, hidden by base or shoe and never pinned by a fastener. Here the gap existed, but the fasteners meant the floor could not reach it. See related: Expansion Space at Walls and Fixed Objects.
- No break at the doorway tied a hot floor to a cool one, so the sunroom field was held from the family-room side as well.
- No sun control on a wall of south glass let the planks near the window run 40 degrees above the shade.
With the edges pinned and the far end held by the family-room floor, the growth in the sun-heated band had no path except up, where the planks were hottest. In plan, with every fixed point marked x:
south glass over the knee wall
+=x==x==x==x==x==x==x==x==x==x==x==x==x==x=+
| quarter round nailed through the planks |
| ^^^^^^^^^^^^^^ tent, 9 ft long, |
| 2 ft in from the sill | sunroom,
| | 14 ft deep
| shoe nailed through the planks |
+=x==x==x==x==+ 6 ft opening, +=x==x==x==+
| no break here |
| floor runs on 20 ft to |
| the far family-room wall |
The repair
1. Free the whole perimeter. The shoe on the opposite wall came off, nails were pulled from the planks, and both walls' trim was refastened into the baseboard or knee wall only, never down into the floor. Removing trim is low-hazard, but flat bars at a window wall stay low and bear on a block, and the old nails are pulled, not hammered flat, so no point is left in the floor.
2. Restore the gap where it was short. The 4-foot section at the bowed knee wall was trimmed back to the maker's gap with an oscillating tool and a guide. Cutting the plank's mineral-filled core throws fine dust: dust extraction on the tool and N95 respirators while cutting.
3. Replace the damaged planks. The five planks with cracked joints were replaced from the job's leftover cartons, using the method in 'How to Replace One Plank in the Middle of a Floating Floor'.
4. Add the break at the doorway. The crew cut the floor in the center of the cased opening, removed a strip for the T-molding track, and fastened the track to the subfloor only. The sunroom and family-room fields now move independently, each with its own expansion space under the cap.
5. Settle the sun control. This part was the homeowner's, and the crew lead put it to her in plain terms.
The window conversation
He showed her the three temperature readings and the page of the plank maker's instructions calling for window treatments in high-sun rooms. The floor could not be warranted in that room with the glass uncovered on summer afternoons, he told her, because the maker made sun control a condition of the installation; the repair freed the floor to move, but it could not make a 124 F plank stop growing.
She asked what counted. He named what the maker's guidance described in substance: blinds, shades or drapes used during peak sun, or a window film that cuts solar heat, and noted that rugs laid over the floor in the sun band trap heat under them, so they were not a substitute. She chose solar shades on the south glass. The shop recorded in the job file that the shades were the homeowner's to install and use, with the maker's page attached.
How it was confirmed
The crew lead returned on a sunny afternoon two weeks after the shades went up: shades down, plank surface near the glass 88 F, the T-molding cap sitting flat on both sides, a putty knife sliding freely under the trim on both sunroom walls, and no tent. A second check in August found the same.
References
- Rigid-core flooring manufacturer's installation instructions: expansion space, required breaks, trim fastening, heat and direct-sun guidance, and room exclusions.
- See related: Expansion Space at Walls and Fixed Objects; How to Install Laminate or Rigid Core Floating Floors; How to Fit the Last Row and Door Jambs When Floating; How to Replace One Plank in the Middle of a Floating Floor.
- See related: What Is Inside a Laminate or Rigid-Core Plank; How to Install Baseboard and Shoe Over a New Floor.