Floating Floor Installation

A Raft, Not a Floor Covering

A floating floor is a raft. Nothing fixes it to the building — no cleats, no adhesive to the substrate, no anchors through the surface. It rests on the subfloor, held by its own mass and by friction against whatever separation layer sits beneath it, and it grows and shrinks as one continuous assembly. Fix that idea before the first carton is opened, because every dimension, every void and every transition detail on the job exists to protect the raft's freedom to change size.

Cores behave differently but end in the same place. Laminate HDF, engineered plywood and cork respond mostly to humidity, gaining width through a humid summer and giving it back through a heated winter. Rigid vinyl and SPC respond mostly to temperature, stretching under direct sun and drawing in when a house is left unheated. Two drivers, one consequence: the floor laid in March is not dimensionally the same floor in August.

Peaking at end joints, a squeak that travels, a plank crept clear of a doorway, a click seam opened to a hairline — those are rarely manufacturing defects and rarely sloppiness in the ordinary sense. They describe a floor denied somewhere to expand, or a floor held at one point while the rest of it stayed free.

What the Substrate Gives the Raft

Moisture in the substrate becomes moisture in the floor, and moisture is movement. Concrete releases vapour for months after it stops looking wet. Relative humidity testing to ASTM F2170 Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes reads the slab at depth and in service conditions; calcium chloride testing to ASTM F1869 Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride reads only the surface layer. Run the tests before ordering rather than on installation morning, because a failing slab puts a mitigation membrane and a cure period into the programme.

Flatness governs whether the raft can slide and whether its joints survive. A hollow beneath a click seam turns every footstep into a hinge, and the joint works loose long before the wear layer gives up. Manufacturers publish their own tolerance over a stated span — take theirs rather than quoting a trade habit, because rigid core and HDF are not held to the same figure and the warranty follows the printed one. Grind the high spots, fill the low ones with a leveller mixed and poured to its own instructions, and let it cure on its schedule. Concrete preparation itself is covered by ASTM F710 Standard Practice for Preparing Concrete Floors to Receive Resilient Flooring.

Timber subfloors carry a separate moisture story. Meter the deck and the flooring, note the difference, and refuse a deck that reads wet; a joisted floor over a damp crawl space keeps feeding the raft above it. Refasten every squeak, punch proud nails and plane down any seam lip while it is still reachable — none of that is fixable through 8 mm of laminate.

Underlayment does three jobs and one of them belongs to expansion. It carries the vapour retarder where the substrate needs one, it absorbs minor deviation, and it forms the slip plane the raft travels across. Lay it with laps taped as the maker specifies, never doubled up to disguise a flatness problem, and never let it climb the wall as a soft filler in the perimeter gap where a fitter later trims it flush and blocks the void.

Conditioning Stock to the Room It Will Live In

Acclimation is a dimensional exercise, not a ritual. Cartons are conditioned to the temperature and humidity the floor will actually experience in service, which means heating or cooling running and the building closed — not a shell with open doorways and plaster still drying. Follow the maker's instruction on whether cartons stay sealed, get cross-stacked or are opened; the answers differ by core, and getting it wrong changes the plank's width at the moment you lock it into a row.

Cold delivery deserves particular attention on rigid core. Planks unloaded near freezing and installed before they warm will expand as the room comes up to temperature, and a perimeter gap cut for a normal season absorbs movement it was never sized for. The reverse case is quieter and worse: warm planks fitted tight in a heated house, then a vacant winter with the heating off, and every end joint opens.

Site conditions worth arguing about before a date is agreed: wet trades finished, glazing in, heating commissioned, and a slab tested rather than assumed. A floor installed into a drying building takes on moisture it will later give back, and the gaps that show up in the second season were built in during the first week.

Working Out the Quantity

Take off the area room by room rather than as one gross figure, because the raft is broken at doorways and thresholds and each field is cut separately. Add waste against the geometry actually being laid: a straight run in a rectangular room recovers most offcuts into the next row, a diagonal or herringbone layout does not, and a plan with several bays or a stair well generates cuts nothing else uses.

Order the full quantity in one batch and leave a spare carton on site after handover. Shade and gloss shift between production lots, and the plank needed in year three is usually the one damaged by a fridge dragged across the floor — itself a movement problem, since a heavy appliance shoved over a raft can drive a whole field into the wall.

Underlayment gets counted separately and rarely lands on the same number as the flooring. Rolls come in fixed widths and lengths, laps and turn-ups consume coverage, and part rolls do not stretch across a corridor. Work the roll count from room dimensions and the running direction you intend to use.

Setting Out So the Last Row Still Fits

Measure the cross-run of the room before ripping a single board. Divide the width by the plank width and look at the remainder; if the last row lands as a sliver, take the difference off the first row so both edges finish wide enough to hold a spacer, carry a perimeter gap and take a pull bar without splitting. A 20 mm final row does none of those things.

Square off a line struck from a measured centre rather than from a wall you have not checked. End-joint stagger follows the manufacturer's minimum offset, and the stagger sequence doubles as a waste strategy: the offcut ending one row starts the next wherever room length allows. Watch that adjacent rows do not repeat a joint position across the field, which reads as a stripe under low-angle light.

Plan direction against the light and against the run. Boards laid down the length of a long room look right and also put the greatest accumulated expansion along the longest dimension — the wall that then needs the most generous gap and the earliest check.

The Gap Is the Detail

Perimeter gap scales with the run it serves. A manufacturer states a minimum and often an increase per metre of continuous dimension; that number belongs to the product, so take it from the sheet in the carton rather than from the last job. Gap goes at every vertical the raft can reach: walls, columns, pipe penetrations, hearths, tub aprons, door jambs, and the front edge of a stair nosing.

Undercut jambs and casings instead of scribing planks around them, and slide the board beneath. Pipe collars are cut oversize and covered with a two-part rose that sits on the plank rather than screwing through it. Skirting fixes to the wall, never down through the floor into the subfloor; scotia or quarter-round fixes to the skirting for the same reason.

Leave the gap open. Foam backer with a flexible sealant is acceptable where a wet area needs closing, provided the material stays compressible — a bead of hard adhesive, or mortar butted against tile, converts a free edge into a fixed one and the movement reappears somewhere less convenient. Walk the perimeter with spacers still in, count them out as they are pulled, and confirm the gap survived before skirting covers the evidence.

Everything That Can Pin a Raft

Kitchens are the usual scene. A floor run under a cabinet bank carries the cabinet's weight and the loaded worktop above it; an island bolted through the plank into the slab spikes the raft at a point and the field tears around it. Decide early whether the floor stops at the cabinet toe with a gap and matching trim, or runs under with the cabinets shimmed to bear on the subfloor — both details work, guessing does not.

Sanitaryware repeats the problem in miniature. A pan bolted through a floating floor into the slab, a pedestal grouted down, a partition track screwed through: each one is a fixed point on a moving field. Transition profiles fasten to the subfloor inside their own channel and never through the flooring; a screw driven through a plank to hold a threshold has effectively nailed the floor to the building.

Loads matter as much as fixings. Tall wardrobes with feet standing on two rows, a piano, a bank of shelving against a wall — anything heavy enough to hold one part of the raft while the rest moves will telegraph as a raised seam elsewhere. Furniture that must sit tight to a wall goes on the far side of a break, or the floor stops short and the unit lands on the subfloor.

Where the Field Has to Break

Manufacturers publish a maximum continuous run and a maximum continuous area, and those are the figures clients argue with hardest. Asking for an unbroken floor from a hallway through two bedrooms is asking for a raft longer than the product is rated to move as one piece. The break belongs in a doorway under a T-profile, where it reads as a deliberate line rather than a compromise.

Shape counts as much as length. An L-plan with a narrow neck funnels the movement of two large fields through a metre of floor, and that neck peaks first. Break at the neck. Long corridors feeding rooms on both sides need the same treatment at each doorway, and every break needs its own gap on both sides of the profile — a T-moulding pushed tight to one edge is a fixed point in disguise.

Put it in writing at quotation stage. A break added after the client has pictured a continuous floor becomes a change; a break priced and drawn from the start is a specification. The alternative turns up as a callback in the first hot spell, and once a click seam has peaked the repair is usually to lift back to the wall and refit.

Heat, Sun and the Floor That Moves Twice a Day

Underfloor heating shifts the driver from seasonal to daily. Confirm the product is approved over the system, that output stays inside the maker's limit, and that a floor sensor with a high limit is fitted rather than air sensing alone. Commission the screed through its own heat-up and cool-down cycle before the floor goes down, then bring the system up gradually — the surface temperature ceiling is the manufacturer's figure, and exceeding it degrades the core and the joint, not merely the finish.

Insulating a heated floor is a hidden fault. A thick rug over a heated bay traps output beneath it and lifts local surface temperature above the surrounding field, so the trapped area expands against neighbours that have not moved. Hand that over as a written instruction to the occupant, together with the underlayment's thermal rating, which has to suit the system rather than be chosen for footfall comfort.

Solar gain does the same thing without a thermostat. Full-height glazing on a sun-facing elevation can drive a rigid core plank well above room temperature by mid-afternoon, and an empty house with blinds up through a heatwave is the worst case anyone builds. Where glazing dominates, argue for a lighter surface, a shaded specification or more generous breaks; conservatories and garden rooms sit outside many warranties for precisely that reason.

Handover, and the Two Seasons After It

Check the raft before it disappears. Pull the spacers, verify the gap survived at every wall, confirm transitions are fixed to the subfloor, and press the field along seams to find any hollow a leveller pour missed. Photograph the perimeter before skirting goes on — that photograph is the only surviving evidence the gap existed.

Give the occupant the humidity range the manufacturer states and explain what falls outside it. A heated house through a dry winter can run well below the lower bound, and the seasonal gapping that follows is the floor behaving rather than failing. Small movement at end joints that closes again in summer needs no intervention; a seam that has peaked, a plank that has cupped, or a gap widening every season points at moisture from below or a fixed point somewhere in the field.

Keep the file. Slab test results with dates, the acclimation window, flatness readings, the underlayment used and the manufacturer's instruction sheet as printed for that batch — a warranty claim on a floating floor turns on whether the raft was given room and whether the substrate was proved, and both are argued from paperwork long after the site has been handed back.