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 (5/16 in) 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.
Hollows that barely register under a straightedge still drink compound by the bag, and the pour has to be priced off measured area and an honest average depth while the leveller can still be ordered in one delivery.
The total subfloor area to be leveled.
The average thickness of the self-leveling compound pour across the floor.
SLU volume needed
42.1 gal
Actual SLU consumption depends on subfloor roughness/low spots — an uneven subfloor will consume more material than this average-depth estimate suggests. Order 10-15% extra for irregular subfloors.
They open the calculator with your figures already in it
Self-Leveling Underlayment (SLU) Volume Calculator: 42.08 gal — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- Litres do not buy anything. SLU is sold by the bag, and turning this volume into a bag count needs the product's own yield — the litres one bag makes at its specified water ratio — which runs roughly 12 to 16 litres (3.2 to 4.2 gal) from a 25 kg (55 lb) bag and varies enough between products to shift the bag count by 10% or more on the same floor. Take the yield off the data sheet rather than assuming a density.
- The high spot sets the datum for the whole floor, and this takes an average instead. Every SLU has a minimum thickness it will flow and cure at, and you cannot pour thinner than that over the highest point — so nothing on the floor ends up thinner than that minimum and the low corner takes the minimum plus the whole deviation. There is also a maximum lift, above which the pour must be split or extended with aggregate, and adding aggregate changes the yield again.
- Primer is a separate material with its own coverage rate, and skipping it is how a pour fails rather than how it saves money. A porous slab pulls water out of the compound and pinholes; a sealed or non-porous substrate gives it nothing to grip. Porous slabs often take two coats, and the primer rate per square metre has no relationship to the pour depth calculated above.
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.
Room-by-room area, waste allowance and carton count before the field is split at the doorways.
The length of the room to be floored.
The width of the room to be floored.
Different materials and layouts waste different amounts of material.
Estimated flooring needed
161.5 sq ft
- Room area
- 149.5 sq ft
- Waste factor applied
- 8 %
They open the calculator with your figures already in it
Flooring Calculator: 161 sq ft — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- The waste allowance is fixed by the material and layout you choose — 8, 10, 13 or 15 percent — and there is no field to raise or lower it, so a floor with an unusual amount of cutting, such as a hallway of doorway returns or a run of bay windows, cannot be given more than its category carries.
- Each side is capped at 30 m (100 ft) and only one rectangle is accepted, so an L-shaped or open-plan floor has to be broken up and run piece by piece, and because the percentage was set for a plain room it does not widen for the extra cut perimeter an L-shape brings.
- Nothing is subtracted from that rectangle: a kitchen island, a hearth, a fitted wardrobe or a chimney breast is still counted as floor by length times width, so the figure covers the full footprint between the walls.
- Board width and tile size are never asked for, so every straight-set tile job takes the same 10 percent whether the units are 600 mm (24 in) squares or the small formats the material help itself says cut more heavily.
- The answer is an area in square metres, not a count of boxes, packs or planks, and since flooring is sold in boxes covering a fixed area the quantity you actually order will be the next whole box above this number.
- The cost line prices the whole waste-inflated area at an installed rate per square foot, so the offcut destined for the skip is charged at the same rate as the floor you walk on.
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 (3/4 in) 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.
Plank counts by row, starter rip width and end-joint offsets for click LVP, laminate and engineered board.
Net floor area.
Area covered per pack, from the label.
Complexity drives the offcut you cannot reuse.
Packs required
13 packs
Keep a full pack spare. A damaged plank in a click floor is replaced by lifting back to it, and a batch-matched board is the only invisible repair.
- Area with waste
- 288.9 sq ft
- Coverage ordered
- 307.85 sq ft
- Waste applied
- 7 %
- Spare after the job
- 37.85 sq ft
- Area
- 270 sq ft
They open the calculator with your figures already in it
LVP, Laminate & Engineered Floor Calculator: 13 packs — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- Nothing caps the size of the floating field. Click systems carry a manufacturer maximum for one continuous run — commonly around 10 to 12 m (33 to 40 ft) in either direction, and a break at every doorway regardless — past which an expansion joint and a T-profile are required. An open-plan floor entered as a single area returns the packs for it and shows none of the profiles, none of the doorway breaks, and none of the extra part-rows that start again on the far side of each one.
- Plank length never enters the arithmetic, and it is what decides whether the offcut chain the waste bands assume actually closes. The tail cut off one row starts the next only if it beats the manufacturer's minimum starting piece — commonly 300 mm — and only if it holds the end-joint stagger from the row below, usually 300 mm or a third of a plank. A room whose width in the plank direction lands just past a whole number of planks throws that tail away on every single row, and 7% will not cover it.
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.
Spacers only work if they hold every wall at once, and the bag in the van has never yet held enough — so the count belongs to the room's perimeter rather than to a handful grabbed on the way in.
The total perimeter of the room where the floating floor meets the walls.
The distance between spacers along the perimeter.
Expansion gap spacers needed
68 spacers
- Spacer centres round the room
- 1.16 ft
They open the calculator with your figures already in it
Floating Floor Perimeter Expansion Gap Spacer Calculator: 68 spacers — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- Counts spacers, and says nothing about the GAP they hold. Gap width is set by how far the floor can move, which follows the run length and the product — most click laminate and LVP makers ask for 10-12 mm at every edge and then cap a continuous run, commonly somewhere around 8-12 m, before an intermediate expansion joint is required. Perfectly spaced spacers holding a gap too narrow for the run still peak the floor in the middle of the room.
- The wall line is not the only place the floor has to be free. Door casings, pipe risers, hearths, stair nosings, a kitchen island's feet and every threshold bar need the same clearance, and none of them appear in a wall perimeter. One screw driven through a transition strip, or one radiator pipe grouted tight, pins the whole field and the boards lift somewhere else entirely.
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 (3 ft) 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.
Every break drawn at quotation stage needs a profile long enough for the opening it lands in, and those lengths are ordered alongside the flooring rather than discovered on the day the field is cut.
SettingsSettings for this calculation
Waste is set to 5% by hand. Pick a tier above to replace it, or keep your own figure.
How many doorways or flooring-material changes need a transition strip.
The typical width of each doorway or transition opening.
Extra material for cuts and fitting at each transition.
Transition strip needed
15.75 ft
They open the calculator with your figures already in it
Floor Transition Strip Calculator: 15.75 ft — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- Every transition is measured from the single Average Opening Width figure, so a job that mixes standard 0.9 m (3 ft) doorways with one wide archway comes out short, and the field itself stops at 1.5 m (5 ft) — narrower than a set of double or sliding doors, which have to be entered as their own separate run.
- The opening width is treated as the finished strip length, with nothing added for a strip that overruns the jambs, tucks under the door stop, or turns a corner where the material change runs L-shaped into the room.
- Waste is one flat percentage applied to the whole total rather than to each opening, and it caps at 15% — at the default 5% across five doorways that is around 22 cm (8.5 in) of spare material for the entire job, not a guaranteed second attempt at any one threshold.
- The answer is a single continuous length in metres with no piece count derived from it, so a 4.73 m (16 ft) total still usually means buying a strip per doorway rather than splitting 4.73 m (16 ft) of stock between them.
- Fixing hardware falls outside the arithmetic entirely: the screw-down track or channel that two-part strips clip into, the plugs and screws, and any adhesive all have to be quantified separately from the linear length shown here.
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.
Before the first carton opens
Five checks that decide whether the raft can move, taken in the order they bite on site.
- Slab moisture record — In-situ RH probes to ASTM F2170, read in service conditions and dated before the order goes in.
- Flatness survey and fill — Manufacturer's tolerance over their stated span; grind highs, pour leveller for hollows and let it cure on its own schedule.
- Underlayment / vapour retarder — One system, taped laps, thermal rating suited to any heating below; count rolls from room dimensions, not floor area alone.
- Perimeter spacers — Enough to hold every wall simultaneously, sized to the gap the product's run length calls for.
- Transition profiles — One per doorway and per planned run break, fixed into the subfloor within their own channel — never through a plank.
- Waste allowance — Set against the actual layout: straight run recovers offcuts, diagonal and herringbone do not; order one lot.
Opens the calculators above on one screen with the dimensions from this article already filled in. Quantities only — this site publishes no price list, because local prices vary too much to publish honestly.