The bath comes out on the Monday and the tape goes straight into the void
Two point four by one point nine, first floor, chipboard deck on 220 mm joists running the short way. The bath is going out and a 1500 by 900 walk-in is going in, level access, no upstand. Everybody is thinking about tile. The only measurement that matters that morning is how far it is from the top of the deck to the bottom of the ceiling below.
A wet room is not a cubicle with the tray left off. A cubicle holds its water in a moulded vessel with a rim, and everything outside that rim tolerates a splash. Take the rim away and the containment moves into the build-up: a bonded membrane under the whole floor and up the walls, with a drain at the bottom of a slope you have to construct. The tile stops being the finish over a waterproof thing and becomes a wearing surface over it.
That shift is where retrofits go wrong. Nobody costs the fall, because a fall is invisible on a plan. Nobody costs the tanking properly, because it gets quoted per square metre of floor when the real area is nearly three times that. And nobody checks the gully body against the joist depth until the plumber is holding it.
Everything above the joist is a budget of millimetres
Settle the budget first. Below the deck sits the gully body, the trap and enough fall on the waste run to reach a stack; above it sits the board, the screed or former, the membrane, the adhesive and the tile. Both halves compete for the same joist depth, and the loser is usually the ceiling below.
On a hand-cut floor a realistic stack above the deck is 6 mm of board, a screed running from a few millimetres at the drain to twenty-five or thirty at the far edge, a membrane you can ignore dimensionally, 3 to 5 mm of adhesive and 8 to 10 mm of tile. Call it 45 mm at the high side. That is what the threshold, the door leaf and every adjoining finish have to absorb, and on a retrofit into a hall with engineered board it is exactly what is not available.
So the budget is settled by subtraction rather than preference. Whatever the finished floor level has to be, and whatever the ceiling below can lose, the difference is what the gully fits into. Former or hand-cut, point gully or channel, mosaic or large format — all of it follows from that arithmetic.
What a level-access wet room floor is made of
- Tile and grate — a wearing surface, not the waterproofing; small enough in the fall zone to follow a plane that tilts two ways at once, and cut around the grate on all four sides Floor & Wall Tile Calculator
- Adhesive bed — two notch sizes on one floor because the tile changes size at the edge of the fall zone, and two consumption rates with it Thinset & Tile Adhesive Calculator
- Tanking membrane — the only continuous barrier in the assembly, dressed into the gully flange, banded at every internal corner and carried up the walls past the wettest line in the room Cold-Applied Liquid Waterproofing Membrane Volume Calculator
- Screed cut to falls — thickest at the far edge of the catchment and thinnest at the drain flange, which is why its volume is an average depth rather than a slab thickness Wet Room and Curbless Shower Floor Fall Calculator
- Tile backer board — the rigid, dimensionally stable plane a bonded membrane needs, screwed down at the spacing the board maker states and taped at every joint Tile Backer Board Calculator
- Joisted deck — the depth from here down to the ceiling below is the whole budget the gully body, the trap and the waste fall have to fit inside
The gully fits from the bottom up, and it is the trap that sets the floor
A wet room gully is two parts: a lower body carrying the trap and the outlet, and an upper frame that adjusts to the finished floor. Shallow-body versions are genuinely shallower, but there is a limit under how shallow they go. The trap has to hold a water seal, and the International Plumbing Code requires that seal to be not less than 2 inches — about 51 mm — with a stated upper limit too. A body carrying a compliant seal, a removable insert and an outlet is not going to be 30 mm tall in anybody's catalogue.
Then the outlet has to leave. A vertical outlet is the shallowest arrangement in the room and the deepest in the ceiling below, because the swept bend lives under the joists. A lateral outlet keeps the pipe in the joist zone, which is what most first-floor retrofits need, and immediately raises whether the joist can be cut. Usually it cannot: the notching and boring limits in the floor framing provisions of the adopted code confine notches to shallow cuts near the supports, and a 40 mm waste crossing joists at mid-span with a gradient on it is not a notch.
Sequence follows. The gully body is set to the finished floor level, never to the deck, and the frame adjusted last against a datum already marked on the wall. Setting a gully off the deck and hoping the build-up lands on it is the most expensive error available here: the correction is a screed running thicker everywhere to catch up, or a gully coming out after the pipework is boxed in. And where the outlet can go decides where the drain sits, which decides the shape of the fall.
Former or hand-cut: the deck usually decides, not the tiler
Two ways to make a floor fall. Buy a pre-formed sloped panel — rigid polystyrene with the gradient moulded in, sold as a system with a matched gully and often its own bonded waterproofing, of the kind wedi publishes for Fundo and Schlüter-Systems for KERDI-SHOWER. Or cut the fall by hand in screed or a proprietary fall mortar, to a gully you positioned. The first is a component; the second is a skill.
The honest version is that the former wins on timber and the hand-cut fall wins on concrete. On a joisted deck the former gives a factory gradient, a stated depth, a drain with a documented body height and a warranty covering the interface between fall and waterproofing — worth real money when the budget is tight and the deck moves. On a slab, where depth is cheap, cutting by hand costs a day, fits any shape, and does not lock the shower to a catalogue size.
| Pre-formed sloped panel | Fall cut by hand | |
|---|---|---|
| Gradient | Fixed by the mould; read it off the data sheet before buying, because it is what proves compliance | Yours to set, and yours to hold across the whole catchment with a straightedge |
| Depth used above the deck | Stated by the maker as one number, matched to a known drain body | Whatever the longest run demands, plus the flange standoff at the drain |
| Shape | Rectangles and a small set of drain positions | Any shape, any drain position the drainage will accept |
| Weight added | Negligible — a rigid foam panel | Roughly 2 tonnes per cubic metre of screed, worth checking on old joists |
| Where it fails | A perimeter feathered badly into the surrounding floor, or a former cut down and its fall ruined | A gradient that flattens or reverses part-way, found only when the floor is flood-tested |
| Best fit | Timber decks, tight depth budgets, standard shower sizes | Concrete slabs, irregular rooms, linear channels along a wall |
The gradient band is far narrower than the trade thinks it is
Ask five fitters what fall a wet room needs and you get five ratios. The codes are less relaxed. The International Plumbing Code sets the shower floor slope toward the drain at not less than one-quarter unit vertical in twelve horizontal and not more than one-half in twelve — 1:48 to 1:24. Below the minimum a film of water sits rather than runs; above it a wet floor is something you can slide on.
Now add accessibility. ICC A117.1, Accessible and Usable Buildings and Facilities, limits the floor of an accessible shower compartment to a slope no steeper than 1:48 in any direction. Set that beside the plumbing minimum of 1:48 and the band collapses to a single value with no tolerance either side — a real constraint on a hand-cut fall, and a very good reason to buy a former whose gradient is a printed number rather than a promise from a trowel.
The gradient then decides the tile, not the other way round. A point gully in the middle of a square catchment produces four triangular planes meeting at hips, and a rigid tile cannot follow two of them at once — which is why the fall zone gets mosaic on sheets, and why 600 mm porcelain there is a lippage complaint waiting to be written. A channel against one wall makes the fall a single plane in one direction, which a large tile follows comfortably. That is the real reason linear drains took over the domestic market.
The trade-off is depth. A 1.5 m square catchment — on the example floor that is the 1500 by 900 shower plus the strip beyond the screen line that falls with it — with a centre gully needs about 22 mm at the corner to hold 1:48, because the diagonal run is only 1.06 m. Put a channel along one wall and the run becomes the full 1.5 m, so the far edge needs 31 mm — nine millimetres out of a budget that did not have them, which is how a linear drain quietly turns into a dropped ceiling.
Cutting the fall by hand, and why the volume comes back short
A hand-cut fall is a bonded topping laid to a gradient, and it is governed as a screed: BS 8204-1, the code of practice for concrete bases and cementitious levelling screeds to receive floorings, sets how flat it has to be along the plane of the fall. A floor can drop 22 mm to a gully and still owe the tiler a plane straight enough to bed mosaic on without hollows.
The volume looks like the easiest number on the job and is the one most often under-ordered. Take that catchment, centre gully, 22 mm at the walls. The usual take-off treats the screed as a linear taper from its maximum at the perimeter to zero at the drain, so the average depth is half the maximum: 2.25 m² times 11 mm, about 25 litres. That is exact for a straight-line fall to a channel, and wrong here.
For a point gully it is short twice over. Four planes falling from level walls to a centre drain make a shallow upside-down pyramid, and a pyramid holds a third of its box — so the fall averages two-thirds of its drop, not half, because most of the area sits out towards the deep edges. And the screed never reaches zero: it lands on the drain flange, which stands proud, so a few millimetres of constant thickness runs under the whole zone that the taper model throws away. Put 4 mm of standoff and the two-thirds into the same square and the honest figure is about 42 litres, not 25.
The channel version lands close by — a 1.5 m run at 1:48 averages 15.6 mm across 2.25 m², about 35 litres, and 44 with the same 4 mm under it. It costs depth at the far edge and almost nothing in material, and what it buys is a single-plane fall.
Enter the catchment and where the gully sits: the page holds the fall along the valley from the farthest corner, carries the flange depth under the whole floor, and averages a point gully at two-thirds of its drop — the forty-two litres above, not the twenty-five a straight taper gives.
The length of the floor that falls to the drain, between its two end walls.
The width of the same floor, between its two side walls.
Where the water leaves the floor, which decides the shape of the fall.
How far the centre of the drain is from one of the end walls, measured along the length.
How far the centre of the drain is from one of the side walls, measured across the width.
The flattest the floor may be anywhere: 1 unit of fall in N units of run — enter N.
The thinnest the screed gets — at the drain's flange.
How far the waterproofing turns up every wall around the floor.
Screed depth at the walls
2.25 in
The walls stay level, so every wall starts its tiles on one line. The floor falls in four faces to the drain; water crossing a face runs into the valleys from the corners, so the fall is held on the longest valley and the faces are steeper. The steepest face falls at 1:29, against 1:48 along the valley from the farthest corner.
- Screed depth at the drain
- 1 in
- Drop from the walls to the drain
- 1.25 in
- Longest run, from the farthest corner to the drain
- 5 ft
- Fall along that valley — the flattest line on the floor
- 2.08 %
- Fall on the steepest face, towards the nearest wall
- 3.47 %
- Screed volume, with the depth at the drain under all of it
- 54.86 gal
- Floor area
- 48 ft²
- Area to tank — the floor and its upstand
- 62 ft²
- Floor-to-wall junction to reinforce
- 28 ft
They open the calculator with your figures already in it
Wet Room and Curbless Shower Floor Fall Calculator: 2.25 in — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 2.25 in — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- One rectangular floor and one drain. An L-shaped room, two drains, or a shower zone that falls while the rest of the floor stays flat is set out zone by zone, and the flat part is a levelling job, not a fall.
- The depth at the drain is the reader's: the screed's own least thickness for how it is laid, or the drain flange's height above the deck, whichever is greater. On a timber floor the joists usually have to be dropped or the deck lowered to find this depth, which is structural work this page does not check.
- The tanking area is the floor and an upstand around it. The walls inside the showering area are tanked much higher and are not included unless entered as the height; primer, membrane and tape are then bought from these areas and lengths at the product's own coverage.
- It reports the falls and does not judge them. An accessible shower has its own limit on how steep its floor may be, and a point drain's faces are always steeper than its valleys — which a floor held to its minimum along the valleys cannot avoid.
Outside the fall zone the floor is flat, and that is a different pour entirely
The fall belongs to the catchment, not the room. Outside the shower zone a wet room floor is a normal tiled floor held to a tolerance rather than a gradient: ANSI A108.02, the general requirements for subsurfaces and preparations by other trades, sets the substrate tolerance at a quarter inch in ten feet for tiles with an edge under fifteen inches, tightening to an eighth at fifteen inches and above. A 1990s chipboard deck with a bath scar across it meets neither.
Self-levelling underlayment is the usual answer, with three rules that get broken on bathroom jobs. It needs a primer matched to the substrate, and on timber a grade rated for timber — a different product from the slab version. It has a minimum thickness, typically 3 to 5 mm, and feathering to nothing at the edge of a pour is how you get a scab that debonds eighteen months later. And it is not waterproof, so inside the tanked area it goes under the membrane, never over it.
Cost the weight as well as the volume. Levelling compound sits around two tonnes per cubic metre, so a 10 mm average pour is roughly 20 kg on every square metre — about 0.2 kN/m² added to a floor already carrying a full mixer and a tiler. On sound modern joists that is noise; on a Victorian floor it is worth putting in front of whoever signs off the structure.
Level the room outside the catchment, not the catchment itself: take the average depth from straightedge readings across the field and keep the pour clear of the zone where the fall does the work.
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.
Primer, bandage, two coats — the tanking is a system, and it is sold as one
A liquid tanking membrane is not paint and is not sold by the coat. The system is a primer matched to the substrate, a reinforcing tape set into the first coat at every internal corner, wall-to-floor junction, board joint and change of plane, collars at each penetration and at the gully flange, and a stated dry film thickness built up in the number of coats named. Skip the bandage and the membrane is bridging a moving joint with nothing in it; skip the primer and it is bonding to a board that drinks the first coat.
Primer is the item most often left off a quote. Its job is to control the substrate's absorbency and give the membrane something consistent to key into, and its dilution is substrate-specific — a dense slab and a lightweight backer board do not take the same mix. Test with a drop of water first: if it disappears in seconds, the primer is doing real work. Price it against the same area as the membrane, which is the whole tanked area and not the floor.
That area is what surprises people. The example room is 4.56 m² of floor. The shower sits in a corner, so two walls are tanked full height for the width of the zone plus the overrun the manufacturer requires — call it 3.6 m of run at 2 m high, about 7.2 m². The rest of the perimeter takes a 150 mm turn-up behind the skirting, another 0.7 m². Total 12.5 m², nearly three times the floor, bought twice over.
Then the wet volume, where solids content stops being a data-sheet curiosity. Membranes are specified by dry film thickness — what is left after the water or solvent has gone — and bought by wet volume. A single-part dispersion at around 60% solids needs 1.27 mm of dry film applied as roughly 2.1 mm wet, which over 12.5 m² is about 26 litres. A two-part cementitious slurry at 90% solids reaches the same dry film in about 18 litres. Same area, a third less material.
Specify against a document rather than a brand: BS EN 14891 in Europe, for liquid applied water impermeable products used beneath ceramic tiling bonded with adhesives, which tests the cured film for crack bridging and for adhesion after water contact; ANSI A118.10 in North America, for load bearing, bonded, waterproof membranes for thin-set ceramic tile and dimension stone. A product carrying neither is not a tanking membrane, whatever the tin says.
Put in the full tanked area — floor plus wall run plus turn-up, not the floor alone — then the dry film thickness and the solids content off the technical data sheet — the thickness field is in mils, so the 1.27 mm above goes in as 50 — because those two together decide whether the same specification costs eighteen litres or twenty-six.
The total surface area to be waterproofed.
The cured (dry) membrane thickness required by the manufacturer's spec.
The percentage of the wet product that remains after solvent/water evaporates during cure.
Liquid membrane volume needed
25.2 gal
Solids content and dry film thickness requirements vary by manufacturer and application (horizontal vs. vertical, exposure condition) — confirm your specific product's technical data sheet for exact coverage rates.
- Required wet film thickness
- 75 mils
They open the calculator with your figures already in it
Cold-Applied Liquid Waterproofing Membrane Volume Calculator: 25.25 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
- A cold-applied system is resin plus reinforcement, and the reinforcement drinks far more than the field coat does. Polyester fleece has to be wetted out from underneath and above to be properly embedded, and every upstand, internal corner, outlet, penetration and movement joint takes a reinforced detail band laid before the field goes down. Detailing consumes material at a rate that has nothing to do with the flat area, and it is where jobs run short.
- The area entered is a flat surface, and a waterproofed roof is not one. Upstands of 150 mm (6 in) and more right round the perimeter, kerbs to every rooflight and plant base, gutter cheeks and the flange of every outlet are all additional surface, and on a small busy roof that detail area can rival the field it surrounds.
When a bonded sheet beats a brush
Sheet tanking — a polyethylene core fleeced both faces and bedded in the tile adhesive — solves the two problems liquid membranes have on a small domestic job. There is no wet film thickness to police, because the thickness is manufactured rather than applied, and no cure clock between waterproofing and tiling, because the sheet bonds with the same mortar the tile goes on with. Over a timber deck that will always move a little, a sheet also decouples in a way a thin brushed film does not.
It loses where the geometry gets complicated. A hand-cut fall to a point gully has four hips and a compound corner at the drain, and folding a sheet into that neatly is a genuine skill. Systems answer with preformed corners, pipe collars and a bonding flange moulded into the drain, and those are not optional extras — they are what makes the system a system.
The roll take-off has one trap. Roll count is area divided by coverage and rounded up, and the coverage figure on the data sheet may or may not already allow for lap. Sheet systems typically want 50 mm side laps and preformed pieces doubling up at every corner, so the 12.5 m² example is bought against something closer to 14.5 m². Against 5 m² rolls that is three; against a single 30 m² roll it is one, and half of it left in the van.
The arithmetic is the same wherever the sheet goes — area against the roll's rated coverage, rounded up to a whole roll — so inflate the area for laps and corner overlays first, unless the data sheet says its coverage figure is already net of them.
The total below-grade foundation wall area to be waterproofed.
The coverage area of a single membrane roll, per the manufacturer.
Membrane rolls needed
9 rolls
They open the calculator with your figures already in it
Below-Grade Waterproofing Membrane Roll Calculator: 9 rolls — 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 here asks whether the wall will actually accept the membrane. Self-adhered and torch-applied sheets bond only to a sound, dry, clean substrate, and most carry a minimum application temperature. Honeycombing, unfilled tie holes, form release still on the face, or a wall that is too green or too cold all have to be dealt with first — an unbonded sheet is a channel that carries water sideways behind it, which is worse than no sheet at that spot.
- The membrane still has to survive backfill, and what protects it is not in this count. Sheet membrane punctures easily on angular fill and under the machine placing it, so a protection board or drainage composite normally goes on over the same area. Unlike a miscounted roll, a sheet torn during backfill is buried: it cannot be found or repaired without excavating the wall a second time.
Flood it while you still can
There is exactly one moment when a wet room floor can be tested: after the tanking has cured and before a single tile goes down. After that the test is destructive. The shower liner test in the tests and inspections provisions of the International Plumbing Code is the model — plug the drain, fill the receptor, hold it 24 hours, look. There is no good reason to skip it in a house, where a pinhole means a stained ceiling over a living room.
Mark the water line in pencil rather than trusting your memory of it, and photograph the whole floor first — laps, bandage runs, collars, the gully flange. A damp patch in year three is diagnosed from those photographs, or by lifting tile.
- Confirm the membrane has reached the manufacturer's stated cure time, not just that it has stopped feeling tacky.
- Plug the gully with the test plug the drain maker supplies, or a proper expanding plug — not a rag.
- Fill to the highest point of the tanked floor that will still hold water, and pencil the line at both ends of the room.
- Leave it 24 hours, and check the ceiling below at the same time as the water line above.
- Photograph every junction, collar and lap before releasing the plug, and keep the photographs with the job file.
- Re-test after any remedial coat, since a repair to a membrane is a new membrane at that spot.
Small format in the fall, and what that does to a take-off
Inside the catchment the tile has to follow a surface tilting two ways at once, which is a job for mosaic on sheets or a genuinely small format. Outside it, the field can be whatever the client chose. One floor, two tile schedules, two waste factors — and a take-off that averages them is wrong in both directions.
Waste is higher in the catchment than the field, not lower, despite the tiles being smaller. Mosaic sheets get cut on all four sides of the grate, again along the transition to the field, and again at the hips where two planes meet. Ten per cent is a field number; the catchment wants closer to twenty.
Slip resistance is a specification item here, and a wet room is a barefoot wet area rather than a shod one. ANSI A326.3 gives the dynamic coefficient of friction test behind the widely specified minimum of 0.42 for level interior floors walked on when wet. DIN 51097 is the ramp test written for wet barefoot areas and returns classes A, B and C; DIN 51130 and its R ratings describe shod workrooms and are the wrong thing to quote here. In the UK it is the pendulum test to BS 7976 with the UK Slip Resistance Group guidelines, where a value of 36 or above is the usual threshold for low slip potential. And a sloping floor is being asked for more than a level one — the second argument against the steep end of the band.
One consequence of a two-schedule floor: buy both tiles at once and box an offcut of each. The transition between them is the joint most likely to be disturbed later, by a screen fixing or a replacement grate, and a sheet of discontinued mosaic is a very expensive small thing.
Run it twice, once for the catchment and once for the field. Waste here follows the lay pattern rather than a figure you type, and the ten per cent a straight lay carries is a field allowance — so add the cuts around a grate and along a hip on top of what it returns for the catchment, because they are what make the small tile the expensive half of a small floor.
Net area of floor or wall.
Long side of the tile.
Short side of the tile.
How the tiles are laid.
From the supplier's data.
Tiles required
36 tiles
Keep the spare tiles. A batch-matched replacement in five years is worth more than the box it came in.
- Boxes to order
- 6 boxes
- Tiles before waste
- 32.5 tiles
- Waste applied
- 10 %
- Area covered by the boxes
- 144 ft²
- Spare after the job
- 0 tiles
They open the calculator with your figures already in it
Floor & Wall Tile Calculator: 36 tiles — 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 checks that the tile and the pattern belong together. A tile with any edge of 15 in (about 380 mm) or more falls under a tighter substrate flatness rule — TCNA asks for 1/8 in in 10 ft — and the offset on a long plank is held to roughly a third of its length, because a half-bond sets the neighbouring tile's high point against your tile's crown and produces lippage. This will happily return a herringbone count for a 1200 mm plank over a floor that cannot take it.
- Movement joints sit outside the count. The same TCNA and BS 5385 documents cited above for the waste figures also call for a soft joint at every perimeter and wall abutment, around columns and fixed penetrations, and across the field at intervals — roughly 8 to 10 m indoors, and much closer over a heated screed or outside. Those joints take sealant and a profile rather than grout, and a floor tiled hard into its walls tents at the first hot spell.
Ninety-five per cent, not eighty
The coverage requirement changes as soon as the floor is a wet area. The TCNA Handbook asks for 80% mortar contact in a dry interior and 95% in wet areas and exteriors. A void under a tile in a dry hallway is a hollow sound; a void under a tile in a shower fall is a reservoir that fills and sits against the membrane. Back-buttering is not a refinement here, it is how the number gets met.
Two notch sizes on one floor means two consumption rates. A 6 mm notch under mosaic is around 3 kg per square metre, a 10 mm notch under a 600 mm field tile around 5, and the back-buttering needed to reach 95% pushes the small-tile figure up rather than down — so the catchment is not the cheap half. On the example floor that is roughly 10 kg over 2.25 m² of catchment and 12 kg over 2.3 m² of field: about 22 kg, or two bags once the bond coat behind a sheet membrane joins the order.
Class the adhesive against a document and against the membrane. BS EN 12004-1 classifies tile adhesives — a C2 cementitious adhesive with improved characteristics is the normal minimum over a membrane, with S1 or S2 deformability where the substrate moves — and ANSI A118.4 is the modified dry-set specification. Then check the membrane maker's own list, because a bonded sheet system names the adhesives it is warranted with, and using something else voids the only warranty on the job that matters.
Trowel notch is what sets the rate, so run the catchment and the field separately at their own notches and then add for back-buttering, because 95% coverage in a wet area is not achieved with the notch alone.
Net tiling area.
Notch size governs how much adhesive stays on the wall.
A skim coat on the tile back as well as the substrate.
Weight of the bags you are buying.
Adhesive required
3 bags
Coverage rates are typical for cement-based adhesive. Check the specific product — rates vary by 20% between brands and between standard and lightweight formulations.
- Adhesive required
- 133.13 lb
- Coverage rate
- 1.02 lb/ft²
- Bag size
- 50 lb
- Surplus in the last bag
- 16.87 lb
They open the calculator with your figures already in it
Thinset & Tile Adhesive Calculator: 3 bags — 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
- An uneven substrate consumes considerably more than these figures. Levelling first is cheaper than filling with adhesive, and it performs better.
- Does not cover a levelling compound, a decoupling membrane or a waterproofing layer, all of which are separate materials.
The joints that are not grout
Grout is a filler and a wearing surface. It is not waterproofing, it never was, and a wet room built on the assumption that it is will be dry for about a year. Specify it against BS EN 13888 for grouts for tiles or the ANSI A118 grout specifications, pick a formulation domestic cleaning products will not attack, and stop expecting it to do the membrane's work.
It also costs more than a per-square-metre habit suggests, because grout is consumed by joint length rather than by area. A 50 mm mosaic at 2 mm joints carries close to 38 metres of joint in every square metre; a 600 mm tile at 3 mm joints carries about 3.3. Multiply through the joint width and depth and the mosaic takes roughly six times as much grout per square metre as the field tile beside it — on a small floor, the difference between one tub and three.
Then the joints that must not be grout at all. Internal corners, the wall-to-floor junction, changes of plane and the perimeter against a fixed element are movement joints and take a flexible sealant — grout a corner and it cracks, and the crack is the route water takes to whatever the membrane did not cover. The perimeter of the catchment is itself a change of plane. The movement joint guidance in the TCNA Handbook is the reference, and those joints belong on the setting-out sketch before the first tile is bedded.
What the client has to be told before you leave
Two handover items, neither of them construction. The gully trap is a maintenance component: it comes out, it needs to come out, and it will fill with hair. Show whoever lives there how the insert lifts, leave the key on site, and say plainly that a wet room with a trap nobody can reach has a smell in it by the second winter.
Then ventilation, because a wet room puts far more water into the air than a cubicle does — the whole floor is a wet surface drying into the room, several times a day. Approved Document F, Ventilation, sets the extract performance a bathroom has to achieve in England, and equivalent provisions elsewhere do the same, but all were written around a bathroom with the shower in a box. Fit the higher end of what the room qualifies for, run the fan on an overrun, and treat condensation staining on newly tiled walls as a ventilation defect rather than a tanking failure.
The seven numbers that decide this floor
Settled in this order, because each closes off options in the next: the depth budget rules out fall types, the fall type rules out tile formats, and the tile format is what the adhesive and grout get bought against.
- Deck to ceiling, in millimetres — Measured on the first visit. Everything below it is gully body, trap and waste fall; everything above it is board, fall, membrane, adhesive and tile.
- Gully body height and outlet direction — From the drain manufacturer's data sheet, checked against the joist depth and against whether the joists can legally be cut where the pipe wants to run.
- Design fall, as a ratio — 1:48 to 1:24 under the plumbing code, and no steeper than 1:48 if the shower has to be accessible — which leaves a single value with no tolerance.
- Screed volume, catchment by catchment — The run from the farthest corner to the drain times the gradient gives the drop; the screed is the flange depth under the whole catchment plus two-thirds of the drop for a point gully, or half of it for a channel.
- Total tanked area — Floor, plus full-height wall for the shower zone and its overrun, plus the turn-up behind the skirting elsewhere. It is bought twice — once in primer, once in membrane.
- Wet volume or roll count — Dry film thickness divided by solids content for a liquid; area inflated for laps and corner overlays for a sheet. Preformed corners and collars are counted separately.
- Two tile schedules and two adhesive rates — Catchment and field are different formats, different notches, different waste. Grout follows joint length, so the small tile carries most of it.
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.