Notched Trowels: Notch Sizes, the Half-Bed Rule, and What BS 5385 Actually Demands
The UK guide to notched tile-adhesive trowels. Square vs V vs U profiles, notch sizing 3mm to 12mm, the half-notch rule, BS 5385 coverage, DITRA, and what to buy.
You've bought a pallet of large-format porcelain for the kitchen floor and a tub of S1 flexible adhesive. Your fitter dropped out, so you walk into the merchants and grab the cheapest notched trowel on the rack: a 6mm square-notch for under a fiver. Three months in, half the tiles sound hollow when you tap them and two have cracked along the grout line. The adhesive was fine. The tiles were fine. The notch was wrong by a factor of two, and nothing on the trowel rack told you that.
This is the tool that controls whether your tiles stick. Get the notch right and the bed compresses to the correct thickness; the tiles outlast the building. Get it wrong and the only fix is a chisel.
What it is and when you need one
A notched trowel is a flat steel blade with teeth cut into two edges, set into a handle. You scoop adhesive with the smooth long edge, spread it on the substrate, then comb the bed with the toothed short edge. The teeth leave parallel ridges of adhesive. When the tile presses down, the ridges collapse sideways into the gaps, and the resulting bed is roughly half the height of the original notch. That collapse is what the tool is designed to control.
You need one for every tiling job; there is no useful alternative. The size of the notch on the trowel you pick up is the single most important technical decision in any tiling job, more so than the brand of adhesive or the tile itself.
The notch profile and size depend on what you're sticking down. Mosaic tiles need almost no adhesive. A 1200mm x 600mm porcelain plank needs a deep bed and back-buttering. A Schluter DITRA decoupling membrane needs its own specific notch, different from the trowel you use to bed tiles on top of it. You'll need at least two trowels for a typical extension fit-out: one for walls and small tiles, one for floors and large tiles.
Notch profiles and what each one does
Three profiles dominate the UK market. Each cuts a different ridge shape and each suits a different job.
Square notch. Right-angled rectangular teeth produce flat-topped ridges with vertical sides. Square notch is the most common profile in UK retail and the default for floors. It deposits the most adhesive per square metre, which is what you want when full coverage matters: floors, wet areas, large-format tiles, anywhere BS 5385 demands solid bedding. The half-notch rule (covered below) applies cleanly.
V-notch. Triangular teeth produce sharp ridges. V-notch deposits less adhesive than square at the same nominal size, intended for thin, lightweight tiles where you don't want adhesive squelching out: wall mosaics, ceramic wall tiles in dry areas. V-notch is the wrong choice for any floor or wet area.
U-notch (round-notch). Semicircular teeth produce rounded ridges. U-notch sits between square and V in adhesive volume and is favoured by some professional tilers for floors because rounded ridges collapse into voids more easily. UK retail stocks fewer U-notch options, so most homeowners default to square. Both work; square is simpler to predict.
A fourth profile, the Y-W trowel, has dual-height teeth and is sold for very large format porcelain (anything above 1m²). It is a specialist purchase and outside the budget for most homeowner jobs.
The half-notch rule, explained
A square-notch trowel cuts equal-width ridges and gaps. When you press a tile onto the ridges, the adhesive compresses sideways and fills the gaps. Because the gaps are the same width as the ridges, the final bed is roughly half the height of the original notch.
A 12mm square notch produces a 6mm bed. A 10mm notch produces a 5mm bed. A 6mm notch produces a 3mm bed. This is the maths every tile manufacturer's instructions assume.
V-notch and U-notch behave differently. V-notch holds about a third of the adhesive volume of a square notch at the same size, so a 6mm V-notch produces something closer to a 2mm bed. Square notch is the only profile where the half-rule works cleanly.
The reason this matters: adhesive manufacturers specify minimum bed thickness on the tub. S1 flexible adhesives typically need 3mm minimum to flex without cracking. Above 6mm with a standard cement-based adhesive and you risk the bed slumping before the tile sets. The notch you pick keeps the bed inside that window.
Notch sizing matrix
Pick your notch from the tile size first, then check whether the substrate condition or wet-area rules push you up a size.
| Notch size | Profile | Tile size / use case | Resulting bed (square notch) | Back-butter required? |
|---|---|---|---|---|
| 3mm | Square | Schluter DITRA bedding to substrate (4-4.5mm is the Schluter spec; 3mm is acceptable for thin-set under decoupling membrane) | 1.5mm | No |
| 4mm | V or square | Mosaic tiles up to 100mm; thin wall mosaics on plasterboard | 2mm | No |
| 6mm | Square or V | Standard wall tiles 100-300mm in dry areas (kitchen splashbacks, bathroom walls outside the shower zone) | 3mm | No |
| 8mm | Square | Medium floor tiles 200-400mm; wet-area wall tiles up to 300mm in showers | 4mm | No for floor tiles up to 300mm; yes for wet walls if substrate is uneven |
| 10mm | Square | Standard floor tiles 300-600mm; the default trowel for most domestic floor jobs | 5mm | Recommended for tiles above 450mm; required for porcelain above 600mm |
| 12mm | Square | Large-format tiles 600-900mm; underfloor heating substrates; uneven wood floors | 6mm | Required |
| 15-20mm | Square or U | Oversize porcelain 900mm+ and slabs; heavily uneven substrates | 7-10mm | Always |
A few caveats sit on top of the matrix.
If your substrate is uneven, jump up one notch size to compensate for low spots. A flat self-levelled floor tolerates a smaller notch than an old kitchen subfloor with a 3mm dip across each plywood sheet. The notches need to deposit enough adhesive to fill the dips and still leave a uniform bed under the tile.
If you're tiling onto a decoupling membrane like Schluter DITRA or Dural Durabase, you need two notches in your toolkit. The membrane bonds to the substrate with a 4mm to 4.5mm square notch (Schluter's own spec is 4.5mm x 4.5mm). The tiles then bond to the top of the membrane with a 6mm to 10mm square or U-notch depending on tile size. Don't try to use the same trowel for both stages.
If the adhesive tub specifies a notch size that conflicts with the matrix above, follow the tub. Some rapid-set and large-format-specific adhesives are formulated to behave differently, and the manufacturer's spec is the binding number.
What BS 5385 demands
BS 5385 is the UK standard for wall and floor tiling. The two parts that matter are BS 5385-1 (internal wall tiling) and BS 5385-3 (floor tiling, internal and external). Both speak to coverage, the percentage of the back of the tile in contact with adhesive.
BS 5385-3 is the strict one. All floor tiling and all external tiling must be solid-bedded with no voids except minor bubbles. In practice that means 100% coverage on the back of every floor tile. If you lift a freshly laid floor tile and see a clean square of bare back, the bed is wrong.
BS 5385-1 changed in 2018, moving from flat coverage percentages to a weight-based approach: any tile under 0.1m² that weighs more than 70% of the substrate's load capacity must be solidly bedded. A 9mm porcelain wall tile at ~18kg/m² bonded to gypsum plaster rated 20kg/m² is over the threshold and now needs solid bed. Lighter ceramic tiles on dense substrates can sit at the older partial-bed percentages.
UK industry has settled on practical benchmarks that align with the standard's intent:
- 85% minimum coverage for internal wall tiles in dry areas (kitchen splashbacks, downstairs WC walls outside the splash zone)
- 95% minimum coverage for internal wall tiles in wet areas (showers, splash zones, wetrooms)
- 100% (solid bed) for all floors and all external tiling
The 50% and 65% figures that turn up in older online guides are misreadings of pre-2018 versions or imports from US standards (ANSI A108). Don't use them as targets in 2026.
The notch size is your primary lever for hitting these coverage numbers. A bigger notch deposits more adhesive, which means more material to fill voids when the tile lands. The other lever is back-buttering, covered below.
Tip
You don't need to memorise the percentages. The practical rule is simpler: any floor tile, any wet-area tile, and anything above 600mm needs solid bedding, which means a bigger notch (at least 10mm) and a back-butter. Treat dry wall tiles in a kitchen splashback as the only place where a 6mm notch and standard combing is enough.
How to use one properly
The technique is short, the mistakes are unforgiving, and most homeowner videos skip the steps that prevent failures.
Mix the adhesive correctly and only mix what you can use
Tile adhesive has an open time, the period after spreading during which a tile can still bond properly. Standard cement-based adhesives like Weber Weberset Plus give around 20 minutes; rapid-set adhesives give 10. Heat, draught, dry substrates, and direct sunlight cut these times sharply. Mix one bucket at a time. If it starts to thicken, it's past pot life. Discard, don't water down.
Spread no more than 1m² at a time
Beyond 1m², you risk the adhesive skinning over before you can get the tiles down. Skinned adhesive looks fine but no longer transfers properly to the tile back, and the bond fails. On a hot day in a draughty shell, drop to 0.5m². If the adhesive looks dull or matte, scrape it back and respread.
Apply the flat side first, then comb with the notched side
Hold the trowel at roughly 30 degrees and press the smooth long edge against the substrate to push a thin contact coat into the surface. Then turn to the notched edge and comb the bed at roughly 45 degrees, with all ridges running in the same direction. Same direction matters: parallel ridges collapse into parallel gaps cleanly. Cross-hatched ridges trap air.
Hold the notched edge at 45 degrees and press hard enough to scrape the substrate
The notch only deposits the correct ridge height when the teeth cut all the way down to the substrate. Held flatter or pressed too lightly, the ridges sit on top of an uneven layer of adhesive and the bed comes out thicker than designed. The teeth should leave clean lines of bare substrate at the bottom of each gap. If they don't, the angle is wrong or the trowel is worn.
Set the tile with a slide-and-press motion, not a drop
Lower the tile onto the wet ridges, then slide it across the ridge direction by 5-10mm before pressing down. The slide breaks up the ridge tops and helps the adhesive flow into the gaps. Press firmly with the heel of your hand to push trapped air out. For tiles above 300mm, also tap the centre gently with a rubber mallet.
Lift the first tile and check coverage
Lift the first tile of every new session, every new bucket, every change in conditions. Inspect both the back of the tile and the bed it came off. You're looking for full coverage with no clean rectangles of bare back. If you see ridge lines still visible or bare patches on the tile, your notch is too small, your angle is too flat, or your adhesive has skinned. Fix the cause before laying more.
Back-butter large or heavy tiles
For any tile above 450mm, apply a thin layer of adhesive to the back with the flat side of the trowel before setting. This is back-buttering and it's not optional for large format. The contact coat fills the natural bow in porcelain tile backs, guarantees full perimeter contact, and helps hit the BS 5385-3 solid-bed requirement.
Warning
Hollow-sounding tiles (the dull thud you hear when you tap a poorly bedded tile) are not a cosmetic issue. They are a structural failure waiting to happen. Hollow tiles crack along grout lines under any movement (foot traffic, thermal expansion, underfloor heating cycles), water ingress travels through the cracks into the void, and the bond eventually lets go. There is no fix that doesn't involve lifting and relaying. Do the lift-check on tile one, not on tile fifty.
Stainless or carbon steel?
Two blade materials dominate the UK market. The choice is not interchangeable.
Stainless steel. Won't rust between jobs. Easier to clean. Mid-range and budget UK trowels (OX Trade, Faithfull, Vitrex, Tile Mountain, Tiling Supplies Direct) are mostly stainless and sit in the £8 – £14 bracket. For occasional homeowner use, this is the right choice. A stainless blade tolerates being wiped, dried, and forgotten about between weekend sessions.
Carbon steel. Stiffer than stainless and holds an edge longer. Premium tilers' trowels (Marshalltown's M710 series, some Refina Huds NotchTile configurations) use carbon steel. The catch is rust: carbon steel must be cleaned, dried, and oiled after every use. Leave one damp in the toolbox overnight and the notches pit, the edges blunt, and the tool is finished. A homeowner using one twice a year will see it rust before the second use.
For homeowner jobs: buy stainless. Save the carbon-steel debate for tilers doing 30 hours a week.
What to buy
Notched trowels sit in a narrow price band. There is no high-end gain that translates to better tile bedding for a DIY user; the differences at the top end are about blade replaceability, longer working life, and ergonomics over hundreds of hours of use.
Budget tier
Budget notched tile trowel (Tile Mountain DIY, Vitrex, Magnusson, Marshalltown QLT)
£3 – £10
Tile Mountain DIY 6mm, Magnusson 4mm V-notch at Screwfix, Vitrex 6mm at Toolstation, Marshalltown QLT combo. Carbon or lacquer-coated steel, plastic handles, fine for a single bathroom splashback or small floor. Notches wear quickly under heavy use and the blade will rust if left damp.
Mid-range tier
Mid-range stainless notched trowel (OX Trade, Faithfull, Tiling Supplies Direct, Rubi)
£8 – £14
OX Trade 6/8/10mm stainless at Screwfix, Faithfull cold-forged stainless, Tiling Supplies Direct's stainless range, Rubi stainless. The sweet spot for homeowners: stainless blades, riveted handles, notches that stay sharp through several weekend jobs. OX Trade is the most consistently recommended budget-pro brand on UK tiling forums.
Professional tier
Marshalltown wooden-handle and DuraSoft notched trowel
£18 – £44
Refina Huds NotchTile (replaceable laser-cut blade)
£31 – £51
Marshalltown wooden-handle 6mm and the M710SD carbon-steel large sit at the top of the trade Marshalltown range. Refina Huds NotchTile uses a laser-cut H45 two-step notch profile and replaceable stainless blades (A1, A2, B11), so the handle lasts indefinitely. Overkill for one bathroom; genuinely cheaper than mid-range over a whole-extension project.
What a typical homeowner kit looks like
Homeowner kit: 6mm + 10mm stainless pair
£18 – £25
A 6mm and a 10mm stainless pair from Screwfix or Toolstation covers every tiling job in a domestic extension: kitchen splashbacks (6mm), bathroom walls (6mm), shower walls (8-10mm for porcelain or large format), bathroom and kitchen floors (10mm). Add a 4mm if using DITRA. Add a 12mm if using porcelain above 600mm. The 6mm + 10mm pair is genuinely all most homeowners need.
A note on resharpening. Square-notch teeth cannot meaningfully be resharpened at home. Once the notches round off, the trowel is finished. Premium ranges like Refina solve this with replaceable blades. Mid-range and budget trowels are economical enough to discard and replace per project. Don't waste time trying to file old notches back to spec.
External resource
BAL Adhesives technical helpline
When the tile manufacturer's instructions and the adhesive tub disagree on notch size (this happens with large-format porcelain), call the adhesive manufacturer rather than guessing. BAL, Weber, and Ardex all run free technical helplines for trade and DIY users.
bal-adhesives.com
Alternatives
There are no genuine alternatives for spreading tile adhesive at the right bed thickness. A flat trowel doesn't comb and produces unpredictable beds. A serrated drywall knife doesn't have the right tooth profile and won't produce the half-bed collapse. A tile adhesive comb (a thin metal or plastic comb sold for very small jobs) works for a 1m² splashback but is too small and too flexible for floors.
The only real alternative is the level of automation: vacuum-suction tile-laying machines and robotic large-format setters used on commercial sites. These are not relevant for any domestic job.
The closest companion tool is the grout float, which finishes the same job by pressing grout into the joints between the tiles you've just bedded. The two tools come out of the bag together for every tiling task and the grout float is just as essential as the notched trowel.
Where you'll need this
- Bathroom tiling and shower waterproofing - bedding wall and floor tiles in the bathroom and shower zone, where BS 5385-1 wet-area coverage applies
- Kitchen splashback tiling - small areas between worktop and wall units where a 6mm trowel and partial-bed coverage is fine
- Floor tiling on screed or substrate boards - large-format porcelain or ceramic on liquid screed or backer boards, where solid bedding is mandatory
The notched trowel is a second-fix tool that turns up at every tiling stage on every extension or renovation project where ceramic or porcelain tiles are being laid. Two trowels (6mm and 10mm) cover the vast majority of domestic work; a third (4mm) is needed if the floor build-up uses a decoupling membrane.
Common mistakes
Using the same 6mm trowel for floors as for walls. The single most common cause of hollow floor tiles. A 6mm notch produces a 3mm bed, not enough for floor tiles above 200mm and never enough for solid bedding. Use 10mm minimum for any floor.
Holding the trowel flat instead of at 45 degrees. Flat-angle combing produces shallow ridges and a thicker overall bed. If the substrate isn't visible between ridges, the angle is wrong.
Spreading more than 1m² at a time. Adhesive skins over inside the open time. Work in panels and pace yourself.
Skipping the lift-and-check on tile one. Every coverage problem could have been caught by lifting the first tile. Do it on tile one, not after you've laid twenty.
Mixing notch profiles for the same job. Using a 6mm V-notch on a floor because the rack only had V-notch in stock. V holds far less adhesive than square at the same nominal size and the bed will be too thin. Square notch is the floor profile; don't substitute.
Using a worn trowel without checking. Square-notch teeth round off after 15-20 hours of heavy use. Worn teeth deposit smaller, irregular ridges. If you've used the trowel on more than two jobs, inspect the notches against a new one before starting.
Using the plastic comb that came in the adhesive tub. Some manufacturers include a flexible plastic comb. These are not real notched trowels: the teeth are wrong, the blade is too short, and the comb flexes when you press hard enough to scrape the substrate. Buy a steel trowel separately.