Large-format tiles
Lippage from tile bow in an offset (running bond) layout
- where
- centre bow of each tile (mm)
- edge bow as % of tile length
- tile length along the offset (mm)
- lippage caused by bow at the chosen offset (mm)
- offset as % of tile length
- BS 5385 allowable lippage (mm)
- joint width (mm)
Explain this equation
Use this before setting out long rectified tiles or planks in running bond. It tells you how big a step the natural bow of the tiles will create at the joints, and whether that step is inside the BS 5385 tolerance of 1 mm (joints under 6 mm) or 2 mm (6 mm and over).
δ is the bow: how far the middle of a tile sits above a straight line between its ends. It comes from the flatness figure on the data sheet, given as a percentage of length. f is how far one row is shifted along from the next, as a percentage of the tile length.
The bow is treated as a smooth curve. Where the end of one tile sits beside a point a fraction f along its neighbour, the neighbour is raised by 4δ·f·(1 − f) above it, and that is the biggest step anywhere on the joint. At a half bond the end of one tile lines up with the crown of the next, so the step is the whole bow.
A 1200 mm plank with 0.2% bow has δ = 2.4 mm. A half bond gives a 2.4 mm step, a third bond about 2.1 mm, a quarter bond 1.8 mm. That is why long tiles are kept to a third bond or less and why the flatness figure matters as much as the bond.
Watch out: real tiles do not always bow evenly, and the bed and substrate add their own errors. Levelling clips help but cannot pull a bowed tile flat.
Works out how much step (lippage) a bowed long tile creates at the joint when it is laid in running bond, and compares it with the BS 5385 tolerance.
Enter your values to see the result.
Good to know
Each tile is assumed to bow along its length in a smooth curve, highest in the middle. Where the end of one tile sits beside the middle of its neighbour, the step equals the full bow, which is why a half (50%) bond is the worst case. At an offset fraction f the step is 4δf(1−f), so a third bond gives about 89% of the bow and a quarter bond 75%. ANSI A108.02 limits the offset to 33% on tiles with a side over 15–18 in (380–457 mm) unless the manufacturer allows more. BS 5385 allows 1 mm between adjacent tiles for joints under 6 mm and 2 mm for 6 mm and over. Use the flatness figure from the tile data sheet (often ±0.2–0.3%) or measure the bow with a straightedge.
Common mistake
Laying long rectified planks in a half bond because it looks traditional, then blaming the bed when the joints step.
Limits
Assumes both tiles bow by the same amount in a parabolic shape and the bed is otherwise flat. Real tiles can be twisted or bow unevenly; levelling clips reduce but do not remove the step. Lippage tolerances quoted from BS 5385 via a published transcription; ANSI A108.02 adds the tile’s allowable warpage on top of 1 mm / 2 mm.
Sources
- BS 5385-1 cl. 4.1.8.2 as quoted on Tilers Forums: lippage between adjacent tiles 1 mm for joints under 6 mm, 2 mm for joints 6 mm and over (Part 3 mirrors this) — check against the standard
- CTaSC, Tile and stone lippage: what is acceptable (update) — ANSI A108.02-2023 §4.4 lippage 1 mm (joints 3 to <6 mm) / 2 mm (≥6 mm) plus warpage; running-bond offset ≤33%
- Stone World / CTEF, Do you know the ANSI standards for tile coverage, offset and acceptable lippage? (A108.5 coverage 80% / 95%; A108.02 offset ≤33% for sides over 15 in)
- Porcelain tile technical data sheet (ISO 10545-2 flatness ±0.2% against ±0.5% limit; ISO 10545-8 thermal expansion 6.5 × 10⁻⁶; apparent density > 2.20 g/cm³)
Results are calculations from the published methods and manufacturer figures shown, and depend on the data you enter. Always follow the tile, adhesive and grout manufacturers’ instructions, BS 5385 and the Tile Association guidance. Coverage rates vary by product and substrate. Check prices and VAT rules with your own records and HMRC.