Loads and tolerances
Flatness check against a straightedge tolerance
- where
- measured deviation as a share of the tolerance (%)
- largest measured deviation under the straightedge (mm)
- permitted deviation for that straightedge length (mm)
- remaining margin (mm)
Explain this equation
Compares the worst gap you measured under a straightedge with the deviation permitted for the job. Use it at handover or when there is a dispute over flatness.
dm is the largest deviation you found using the equal-offsets method, and da is the permitted deviation for the straightedge length you used. u shows how much of the tolerance you have used, as a percentage; m is what remains in millimetres.
It is a direct comparison because tolerances are written as a maximum deviation over a given straightedge length, such as ±3 mm in 2 m for walls in the NHBC Standards.
Sense check: a 2 mm gap against a 3 mm tolerance uses 67% of it and passes. A 6 mm gap on a ceiling against ±5 mm is 120% and fails.
Watch out: the tolerance must match the straightedge length you used, and grazing light makes small undulations look worse than they are.
Compares the worst gap you measured under a straightedge (equal-offsets method) with the permitted deviation for the job.
Enter your values to see the result.
Good to know
Common benchmarks: NHBC Standards Chapter 9.1 accepts plastered and dry-lined walls within ±3 mm under a 2 m straight edge with equal offsets, and ceilings within ±5 mm; it also gives ±3 mm under a 450 mm straight edge for local smoothness, and render within ±4 mm in 5 m. BS 8212 and BS EN 13914-2 give other tolerances for drylining and plastering; BS EN 13914-2 notes that thin coats follow the background and cannot have a tolerance of their own. Enhanced tolerances must be specified in the contract. Measure with the straightedge laid on equal blocks (offsets) so both high and low spots show.
Common mistake
Using a tolerance meant for a different straightedge length, or judging flatness with a torch held flat to the wall. Lighting at a grazing angle exaggerates small undulations that are inside tolerance.
Limits
Simple comparison against the figure you enter; it does not pro-rate a tolerance to a different straightedge length. Use the tolerance in your contract or the applicable warranty standard.
Sources
- NHBC Standards 2024, Chapter 9.1 Finishes: plastered and dry-lined walls ±3 mm and ceilings ±5 mm under a 2 m straight edge with equal offsets; render ±4 mm in 5 m
- Specfinish: The lowdown on enhancements (BS 8212 / BS EN 13914-2 tolerances for plastered and dry-lined surfaces)
- BS EN 13914-2:2016 Design, preparation and application of external rendering and internal plastering – Part 2: Internal plastering
Results are calculations from the published methods and manufacturer figures shown, and depend on the data you enter. Always follow the manufacturers’ system and product data, BS 8212, BS EN 13914 and Building Regulations. Coverage rates vary by product, background and thickness. Check prices, VAT and CIS rules with your own records and HMRC.