Flatness tolerance compared
SR1 vs SR2 Screed Flatness Tolerance
SR1 and SR2 are the two flatness standards you'll see quoted, but they only matter in relation to what's actually going on top of the screed.
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SR1 and SR2 describe how flat a screed surface is once it's cured, measured against a straight edge over a set distance. They're not a marketing term — they're a genuine, measurable tolerance that flooring manufacturers reference in their own installation guidance.
Liquid screed is capable of achieving SR1 as standard, which is part of why it's specified so often ahead of tiling or resin finishes. SR2 is a looser tolerance that's perfectly adequate for many floor coverings and is what traditional sand-and-cement typically achieves at best.
This page explains the practical difference and which finishes actually demand SR1.
SR1 vs SR2 defined
| Standard | Tolerance |
|---|---|
| SR1 | ±3mm gap under a 2m straight edge (often achieved tighter with liquid screed) |
| SR2 | ±5mm gap under a 2m straight edge |
Finishes that need SR1
- Large-format porcelain tiles (600mm+) where lippage becomes visible
- Polished or power-floated concrete finishes
- Resin and epoxy flooring in commercial units
- LVT and vinyl where subtle undulation telegraphs through thin coverings
Confirm the flatness tolerance you need
Tell us your floor covering and we'll confirm whether SR1 or SR2 applies and quote your pour accordingly.
Residential liquid screed
- Level floors for extensions and self-builds
- Screed suitable for underfloor heating
- Build-up planned around your floor finish
Finishes where SR2 is generally acceptable
- Carpet and underlay, which absorb minor undulation
- Engineered wood floated over a foam layer
- Standard-size ceramic tile with a forgiving adhesive bed
- Smaller-format tiles less prone to visible lippage
SR2 concrete finish vs SR1
SR2 is the looser of the two tolerances and is what a traditional hand-finished sand-and-cement floor typically achieves at best. It is perfectly adequate under carpet, floated engineered wood and forgiving tile sizes. SR1 is the tighter class that liquid screed can achieve as standard with correct laser levels and finishing, and it is the one to specify where polished or power-floated concrete finishes, large-format tiles, resin or thin LVT will show every undulation. Both are checked the same way, with a 2m straight edge.
How to specify and check floor flatness
Choose the tolerance from the planned floor covering rather than asking for the tightest class by default. Thin finishes such as LVT and reflective or large-format finishes reveal small undulations more readily, while carpet, underlay and floated timber systems can accommodate more variation.
The finished screed is checked with a 2m straight edge laid in different positions and directions across the floor. The largest gap beneath it is compared with the specified tolerance. Carry out that check before the floor covering is fitted, so any isolated high spot can be ground and any low area can be corrected with a suitable levelling treatment.
SR1 wins whenever the floor covering is thin, large-format, or reflective enough to show every undulation. SR2 is perfectly adequate for carpet, floated wood floors and forgiving tile sizes, and specifying SR1 unnecessarily just adds cost. Test it against the flooring manufacturer's own installation tolerance, not a general preference for 'the best'.
— Exeter Liquid Screeders
SR1 and SR2 on concrete floors
The same straightedge classes are quoted for concrete floors as for screed. A power-floated concrete slab is typically discussed against the looser classes, while an SR1 concrete finish needs the pour, the floating and the curing all controlled with the tolerance in mind from the start. The covering-led logic is identical either way: decide what is going on top, then ask for the class that covering needs.
Where a concrete slab has not come out flat enough for the planned finish, a self-levelling screed over it is the usual route to the tighter class. Our screed vs power-floated concrete comparison covers that choice, and floor levelling explains the correction options.
Flatness over underfloor heating
Over underfloor heating, flatness starts before the screed does. The insulation has to sit flat and the pipework has to be laid to a consistent level, because the screed follows whatever is underneath it. A consistent cover depth over the pipes gives the finish its best chance of hitting the specified class across the whole room.
The checks are the same as any other floor once cured: a straightedge in several positions and directions, before coverings go down. Our underfloor heating screed guide covers the build-up and commissioning side.
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Confirm the flatness tolerance you need
Tell us your floor covering and we'll confirm whether SR1 or SR2 applies and quote your pour accordingly.