Screed Testing Guide: Methods, Standards & Interpretation
Testing screeds at various stages — during installation, during curing, and before applying floor finishes — is essential for quality assurance. Proper testing confirms that the screed meets its specified performance requirements and is ready for the next stage of construction. This guide covers the key testing methods, what they measure, and how to interpret results.
BRE Screed Tester (Drop Hammer Test)
The BRE screed tester — commonly called the drop hammer test — is the standard site test for assessing screed compressive strength. A defined weight is dropped from a set height onto the screed surface, and the resulting indentation diameter is measured.
The test is quick, non-destructive (the indentation is minor), and can be performed at any point after initial set. It's specified in BS 8204 and is the primary method used by screed testing laboratories to verify strength on site.
How it works: A 4kg weight drops from 1 metre through a guide tube onto a hardened steel ball resting on the screed surface. The diameter of the resulting circular indentation is measured with calipers. Smaller indentation = higher strength.
Interpreting results: Results are compared against tables in BS 8204 that correlate indentation diameter to approximate compressive strength. For a C25 screed, the indentation should typically be under 5mm diameter at 28 days.
Moisture Testing
Verifying that screed moisture content is within acceptable limits before laying floor finishes is critical. Laying finishes over wet screed causes adhesive failure, mould growth, finish degradation, and costly remediation.
Calcium Carbide (Speedy) Moisture Test
The most widely used site test for screed moisture. A sample of screed is crushed and mixed with calcium carbide reagent in a sealed pressure vessel. The reagent reacts with any moisture present, generating gas. The resulting pressure is read from a gauge and converted to percentage moisture content by weight.
Acceptable levels: For cement screeds, typically under 0.5% moisture by weight (or under 75% relative humidity). For anhydrite screeds, under 0.5% is required — anhydrite is more moisture-sensitive than cement.
Relative Humidity (Hygrometer) Testing
A more precise method. A sealed hood or sleeve is placed on the screed surface for a defined equilibration period (typically 72 hours minimum). A calibrated hygrometer then measures the relative humidity of the air trapped above the screed.
Acceptable levels: Typically under 75% RH for cement screeds, under 75% for anhydrite. Some floor finish manufacturers specify tighter limits — always check the finish manufacturer's requirements.
Surface Regularity Testing
Assessed using a calibrated 2-metre straight edge placed at multiple points across the floor surface. The maximum gap between the straight edge and the floor determines the SR classification:
- SR1: Maximum gap ±3mm (high standard — required for thin finishes)
- SR2: Maximum gap ±5mm (normal standard — suitable for most finishes)
- SR3: Maximum gap ±10mm (utility standard — for basic applications)
Testing should be performed after the screed has fully cured and any initial shrinkage has occurred. For critical projects, independent testing by a qualified surveyor is recommended.
Strength Testing (Laboratory)
For definitive strength verification, cube samples (typically 100mm cubes) are taken during screed installation and sent to an accredited laboratory for 28-day compressive strength testing. This is the reference test specified in BS EN 13813.
Laboratory testing is particularly important for large commercial projects, projects with specific strength requirements, or where disputes arise about screed quality.
When to Test
During installation: Take cube samples for laboratory testing. Perform initial BRE drop hammer tests at 3-7 days to confirm early strength development.
Before floor finishes: Moisture testing is mandatory. Surface regularity testing should be performed and documented. Final BRE tests confirm 28-day strength has been achieved.
Getting Expert Advice
Related Reading
Proper testing protects your project from costly defects. Call us on 0118 370 2060 for advice on testing requirements for your specific screed and finish combination. We offer free delivery on orders over £600 ex-VAT.
