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The purpose of this paper is to analyse two parameters relevant to the physical‐mechanical performance of external renders, mainly, bulk density and apparent or open porosity, by performing laboratory tests on samples collected under real in‐service conditions.

The test procedure (sample preparation and techniques used) and its relationship with the overall performance characteristics and with any anomalies are discussed for each parameter. The analysis of the results covers seven case studies and 15 walls with renders made on site (cementitious and cement‐lime based) and non‐traditional or pre‐mixed renders.

It was concluded that reliable relationships could be established between the test results and other physical parameters relevant to the mechanical and water‐related behaviour of different types of cementitious renders. Notwithstanding the usual drawbacks of laboratory tests (more time‐consuming, higher costs and greater destruction of the walls), the use of small samples and the additional data obtained in terms of in‐service performance justify the use of this type of test.

The originality of this approach comes from the advantage of conducting both laboratory tests (reliability and standardization) and in situ tests (testing real materials subjected to in‐service conditions). Current practice is either to test virgin materials in quasi‐perfect laboratory conditions (therefore far from reality) or to test materials on site using standard in situ techniques sensitive to subjectivity and in far from ideal environmental conditions.