BS EN 998 is a harmonised European standard that sets performance requirements for factory-made mortar used in masonry construction. Divided into two parts, the standard addresses rendering and plastering mortars (Part 1) and masonry mortars (Part 2), providing a unified framework for manufacturers and specifiers across member states. Unlike traditional recipe-based specifications, BS EN 998 emphasises final performance characteristics, allowing innovation in formulation while ensuring consistent on-site behaviour.
What Does BS EN 998-1 Cover?
BS EN 998-1:2010 applies to factory-made rendering and plastering mortar based on inorganic binders, intended for external rendering and internal plastering on walls, ceilings, columns and partitions. The standard provides definitions and final performance requirements, ensuring that products can be assessed independently of their precise formulation. As a harmonised European standard, products meeting BS EN 998-1 carry CE marking, simplifying cross-border trade and procurement.
The 2010 edition superseded BS EN 998-1:2003 and was itself later replaced by BS EN 998-1:2016, reflecting ongoing refinement of testing methods and performance thresholds. Factory-made mortars covered by Part 1 are typically delivered ready-to-use or as dry-bagged mixes requiring only water addition, reducing variability compared to site-batched alternatives.
How Does BS EN 998-2 Govern Masonry Mortar?
Part 2 of the standard addresses masonry mortar used in brick and blockwork joints. It distinguishes between designed mortars—factory-made products with guaranteed strength classes such as M6 or M12—and prescribed mortars, which are site-mixed recipes where strength is inferred from proportions rather than tested. This distinction is critical: designed mortars offer certified performance, while prescribed mortars rely on accurate batching and consistent sand quality.
Common UK mixes under BS EN 998-2 include M12 (1:3 cement-to-sand by volume) for engineering brick and exposed locations below damp-proof course, M6 (1:4) for general brickwork and standard blockwork, and M4 (1:5 to 1:6) for internal or sheltered applications. The key principle is that mortar should remain the sacrificial element in the wall assembly—weaker than the masonry units it binds. When mortar is harder than the brick, moisture becomes trapped in the brick face and freeze-thaw cycles cause spalling. Correctly specified joints can be repointed periodically, protecting the masonry units themselves from premature failure.
Why Performance Matters More Than Recipe
The shift from prescriptive mix ratios to performance-based specification reflects real-world variability. Site-mixed prescribed mortars can drift significantly in strength depending on sand moisture content, grading and batching accuracy; damp sand "bulks," altering volumetric proportions even when the nominal mix ratio remains constant. For commercial or structural work, specifying a designed mortar with certified strength class removes this uncertainty.
BS EN 998-2 also requires manufacturers to declare workable life—the period after mixing during which the mortar remains usable. This practical detail supports site planning, especially on large bricklaying schedules where extended open time reduces waste and rework.
Durability and Air Entrainment
For exposed or severe-exposure masonry, air-entrained (plasticised) mortar is commonly specified. Entrained air provides microscopic voids within the set mortar, accommodating frost expansion and improving freeze-thaw durability. In the UK climate, this feature is essential for external walls and parapets subject to repeated wetting and freezing cycles.
Pre-1919 solid-wall buildings typically require softer lime mortars (natural hydraulic lime, NHL) rather than cement-based formulations, to match the strength and breathability of historic brick. Using overly strong mortar in heritage masonry traps moisture and accelerates brick deterioration, underlining the importance of fit-for-purpose specification.
Relevance to Construction Chemistry and Sustainability
Factory-made mortars governed by BS EN 998 increasingly incorporate supplementary cementitious materials and optimised binder blends to reduce embodied carbon while maintaining performance. The standard's performance-based approach permits these innovations without mandating specific ingredient lists, provided final properties meet classification thresholds. For architects and engineers specifying low-carbon masonry systems, designed mortars with Environmental Product Declarations offer transparent lifecycle data and regulatory compliance under evolving sustainability frameworks.
Further guidance on related topics can be found in articles on polymer-modified grouting mortars and shrinkage behaviour of mortar and concrete, both of which explore performance characteristics critical to long-term durability.
