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Specifying mortar

• Selecting the appropriate mortar mix
• Specifying the mortar mix
• Testing the mortar mix

Selecting the appropriate mortar mix

Advice on selecting the appropriate mortar mix for various types of masonry unit, in a wide range of conditions or situations, is available from a number of sources, the most commonly-available of which are:

• BS 5628-3 Table 12 : This includes a wider range of masonry types and conditions than the two documents referred to below. Table 12 refers to mortar mixes using the (i), (ii), (iii), (iv) designation from BS 5628-3 Table 13. (The "Roman Numeral" designation is taken from the now-superceded BS 4721 - see below. Table 12 should be taken as the definitive reference standard for mortar selection unless brick/block manufacturers specifically recommend otherwise for their products.
• NBS Building, Section F10, General Guidance 9.4 : Table 7 is "derived from BS 5628-3 Table 12". It is, essentially, a selection of the more commonly-occurring masonry type/ condition/ situation variables from Table 12, although it uses the mortar designations 1, 2, 3, 4 rather than (i), (ii), (iii), (iv).
• NBS Shortcut 03 - A Course in Mortar : This presents, graphically, a selection of the more commonly-occurring masonry type/ condition/ situation variables from BS 5628-3 Table 12. This is probably the easiest of the three documents to understand, for the limited number of variables displayed.

Specifying the mortar mix

Relationship between BS EN 998-2:2003 and BS 4721:1981

From BS EN 998-2:2003, National Annex NA.1.

EN 998-2 specifies the constituent materials and performance requirements for mortar for masonry. It is based on performance rather than prescription and in this way differs fundamentally from BS 4721, which includes a mixture of prescriptive and performance requirements.

The following relationships, based on the prescriptive mix proportions historically used in the UK codes of practices and standards can be regarded as authoritative:
BS 4721:1981 - Mix designation (i) » EN 998-2:2003 - Mortar class 12.
BS 4721:1981 - Mix designation (ii) » EN 998-2:2003 - Mortar class 6.
BS 4721:1981 - Mix designation (iii) » EN 998-2:2003 - Mortar class 4.
BS 4721:1981 - Mix designation (iv) » EN 998-2:2003 - Mortar class 2.

Using NBS Building Section F10

Mortar mixes (Clauses F10/110 to F10/395 inclusive)

The Mortar/Standard and Mortar/Mix selection boxes in these clauses are not particularly user-friendly, and the NBS Guidance could be more helpful.

The Mortar/Mix selection boxes are actually three separate lists (which appear as one):

• Site batched and mixed mortar: Select from: (9 options)
• Site made hydraulic lime mortars: Select from: (4 options)
• Mortar to BS EN 998-2: Select from: (7 options)

NBS offers, in principle, four ways of specifying mortar:

• Method 1
• Mortar/Standard: Select Not applicable and delete clause F10/460.
• Mortar/Mix: Select one of the seven site-batched and cement-based options specified by constituents and proportions.
• Method 2
• Mortar/Standard: Select Not applicable and include clause F10/460.
• Mortar/Mix: Select one of the two site-batched and cement-based options specified by Group.
• Method 3
• Mortar/Standard: Select Not applicable and delete clause F10/460.
• Mortar/Mix: Select one of the four site-batched and hydraulic lime-based options specified by constituents and proportions.
• Method 4 (ArchiMentor recommended method for cement-based mortars)
• Mortar/Standard: Select To BS EN 998-2 and delete clause F10/460.
• Mortar/Mix: Select one of the seven BS EN options.

Testing the mortar mix

This section is applicable, primarily, to the completion of NBS Building, Section F10, clause F10/410. The values provided in the NBS Building selection boxes are not helpful and, in certain cases, relate to the values in BS 5628-1 rather than BS EN 998-2.

If you are using either of Methods 1, 2, or 3 (above) to specify the mortar you will need to determine the appropriate values, noting that it is normal to specify the same values for the Preliminary Tests and the Site Tests.

If you are specifying the mortar by reference to BS EN 998-2, insert the value specified in the Mortar/Mix selection (in clauses F10/110 to F10/395 inclusive) for both the Preliminary Tests and the Site Tests. BS EN 998-2 does not specify the maximum allowable strength for a given class but +25% seems a sensible recommendation if using classes 2, 4, 6, and 12 only.

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