English bond is, upon the whole, to be preferred to Flemish bond for strength, as it contains a larger proportion of headers. The only advantage claimed for Flemish bond is its appearance, which is preferred by many, and has led to its use in brick buildings of a superior class.
In walls of 1 1/2 brick in thickness the strength is not so much impaired by using Flemish bond, as it is in thicker walls.
For thick walls English bond should be used, if possible: but, if Flemish bond is required, it should have a backing of English bond, as described at p. 23, unless it is to show a fair face on both sides.
Figs. 83, 84 are plans of two courses of an 18-inch wall, showing the bond frequently recommended in books. It will be noticed that course A has broken transverse joints, but the advantages claimed for these (see p. 20) are neutralised by the straight transverse joints in course B; and upon further investigation it will be seen that the defects caused when all the transverse joints are broken are aggravated by this mixture of the two systems.
Fig. 85 is the plan of two courses (one laid upon the other of the 18-inch wall bonded as shown in Figs. 83, 84. The course A is uppermost, and shown in thin lines; course B below being drawn in dotted lines. Those portions of the joints which coincide in both courses are shown in thick lines.
It will be seen, therefore, that the centre of the wall for more than half its thickness is split up by these coincident or "straight" joints, into vertical slabs having no connections with one another except on the faces of the wall.
Fig. 89. Inferior forms of English Bond, showing Defects.
The bond shown in Figs 86, 87 is frequently recommended for angles formed by walls of considerable thickness, but is also open to objection, although on paper each course presents a symmetrical appearance.
It will be seen by the sections that although each course is well bonded in itself, and appears to be of a strong construction, false headers are used, and there is no part of the wall which is not split up by one or more vertical joints extending throughout its whole height.
The merits of respective bonds and the defects of some forms frequently recommended, are fully discussed in Sir Charles Pasley's treatise on brickwork, from which much of the information given above is taken.
Sir Charles Pasley recommends that a student desirous of thoroughly understanding the various bonds, and of testing their respective merits, should build them for himself with model bricks; or if this cannot conveniently be done, he should at least draw the courses, one above another, as in Fig. 49, in order to ascertain whether any of the joints coincide so as to form splits in the wall.