Types & Styles of Flint Walls


The important legacy of flint construction presents something of a challenge to modern owners and conservators. Too often, repairs seem to be carried out by the uninitiated (and are sometimes specified by them too). The tough, intractable, siliceous nature of flint is the source of its great durability; but this durability is confined to individual units and not necessarily to flint masonry. To a non-expert, flint is not only difficult to split and shape, it is also difficult to raise as a vertical wall even in its rough, nodular, field-flint form. This is why so much mortar is used in which the irregular flints are set like currants in a cake. True, more mortar is needed than for regular units such as brick, but somewhere in the wall the flints need to touch each other and lock together as much as possible within the mortar matrix.

Clearly, the importance of mortar is paramount. The Saxon Shore forts such as Pevensey or Portchester, built in the late third century by the Romans, survive in an aggressive environment because of the quality of their mortar. In these early years the mortars were lime, ash and (commonly) brick aggregate and brick or tile pozzolan. Where no reactive volcanic ash was available (true pozzolana) ceramic pozzolans were used extensively throughout the Roman world to impart a hydraulic set to their mortar. Tile or brick fired at low temperatures and crushed to a fine particle size reacted with some of the lime to form calcium aluminates and silicates, is almost always found during analysis of Roman mortars.

These mortars were ideal for masonry use; their permeability (their ability to allow water to evaporate through their pore structures), adhesion and flexural strength (which enables masonry to accommodate minor movements without cracking), ensured good, long-term performance. Later builders used lime mortars without ceramic powder but still containing significant quantities of kiln residues which could also impart a weak hydraulic set; or they used natural hydraulic limes such as those from Lewes in Sussex or Dorking in Surrey for their flint work. Analysis has shown that additions such as tallow or beeswax were also sometimes included, added when the lime was being slaked.

The use of similar mortars to the traditional types in repair and conservation is absolutely essential to good practice. Yet time and time again, partly misled by the strength characteristics of flint units and partly because the building industry is still locked into the use of cement, flint is raised in cement or masonry cement mortar. Not only is this, visually, a lifeless material, it develops shrinkage cracks around flints and forms continuous cracks through the mortar to accommodate movement inevitably caused by thermal and moisture changes. In addition to these problems, cement rich mortar inhibits the free evaporation of moisture which enters through cracks, and contributes to the overall dampness of the wall. In occupied buildings this is uncomfortable and unhealthy; in freestanding walls the masonry often falls victim to frost.

The most common occurrences of failure in flint walls are detachments of the flint faces, with or without the cracking associated with inappropriate mortars. Where detachment and major cracking occurs, the area affected must be taken down and rebuilt. All situations require that the character of the work is recorded and properly observed. Not all flint work, even of modest character, is as random as it might seem; there is usually coursing and the flints may be set in opposing diagonal lines as in the Roman 'opus spicatum' ('herring bone'). Without careful observation of the flint masonry as found, rebuilding and repair can change its character quite subtly and adversely. Sometimes there is fugitive evidence of jointing patterns and profiles, or the joints may be set with small shards of broken flint called gallets. All such detail needs to be put back.

The proper bonding of the flint face to its backing is a very important detail, even if there seem to be no bulging or loose flints present. Long tailed flints well anchored in hydraulic lime mortar provide the best method of bonding, but stainless steel wire ties or anchors can also be used. The backing or core may also need some consolidation as it varies from well built, well compacted brick and flint ('bungaroosh') to loose, rubbishy fill with little structural capacity ('ragtush'). Sometimes hand grouting of local voids with a proprietary grout or hydraulic lime may be necessary to supplement tamping and pointing. A suitable proprietary grout might contain a suspension of pulverised fly ash (a form of coal ash), hydraulic lime and bentonite.

Inappropriate mortar is cut out in much the same way from flint masonry as from other traditional masonry, although the depth of cutting out may need to be increased. Sharp tungsten tipped chisels should be used. If an aggressive mortar has been placed to some depth, which is unusual, it may help to carry out some 'stitch drilling' with a masonry drill first. The flints and backing need to be well wetted and if new flints are being used they need to be soaked in clean water before being placed. Mortar must be placed with great care, ensuring thorough compaction and filling of all irregular voids, and the final pointing needs to be cured slowly. During the curing process any shrinkage needs to be closed up with pointing keys as it occurs. Successful work cannot be carried out unless a suitable range of tamping tools and pointing irons is available.