The style of the original mortar joints and pointing pattern should normally be replicated. Pointing can take many forms (such as recessed, flush, slaistered etc.). Mortar analysis may assist in matching the original mortar mix.
Under no circumstances should joints be widened to facilitate the repointing work. Raking out should be carried out carefully with hand tools; power tools should be avoided as they invariably cause irreparable damage to the stonework. It is important that the correct tools are chosen and used for specific types of joints.
Mortar should be sufficiently resilient to accommodate minor movements in the masonry and should never be stronger, denser or less porous than the adjoining stones. Pointing with a dense or hard mortar can cause the mortar to crack and will prevent the wall from drying out through the joints; moisture then becomes trapped or is forced into the stones causing them to decay prematurely.
Water vapour egress through the lime mortar joints of masonry needs to be recognised as an important part of how traditional buildings breathe. If this process is prevented from happening a number of damp related problems can occur. The most common cause of such problems is the use of cement-based mortars for repointing so this should be avoided in every circumstance.
HARLING Harling or render was extensively used as a surface coating to protect vulnerable building materials from exposure to the elements or to provide an illusion of a fine masonry finish. Traditionally this would have been a lime harl mixed with local aggregates to provide pigmentation.
New harling should be based on evidence of previous use of the material on the building. Properly specified traditional materials allow the wall to absorb and evaporate moisture efficiently. Old cement renders should be removed if they are found to be causing damage. The application of limewash should also be evidence based and is a process that requires reapplication every 5–10 years depending on the prevailing weather conditions.
PAINT Applying paint to unpainted masonry walls can cause significant long term damage by preventing the evaporation of moisture from the stone. If historic paint is harming the performance of the wall, careful removal is recommended, guided by expert advice.
Craig Wilson LPOC Scottish Conservation Advisor 64 Listed Heritage Magazine January/February 2020
GLOSSARY
Stugged finish: The stone generally has a margin which can either be plain or
Ashlar stone: Masonry of large blocks in regular courses worked to even faces and carefully squared edges: the stones themselves are called ashlars and may have a dressed finish.
droved finished (see below) and the main stone surface is broken up by random chisel point markings. The finish can vary from a light stugged finish to quite a deep broken surface. The technique was employed from the mid-19th century onwards.
Broached finish: Again the stone generally has a margin and the main
stone surface is formed by running parallel chiseled lines in the surface. The lines can vary in depth and frequency.
Harl: Scottish form of roughcast render in which the mixture of aggregate (small even sized pebbles) and binding material (sand and lime) is dashed onto masonry wall; in traditional harls the aggregate is in the mix (wet dash), in non-traditional 20th century harls the aggregate is dashed separately (dry dash).
Rubble stone: Masonry which is not fully dressed: can be of boulders; or random rubble retaining in some degree the natural shape of the stone; or of squared rubble in which the stones are roughly squared and may either be coursed or snecked, i.e. with variations in the coursing brought about by the use of smaller stones or snecks.
Droved finish: The stone is generally finished with a fine vertical or angled line
of wide edged chisel markings. The surface is not broken as with broaching or stugging but it is marked. Popular in the later 18th and early-19th centuries.
Sparrow pecked finish: The stones face is sunken to a depth ¼.” The surface
of the stone is chiseled then picked with a mallet and sharp point.
Slaistering (or sneck-harling): A Scottish term used to describe flush-finished mortar joints in rubble stonework where half the surface of the wall might be mortar joint. Flush pointing would be used to describe the same south of the border.
Split face wallstone finish: The face of the stone is split to provide a
rough texture to the face and the beds and joints are thereafter hammer finished to provide definition.
Reticulated ashlar finished, bush hammered: The surface of the stone
is cut in a serious about 3/8 in deep. The sinkings are worked to a gauge and separated by bands which are all the same width. Once complete the sinkings are picked with a sharp point.
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136 |
Page 137 |
Page 138 |
Page 139 |
Page 140 |
Page 141 |
Page 142 |
Page 143 |
Page 144 |
Page 145 |
Page 146 |
Page 147 |
Page 148 |
Page 149 |
Page 150 |
Page 151 |
Page 152 |
Page 153 |
Page 154 |
Page 155 |
Page 156 |
Page 157 |
Page 158 |
Page 159 |
Page 160 |
Page 161 |
Page 162 |
Page 163 |
Page 164 |
Page 165 |
Page 166 |
Page 167 |
Page 168 |
Page 169 |
Page 170 |
Page 171 |
Page 172 |
Page 173 |
Page 174 |
Page 175 |
Page 176 |
Page 177 |
Page 178 |
Page 179 |
Page 180
