Recessed joints between the brickwork is a sign a wall requires repointing.
The repointing of old brick and stone walls is often done in ways that do nothing to enhance the appearance of a building and little to afford long-term weather protection. Often it is done by untrained operatives. It is a labour intensive operation and therefore expensive, so it is worth giving the matter some thought to ensure the work is done properly.
The types of masonry most commonly met with in the North East are as follows:
Random rubble stone work built with irregular stone as quarried with little attempt at coursing was the standard for agricultural buildings and some town houses up to about 1900. It normally consists of an inner and outer face with a loose rubble core with occasional through stones to tie the faces together. The walls were normally 600mm (2’0”) thick for domestic and farm buildings and 450mm (18”) thick for barns and outbuildings. In older agricultural buildings clay mud was used to bind the walls which were pointed externally with a lime mortar. Dressed stone lintels and sills were used for door and window openings. Over many years, the lime mortar deteriorates, and weathers and moisture penetrates to the internal face. Few such buildings have escaped at least one attempt at repointing and often it is done in a piecemeal fashion ending up like a patchwork quilt. The work was often done by unskilled labourers who thought the best policy was to smear as much mortar over the wall as possible ending up with the centres of individual stones peeping through the mortar matrix.
In later more sophisticated buildings, dressed stone was used in random coursing with square quoin stones at the corners and stone frames, lintels and cills to windows and doors. Again, these were built in a lime mortar but with a much finer joint no thicker than 10mm and often only 5mm.
Landed gentry with country estates built their palatial houses in ashlar which consists of accurately sawn blocks of stone laid in regular courses with fine joints again in lime mortar.
Brick and tile making using the clays from the glacial drift deposits was an indigenous North East industry from about 1750 and often associated with the coal mining industry which supplied the cheap coal for firing the kilns and there were many small brick manufacturers throughout the North East up to the early 1900s. Walls were built solid either 9” thick for outbuildings and rear off shoots or 13 1/2” (1 ½ bricks) for the main walls. Normally there were four or five courses of stretchers (the long face of the brick) to one course of headers (the short face) and invariably they were built in a lime mortar. Cavity walls were introduced around the turn of the twentieth century and began to be common from 1920 onwards. They did not become a universal practise until about 1930.
The function of mortar is not as one might think to stick the bricks or stones together, indeed lime mortar has a very low bond strength, but to provide a bed to even out the irregularities of the individual blocks. The more uneven the surface and shape of the individual units, the thicker the mortar bed and the thickness may vary from 5mm to 50mm with random rubble.
The relative weakness of lime mortar causes the exposed face of the mortar to degrade and ‘weather’ back allowing the ingress of rain into the body of the wall. It is at this stage that repointing becomes necessary.
The stone used in masonry construction in the North East is either sandstone, millstone grit or limestone, and although the face of the stonework also weathers, it is at a slower rate than the lime mortar. If, however the lime mortar is replaced with a modern cement mortar the rates of weathering are reversed and after a while the cement mortar will stand proud of the wall face once again allowing rain to penetrate into the wall.
It is essential therefore that any repointing must be carried out in a mortar that is weaker than the stone or brick with which the wall is constructed and using a ‘good strong mix’ does more harm than good in the long-term.
A common fault in repointing is to apply the mortar straight onto the weathered lime mortar joint without raking out to form a key. This looks fine initially but after a while rain penetrates behind the pointing, freezes and loosens it. At this stage the pointing can be picked out with one’s fingers if it has not fallen out beforehand.
Smear pointing has the same effect. This occurs when the mortar is smeared over the edge of the individual stones and the resultant feather edge of the pointing on the upper stone traps rain running down the face of the stone and again frost action will cause it to spall.
Another fault is ‘ribbon’ pointing where the repointed joints, often in a cement mortar, are left proud of the face of the stonework in the form of ribbons of cement with sharp edges. This provides a ledge for rainwater to collect and make its way into the interior of the wall. It tends to enhance the pointing to the detriment of the stone, whereas the pointing should complement the stone or brick.
From an aesthetic point of view, it is the stone or brick that needs to be emphasised not the mortar and both smear pointing and ribbon pointing have the opposite effect.
There are therefore three basic essentials for repointing:
a) The existing joints must be raked out for a sufficient depth (at least the thickness of the joint and not less than 10mm).
b) The mortar must be weaker than the stone or brick.
c) The mortar joint must be slightly recessed to avoid water penetrating the joint and to emphasise the stone or brick and not the mortar.
When raking out the mortar joints care must be taken to avoid damaging the ‘arrises’ (the corners) of the stone or brick. A Stihl saw is not to be recommended.
The mortar should be a sand:lime mix ‘gauged’ with Portland cement. Only sharp, well washed coarse sands and grits should be used with a minimum amount of water. The mix will depend on the application but generally a 1:1:6 (cement:lime:sand) mix is suitable for brickwork and a 1:2:9 mix for stonework. Cement:sand mixes with mortar additives should be avoided. Sand and lime (otherwise known as ‘coarse’ stuff) can be obtained ready mixed and is then ‘gauged’ on site with cement as and when it is required. Once the gauged mortar begins to set however it can no longer be used although the sand lime ‘coarse’ stuff can be ‘knocked up’ and re-used daily. It is a good idea to order sufficient ‘coarse stuff’ for the whole job to avoid slight variations in colour.
The raked-out joints should be slightly moistened before repointing (an old paint brush and a bucket of water should be kept handy) and the mortar should be pushed into the joints with a pointing trowel so that it is completely filled. The slight recessing can be obtained by ‘bagging down’ with old sacking which also helps to consolidate the surface of the pointing. At the end of the day the stonework or brickwork should be lightly brushed down to remove any traces of mortar adhering to the face of the brick or stone.
Trying to match old weathered pointing is next to impossible and an acceptable mortar mix can only be obtained by trial and error not forgetting that mortar dries out a different colour to the fresh mortar. An experienced bricklayer or mason should be able to advise further.
RM Higgins, July 1994