Stone has a long history as a building material. In the past and indeed today fine ashlar blocks have absolutely minimal pointing; the very fine joints at the front are simply filled with mature slaked lime putty with perhaps a hint of stone dust and you can hardly see them.
By contrast, rough rubblestone outer walls were often built to have all the joints flushed up with a lime mortar and were then covered with a very thin lime render in the form of a slurry that followed the contours of the wall. In that case every stone still shows and can be read through the render, which protects the masonry, together with the sacrificial limewash applied regularly to the outer face.
Conservation builders will press for the same treatment of rubblestone walls in cottages today and resist pressure to leave the random rubble showing because fashion nowadays dictates that exposed stone looks attractive.
“It really depends on the stone and the quality of the mortar,” says masonry consultant Bob Bennett, who runs The Lime Centre near Winchester. “All too often, together with conservation architects, I have had to fight very hard to convince clients that the stone should not be left exposed but reslurried and finished off in the manner in which it was originally built.”
Most of the natural stone going on to the faces of self-build houses today is not rubblestone, but has a pitched and dressed or partially dressed face. Nowadays it is regarded as a premium and usually very expensive material and many architects believe that stone of this sort should be allowed to express itself – indeed planners often demand it. However, the mortar should at all times be subservient and slightly lighter in colour than the stone. The pointing should also be subservient, and follow the outline of the stones.
This means that pointing with a hard-looking cement mortar, which is often made to project a little and is trimmed along both top and bottom, should be avoided. “We try to discourage this ribbon pointing in which the mortar stands proud of the face of the wall,” says Gloucestershire-based architect Edward Tyack of Batterton Tyack in Moreton in Marsh, who specialises in traditional buildings faced with Cotswold limestone. “Apart from being visually very unpleasant this sort of pointing creates ledges that trap water and lead to stone decay.”
All forms of stone absorb a certain amount of moisture in wet weather and in order to dry out again must be allowed to breathe freely through the mass in every direction. Unless this can happen some kinds of stone will fail, and most forms of stone will be adversely affected. Very hard non-porous joints impede aeration, leaving the stone prone to frost damage and should therefore be discouraged.
With coursed stone, because of the irregular nature of the surface, it is impossible to fill the joints flush with the wall face without smearing mortar on the surface. It is therefore better to set the pointing back a little. Whether the finished effect should be wet and smoothed with a flat steel pointing iron, or given a gently brushed surface depends upon your taste, but most experts prefer a slight texture on the finish of the joint to a smooth trowelled finish. “This sort of finish is achieved using a stiff brush, but the action is one of tamping and not brushing,” explains master mason Colin Burns, who has for many years been an instructor on pointing and stone laying techniques on courses run by English Heritage at West Dean College in Sussex.
When extending, the pointing on new stonework should match the original, or else the whole house should be repointed in the same style.
Colin says, “If you wish to avoid smearing and what we call tramlines on the finish of the stonework, the technique is to use a specialist brush called a churn brush to tamp the surface of the mortar after an early set, when it has a leathery feel. The vital thing is not to do this too early, otherwise you will end up with mortar all over the surface of the stone. Timing is everything, because if you leave the mortar to go too far you will need to draw a steel tool across it to remove what we call the latents the early crust on the surface. Getting it right is difficult because in hot weather you might be able to carry out the whole process in a day, while in winter you might need two days.”
As well as being about timing, good pointing is about clean working and using the correct tools. Most specialists use a pointing key a flexible metal blade that fits accurately into the width of the joint. Other people use an American style flitch with a cranked handle and flat blade.
Colin stresses the need to use a hydraulic lime mortar when pointing stonework. “It allows the structure to breathe,” he says. “Over-hard cement-rich mortars should not be used with stonework.”