All masonry buildings have one element in common: mortar, the material that separates the individual masonry units, whether they be brick, stone or some other material, such as concrete bricks or even glass bricks. The mortar’s function is not, as most people suppose, ‘to stick the bricks together’, but to hold them apart — and to fill the irregularities on the bedding faces. Gravity is the element that holds masonry together. At some stage in a building’s life this mortar will – hopefully – show signs of age and wear, and will gradually become recessed. The mortar ought to be softer and more porous than the masonry units and will consequently take the brunt of the weathering process; this is more desirable than the alternative situation where the masonry units are softer than the mortar. In these circumstances the mortar will remain sound whilst the bricks erode, subsequently requiring rebuilding rather than the relatively simple task of renewing the outer portion of the mortar joint, the process known as repointing. It is usually necessary to repoint when the depth of the open joint is approaching the thickness of the mortar bed. The work is generally straightforward but labour intensive, and though materials are cheap, the ultimate cost of employing a builder may be considerable, so wholesale cosmetic repointing may not only be unnecessary but also expensive. One advantage of repointing when necessary is that the preparatory works of raking and cleaning out the joint are minimised. At this point a few words of caution are in order regarding the use of grinders to clean out the joints. Even with great care exercised in their use, slips and over-runs are inevitable and will leave permanent unsightly scars in the masonry. The only time a grinder should be employed is in removing modern over-strong cement-based pointing where the conventional chopping out with a plugging chisel will cause more damage. Here the correct technique is to put a single thin cut in the middle of the joint to the depth of the cement mortar, then, with a sharp 2 1/4” bolster, gently tap the remaining mortar towards the cut groove. This will minimise damage to the adjoining brick faces. When undertaking repointing it is customary to begin the work at the top, gradually working down the wall. This ensures that all the dust and damping down is below any completed work. Preparation, as mentioned, requires cleaning the joints out to a minimum depth of the mortar thickness. On brickwork this will be 3/4” or more, and most importantly the cleaned-out face needs to be a square face, not concave. It takes a little more care and time but does ensure sufficient hold for the new mortar. Clean the joints out with a hand brush and then give a good soaking of water with a spray or hose — the objective being not just to wet the surface, but to create a reservoir of dampness within the wall to facilitate even curing and an even drying out of the new mortar. On dry and porous brickwork this damping process may need repeating. Once the surface has dried, the wall is fully prepared and ready for repointing. Choosing an appropriate mortar mix can be a fraught process and needs to be considered on an individual job basis. Generally it is aesthetically pleasing and sensible to replicate the original mortar in colour, texture and strength, and this may mean mixing different sands together for the desired result. For ensuring consistency of mixes, use a gauging measure, which can be any appropriately sized container, and don’t forget to make a note of the mix proportions! Most Victorian or earlier buildings will be built with lime mortar and, whatever some modern builders say, the use of cement in repointing mixes is generally injurious, causing accelerated decay and damp problems. One characteristic of mortars containing cement is their vulnerability to salt and even sulphate attack, and whilst this sounds esoteric, the destructive effects of salt attack are all too visible in roadside buildings that are splashed by salty water in winter by salt intended for the highway. Mortars containing only lime, either putty or hydraulic, and sand aren’t susceptible to this damage. The mortar used for pointing needs to be firm but workable and pushed firmly into the prepared joints without leaving any voids, and then allowed to firm up before the chosen finish is worked. With all mortars, whether lime or cement, it is preferable to have a slightly open texture finish rather than a dense, smooth ‘ironed’ finish. This might be one of the reasons why the simple ‘rubbed flush’ joint, where the filled joint is gently rubbed with a stick or rubber before a final light brushing removes any surplus mortar fragments, is considered so attractive. - See more at: http://www.homebuilding.co.uk/advice/existing-homes/renovating/repointing-old-brickwork#sthash.UbN15ITr.dpuf
Pointing is the term given to the ‘finish’ that is between the bricks or stone used to build your house. Depending on the age of the building, the mortar used to lay the stone or brick will either be made from lime, or more recently, cement. Incorrect pointing causes irreparable damage to older buildings. It is essential to understand what the mortar joints actually do for the fabric of the house.
The function of the mortar in the wall is to act as a bedding between stones and varies from fine joints in ashlar stonework to larger joints in rubble masonry walls. Joints are effectively reduced in size by inserting small stones and ‘snecked’ pieces of stone. Whilst acting as a bedding the mortar must also perform other functions:
It must prevent water penetration through the joints by its physical presence almost like a masonry‘ sponge’, yet it must allow the wall to breathe and drain, porosity being a key factor in the choice of a repointing mortar.
It must be flexible to allow movement/settlement of the structure due to thermal responses and settlement within the structure. Many earlier large buildings are not designed with the modern expansion/contraction joints of today.
The strength of the mortar should always be less than the surrounding stones and should be considered as a sacrificial element of the wall, and viewed as a maintenance item in need of replacement possibly every century.
The condition of stone walls cannot be viewed in isolation and repointing of any walls will not cure water ingress problems caused by other building failures, such as gutters, roofs, and leadwork. These must be in good condition to maintain the life of the walling elements.
Walls need to breathe – and if the pointing doesn’t allow this, the wall will rapidly deteriorate. Mortar joints are the lungs of a wall – they allow water within the structure to enter and leave freely. If water tries to leave through stone or brick, it will slowly disintegrate. Block the mortar joints, and the wall will destroy itself.
Mortar should be softer than the material the wall is built with. Pointing should be subservient to the material the wall is built with and assume a secondary role, visually.
There are essentially two types of pointing, which work in different ways :
Old houses, generally pre 1920’s, built from brick or stone, using lime mortar, usually solid walls with no cavity
Newer houses, post 1920’s and built from brick or stone, using cement mortar, and having a cavity
If the house is built using lime mortar, it is most likely that the walls are solid – they won’t have a cavity. These walls need to be treated very differently to ones with a cavity. Old walls need to breathe – the stone and brick from which they are built is usually harder, and less absorbent than the lime mortar. Moisture in the wall escapes through the mortar joints. This can be moisture from condensation on the inside of the walls, or water which falls on the outside of the wall from rainfall. If mortar joints are left to breathe, your walls will stay dry. The lime mortar gradually ‘self sacrifices’ over many years, and recedes into the joint, so that eventually, there is a slot between the bricks or stone.
The comonest problem with old houses is flaking brick and spalling stone, which can almost always be traced back to when the joints were raked out, and cement mortar used to re-point them. If cement mortar is used, water will not escape the walls, and you will have damp problems inside the house as well. This is one of the common causes of so-called rising damp – it is nothing to do with rising damp, and the solution is NEVER an injection damp proof course!
This is death to an old house – you must ALWAYS rake out the joints and re-point using lime mortar, which should have NO portland cement in it. If in doubt, you MUST insist on the tradesman or company showing proof that they know how to rake out, mix and use lime mortar, sometimes referred to as hydraulic lime mortar. We are NOT talking about buying a bag of sand, and a bag of lime from the builders merchant – this will not do the job, and is NOT the material we are talking about. Pointing old stonework or brickwork is a specialised job and must be done by people who know what they are doing.
There are several grades of NHL lime – (Natural Hydraulic Lime) – and the three commonest grades are used for pointing stonework, brickwork, and where additional strength is needed, for example chimneys. Do NOT allow anyone to ‘rake’ out such joints with a diamond grinding wheel or abrasive wheel of any sort – these damage the bricks, and cause major problems with flaking and spalling in the coming years. Grinding wheels damage the top and bottom ‘arris’ of the brick, removing natural glaze from when the brick was made. This allows water to enter the brick – in winter this freezes and the brick disintegrates. We have seen entire brick walls crumble from this dreadful practise.
Old houses were built to ‘move’ – lime mortar is flexible. It actually moves and self heals – if a small crack develops because the house settles a tiny bit, or brickwork moves, the crack heals. Lime has the amazing property of actually taking in water and carbon dioxide from the atmosphere, and growing tiny calcite crystals which span the cavity of the crack, and tie it together again. Using cement based compounds is an absolute no-no – this introduces rigidity, and in an old house which needs to move, cement just cracks and falls off within a short period of time – it is useless, and a completely inappropriate material to use.
If the joints are slotted out, but no deeper than around 1cm, leave them – there’s nothing wrong.
If they are deeper, they may need pointing with NHL mortar
If they have obviously hard cement in them, and the brick or stone is flaking away around the joints, the joints need cleaning out and NHL mortar putting back in place of the cement
Lime mortar used should always be softer than the surrounding brick or stone
In summary, if your house has solid walls, and lime mortar was used to build it, you should NEVER use any cement based materials for pointing or maintenance.
If your house is post 1920’s, it may well have been built with brick laid on a cement mortar. Problems with cement mortar generally show themselves as cracks – cement based mortar is incredibly hard and inflexible, and as the building moves, which it will, over time – the mortar cracks around the bricks, and allows water to enter the structure. Cement based mortars can degrade – if badly mixed, or there was insufficient mortar in the mix (often the case when council houses or speculative developments were built in the 1950’s and 60’s before building supervision was tightened up), the joints may erode, and require repointing. In all cases, there is always a case for making the mortar mix relatively flexible, by adding lime, in order that it can move more with the building, and allow moisture to escape. The mix is usually a 4:1:1 mix – that is – 4 sand, to 1 lime, and 1 cement – this should be pointed into joints which are raked out to at least 25mm (1 inch) and the joint should be cleaned of dust and moistened before the cement mortar is pushed into the joint. It can be finished by pointing with a trowel to a flat finish, or cut off and brushed to provide an older, more weathered appearance.
On NO ACCOUNT should a wall built with lime mortar be pointed with cement based materials.
All too often repointing is viewed as a cosmetic exercise. When done in a hard cement mortar, which is placed over the existing joint rather than in it, the incorrect pointing sits on the face of the stonework. This method results in a spiders web effect described as ‘strap’ pointing. It develops hairline cracks due to the strength of the mix allowing water to penetrate, yet prevents the moisture evaporating due to its density. It usually falls off after 10-15 years, due to lack of preparation of the joints, but sometimes spalls the adjoining stone faces with it.
Another common practice is the slap technique whereby cement mortar is plastered over the joint and stone with a trowel and is usually associated with partial or part pointing of a façade. Apart from the disfiguring of the stonework, this also prevents the wall from breathing. Due to the adhesion qualities of cement this sometimes prevents its removal and the next stage of repair is re-rendering due to the damage done.
Repointing should only be carried out if the existing mortar is decayed, cracked or damaged. Sound historic mortar should be untouched. Only in very badly weathered cases should complete repointing be necessary.
If repointing is necessary, care must be taken to avoid damage and the joints raked out to a 10-12 mm depth and fully packed with mortar using a variety of fine edge pointing irons or tools with the joint being left slightly recessed from the face. Mechanical cutting tools should not be used on work to listed buildings as they will damage surrounding masonry.