Mortar mixes

Mortar is commonly made of cement, lime and sand, the proportions varying, dependent on the type of brick being used. This is because the mortar must never be harder than the bricks. Builder’s soft sand is used and if the appearance of the finished work is of importance, ensure that the sand is of a light and even colour, because it will effect the colour of the mortar. Either lime or a proprietary mortar plasticiser can be added to the mix to make it easier to use, or more fatty, as it is called.

A good general mortar mix that is suitable for internal and external walls is composed of 1 part cement :1 part lime : 5 parts sand. If a plasticiser is to be used, simply leave out the lime and add the plasticiser strictly in accordance with the maker’s instructions.

The tops of parapets and free-standing walls must be protected. This can be done by laying overhanging coping stones or the top of the wall can be finished off with a course of bricks laid on edge, in a slightly harder mortar to resist the frost action in winter. For this a mix of 1 part cement : ½ part lime : 4 parts sand can be used.

This stronger mix can also be used below ground level, especially where the ground is generally waterlogged. Even stronger mixes are used with engineering bricks. With such hard bricks it can be strengthened to 1 part cement : !/» part lime : 3 parts sand. The amound of water used is largely a matter of common sense, but the mortar should be soft enough to spread easily, but firm enough to be picked up with the trowel. It should not squeeze out and run down the wall when the bricks are bedded into it.

When using a plasticiser, less water will be needed as the air entrained in the mix reduces the amount necessary, so stop adding water when the mix becomes sufficiently wet to be workable. Do not mix more mortar than you will be able to use in about an hour to an hour and a half. After this time the mortar will have started to set and must not be remixed. On hot days when the sun dries the mortar quickly, an hour may be the limit, but on damp days the mortar may stay workable a little longer.

Mortar must never be stronger than the bricks which are bedded in it, because if it is too strong any natural settlement that takes place can cause cracks through the bricks themselves, whereas softer mortar would let the movement take place along the line of the mortar joints. This would only leave hair-line cracks which would not be detrimental to the wall.

In general, the inner leaf of cavity walls is laid in a 1 : 1 : 5 mix of cement, lime and sand. If calcium silicate bricks are being used then the mix is 1 : 2 : 8, other internal walls are laid in 1:2:8 mortar whether they are clay or calcium silicate bricks.

External walls are laid in a mix of 1 part cement, 1 part lime and 5 parts sand, but if the walls are sheltered the mix can be 1 : 2 : 8 for calcium silicate bricks. Sills and copings in clay or calcium silicate bricks are laid in a mix of 1 : ½ : 4. Parapets and free-standing walls are built in a 1 : 1 : 5 mix because they are prone to saturation with water and need to be able to withstand frost.

Except for these severe exposure conditions, frost damage to finished buildings is rare. Where the mortar is raked out for subsequent pointing, there may be more risk of frost damage if a strong cement mortar is used for pointing over a weak backing mortar. Risk of frost damage is greatest during the building period and it is essential to take precautions to protect the brickwork from unnecessary wetting and to cover it with sufficient insulation material to protect it from frost. Anti-freeze additives such as those used in concrete should not be used with bricklaying mortar.

For general building there is no need for any fancy finishes, but when building extensions to match existing work and when constructing ornamental brickwork in the garden, there are cement colours that can be added to the mortar to give improved effects. These additives must always be used in accordance with the maker’s instructions. Gauge the ingredients accurately if you want to maintain an even colour throughout all the mixes. When the mortar is wet it will be a little different in colour from what it will be when dry, probably a little darker. This means that it will be impossible to match each batch by appearance and newly mixed mortar, when placed against mortar that has started to dry, will be a different colour. These cement colours are obtainable as either powder or liquid.

The mortar joints of the brickwork are finished off in various ways according to the type of building. For a rustic appearance the mortar can be scraped off level with the face of the bricks and left to collect moss and algae.

Structural brickwork of a general kind is given a ‘struck’ or ‘weathered’ finish. This is done by using the trowel to press the mortar in at the top of the joint so that it slopes slightly towards the face of the wall. By this method a small drip is formed under the top brick and the rain is brought forward by the slope so that it runs down the face of the brick to the next drip, instead of soaking into the mortar.

Among the other ways of pointing the mortar joints is running a piece of pipe about 9 mm along the joint to form a curved recess. Another is to take out the mortar to a depth of not more than 9 mm. This method increases the shadow effect making the mortar joint stand out more boldly against the bricks, but it also encourages water to stand on the top of the bricks where it can cause damage, especially if it freezes. This type of pointing should therefore only be used in fairly sheltered positions.

For special effects, the mortar is raked out to a depth of not more than 12 mm and a mix of coloured mortar is used for pointing to one of the conventional finishes. Although rarely used now, tuck pointing provides an interesting finish. This is a method whereby a coloured mortar is inserted into the mortar joint so that it projects in front of the face of the wall. With this method a recess is pressed into the face of the flush mortar joint and then a special tool is used to form a strip of coloured mortar so that it fills the recess and projects a little in front of the wall face.

When preparing pointing mortar, sufficient water is added to the dry-mixed materials to give a stiff doughy consistency. If the mortar is too soft it cannot be handled with the pointing trowel or ironed into the brickwork joints.

Mortar ingredients must be well-mixed before the water is added. Measure out the correct proportion of sand first, then add the lime and mix the two together well. These materials can be mixed in large quantities and left until it is time to make up a new batch of mortar. Then the cement is added in the correct proportion and well mixed until the whole heap is of a uniform colour. Water can then be added slowly while mixing to ensure that the mortar is not made too wet.

Vertical joints are filled first and then the bed joints are filled. Ideally, the ragged edges of the mortar are trimmed off with a tool called a Frenchman. This is simply a thin blade with its end bent at right-angles and filed to a triangle point. Traditionally it is made of an old table knife, heated so that the blade can be bent at the end. A straight edge is used to guide the tool and it is packed off the brickwork with cork or wood so that the surplus mortar will drop clear of the wall.

Window and door frames are positioned in the wall when the correct sill height has been reached. The actual brick or concrete sill can be constructed later, in which case the frame is packed up to its required position and is then built into the wall. Metal ties are screwed to the sides of the window and door frames and their split ends are built into the bed joints of the brickwork. A vertical damp proof course must be built into the wall where the inner brickwork is returned to close the cavity at door, window or other openings. In exposed positions, the straight joint between the brickwork and the woodwork can be protected from weather by setting the wooden frame in a reveal. No wooden frame should carry any of the weight of the wall above the opening, so a lintel must be inserted. This must have a minimum of 150 mm bearing at each side of the opening.

It can be made of metal or concrete. Concrete lintels are heavy and large spans have to be cast in situ. The metal lintel is most suitable for domestic work and for amateurs, because of its light-weight and because the size and shape are designed to suit the span of the opening.

Where appearance is of no importance, simple square lintels which show on the face of the brickwork can be used. In order to improve the appearance of the opening a boot lintel can be used. This has an L-shaped section, the back being as wide as the inner leaf of the wall, 100 mm and a reinforced toe at the front takes the outer brickwork and shows only about 75 mm at the face. If the lintel is kept back behind the face of the brickwork about 25 mm then it will hardly be seen. The effective load that the lintel is considered to carry is that contained in a 60 degree triangle above it. This, of course, includes any floor joists that may be bearing on the wall in that triangle.

Bricklaying can be practised in the garden where the work does not have to be perfect; in fact, the charm of ornamental garden brickwork lies in its imperfections. The uneven courses, chipped bricks and crooked lines have a rustic charm that makes the brickwork seem to belong where it is, especially when moss and algae have become established. This kind of brickwork still needs to be protected against the weather and the inside of a brick planter or a retaining wall should be coated with bitumastic or similar waterproof material to prevent it from becoming, and staying, saturated with water. Open vertical joints must also be left in the lower courses to allow excess water to escape.

The principles for building a coal bunker are the same as those used for building a house; the corners are built up first and the middle of the wall is filled in to a line. The main difference is that the coal bunker does not need to have strip foundations, it can be built up from the concrete floor slab.

A larger structure such as a garden shed should have strip foundations, but it could be built up from the concrete slab if the edges were thickened to take the extra weight.