There is no user manual for all aspects of homeownership, from moving, to taking occupation and to maintaining and understanding common and uncommon defects. Albert van Wyk has more than 38 years’ worth of building experience and has put all he has learned into a concise, easy-to-use reference book entitled, The Proud Home Owner. He has granted HomeTimes exclusive access to republish portions of his book to help homeowners make better decisions around buying and selling, as well as maintaining their properties.
New structures are designed to carry their own weight and imposed loads, so that strains are kept within reasonable limits. Safety factors are included to cater for variations in quality of materials, and construction inaccuracies, and random or accidental forces.
The total weight of the building will gradually exert pressure on the footing and on the soil as construction is taking place. Every building will settle, but if the soil is stable, the concrete footing is strong and the brick work is properly done, then the cracks will be limited.
Flexible historic buildings are often better able to cope with movement than modern rigid structures, thanks to the soft lime mortar which was used. Modern structures with slender walls set in hard cement mortar with brittle plaster are more vulnerable to cracks.
As little as 5% clay in soil can be enough to cause damage to the foundation. It is difficult to imagine the force which the soil can develop. I have read of a figure of 73,000kg/m² which the heaving clay can lift.
The removal of trees can also cause ground heave. That is why trees which are too close to the property should be taken down in stages, very slowly and over a number of years, to allow for the gradual stabilisation of the moisture in the soil. It will not necessarily crack during the first year, but the wet and dry conditions which will follow will cause it to crack.
You should try and control the amount of water next to the building. This is in order to keep the moisture level as far as possible the same. In other words, try to stabilise the moisture content in the soil. Do not make a garden directly next to the house, but construct a 1m wide apron around the house to keep the water away from the building.
I only refer to cracks in surface beds, first floor slabs and roof slabs in houses. Plastic shrinkage cracks appear in the surface of fresh concrete as it dries, but not in an uniform pattern. It is likely to occur when high evaporation rates (wind and sun) cause the concrete surface to dry out before it has set.
The surface is exposed to heat and wind, both of which encourage evaporation on the surface. The resistance of the main body of the concrete to shrinkage, and the surface layer, creates stresses between them. This is relieved by the cracking in the surface layer.
Shrinkage cracks are very common. It is not a structural concern as they are not continuous, and interruptions along the crack line are visible. They are usually parallel to each other on the order of 300mm to 900mm apart, relatively shallow, and generally do not intersect the perimeter of the slab. Plastic shrinkage cracks are unsightly but rarely impair the strength or durability of concrete floors or roofs.
The development of these cracks can be minimised if appropriate measures are taken prior to and during placing and curing of the concrete. Keep it wet as long as possible to slow down the drying process.
The difference between cracks and gaps is sometimes confusing. Cracks are breaks in materials like bricks, mortar, plaster and concrete. Gaps take place in designed construction joints where it was designed to compensate for anticipated movement or expansion.
Expansion joints, also referred to as control joints, allow segments to move independently of each other while retaining the integrity of the structure. Bricks cannot be tied into a concrete beam. It is constructed with a gap and a joint in the plaster which is then filled with a flexible sealant.
When a crack is visible in a joint it is often mistaken as a structural crack, but it is only the filling material which has cracked and not the wall or bricks.
A boundary wall cannot be constructed as one long wall because it will crack in certain places, due to the different soil and moisture conditions below and expansion and contraction of the material. The wall is constructed in different independent panels with gaps in between to allow them to move, so that no cracks will occur.
Albert says: “Never tie a new brick wall into an old wall”
The joints should be filled with polyurethane foam, and a waterproof silicone on top to prevent water ingress, and not a hard filler product or plaster.
Another place where gaps occur is between the cornice and the ceiling and the wall. This is due to the shrinkage of the cornice and the ceiling board over a period of time. The moulded polystyrene cornices are glued to the wall and the ceiling, and the shrinkage can cause the de-bonding. Ask the painter to fill these gaps with an acrylic silicone before the painting is done. The cornice is to be nailed only to the ceiling and not to the wall, to accommodate future movement. Remember that the ceiling is fixed to the roof trusses which are moved by the wind.
Cracks can develop where extensions were done. Never tie a new brick wall into an old wall; create an expansion joint to accommodate the movement. The old building has settled and the soil conditions around it have now also been disturbed. In time the new section will settle.
Cracks can form about 100mm to 150mm in the top corners of window frames. These are plaster cracks at the ends of too short lintels. It is not a structural crack because the window frame is to some extent supporting the lintels and the wall.
Next time: How to repair cracks