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Looking at what lies beneath to find the cause for the crack reveals two coliding walls.

Looking at what lies beneath to find the cause for the crack reveals two colliding walls.


What to consider when building a new wall onto an old wall


1) Foundation: The new wall must sit on the same level foundation as the old wall.

“Where no foundations exist the new wall will not be on solid footings and will sag or crack. A single wall no higher than 1.8 metres requires a 300mm deep x 300mm wide footing, while a double wall, or a wall higher than 1.8 metres should be 600mm deep x 300mm wide.”

“The success of a join in the wall without any cracking still lies in the foundation, however. Any movement of the foundation will result in a crack in the wall. Proper compaction of the new area before digging foundations is important. Alternatively, dig down to solid ground before laying the foundation. There is no short way to do it properly.”


2) Cavity: Any existing cavity must be maintained and continued.

“Before adding a new wall to an old one you must check with your building inspector to see if a vertical damp proof course is required between walls, and whether or not any existing cavity in the walls of the existing construction, needs to be continued into the new construction. Building regulations must be upheld at all times for your own, as well as any future owners, safety.

When tying new extension walls to an existing building it is important that the cavity is maintained. This means cutting into the existing walls to continue the cavity around the building.”


3) Joint: The two walls need a joint between them. There are different types, but basically a joint is a gap between the two building parts.

  • Control Joint

“Most materials experience small changes in dimensions, due to temperature changes, moisture changes, sometimes long-term chemical changes, and loading. Dimensional changes by themselves do not necessarily cause problems, but if the movement is restrained by contact with another part of the construction which is unaffected, or behaves differently, it can result in cracking or overstressing of some elements, and possibly in structural failure.

The usual way of combating relative movements is to provide control joints, which are capable of opening or closing to a certain extent while continuing to provide the structural and enclosure functions of the element.

Movement of the foundations will also cause relative movement of parts of the construction, and is sometimes compensated by the provision of control joints, but this is a different type of movement and its magnitude is more difficult to predict. This is known as articulated masonry, and can be useful for constructing small buildings on relatively unstable sites.

Control joints are by definition a discontinuity in the wall, and thus they reduce the amount of support given to one part of the wall by the remainder of it, or by the building’s frame. In many cases it will be necessary to use sliding wall-ties to transmit some support across the joints.

The joints also must be sealed to maintain the integrity of weatherproofness, acoustic and fire isolation.”

  • Expansion Joint

“Unlike control joints, expansion joints are left completely free of mortar. They are filled with an electromatic sealant to keep them free from water.”

“Brick is the smallest dimension it will be in its long service life when it leaves the kiln. As it is exposed to moisture from a variety of sources including the air, wet mortar, rain and condensation, it will naturally expand since it is a clay product. Temperature will also cause brick to expand and contract. Consequently, it is important to incorporate expansion joints into brickwork to accommodate this movement. Expansion joints should be located where stresses or cracks are likely to develop in brickwork. Prime candidates for expansion joints include long expanses of walls, corners, offsets, setbacks, and parapets.”

  • Slip Joint

“Slip joints are designed to take movement on a load bearing structure such as corbel/slab and brick interfaces where a low friction sliding interface is required. They ensure that the load transfer is correctly through the centre of the horizontal joint thus eliminating any chance of fretting at the edge due to the rotation of the slab. Live load deflection of the slab by means of settlement of adjacent columns/walls and piers is also diminished. Applied in a continuous length they are ideal for both reinforced and post-tension slabs in car parks, shopping centers, airports, hotels and recording studios.”

  • Toothing

“Toothing-in or Toothing-out involves hacking away every other brick in the main building at the point you want to join the extension wall to and then make a seamless connection from the main building with the house extension. For this to be possible, the builder needs to build with the exact brick size and for the bricks to be perfectly aligned with the existing building when constructing the extension.”

“Toothing of the masonry is not permitted in many architectural specifications. Why does toothing provide less strength than raking or stepping back the masonry wall?

Toothing is not as strong because of the difficulty involved in properly filling and compacting the mortar for the full depth of the head and bed joints. Much of the mortar at the tooth portion of the wall must be installed by pointing the joints, and it is difficult to point the mortar in the back portion of the joints. As a result, these tooth joints are often poorly filled, and as a result, create a weak plane within the wall that is susceptible to cracking.

Toothing, however, is sometimes necessary when connecting to an existing wall. If the joint cannot be stepped back, providing a vertical expansion joint at such interfaces may be an alternative to toothing.

When toothing must be done, extreme care must be taken to carefully point these joints to ensure that they are completely packed with mortar for the full depth.”


4) Wall Ties: Wall ties strengthen the bond between the two walls.

“Wall ties are used in cavity walls to connect the outer and inner walls, or to connect a new masonry wall to an existing one.”

“Wall ties should be flexible enough to accommodate the relative movement between both leaves of a cavity wall but stiff enough to transmit axial loads. Stainless steel wall ties should be specified.

Additional wall ties should be placed either side of the movement joint at every block course up the length of the joint and within 150mm of the joint.”


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