One of the most common problems associated with bricks and brickwork is that of ‘spalling’ or ‘spelching’, long after construction has been completed. This may be caused by frost damage – water in the brick freezes and then expands, and this can force the face off the brick (see Fig 267). A less common cause is crypto-efflorescence, but the net effect is the same. This kind of damage is most prevalent in brickwork below DPC level, on very exposed walls or on free-standing walls where rainwater is not thrown clear by the coping and simply runs down the wall face.
Fig. 267 Perished and spalled brickwork.
As part of the maintenance of external walls, it may be necessary to cut out and replace individual bricks or small areas of brickwork that have suffered spalling. The removal of damaged bricks is usually achieved by chopping out the mortar joints that surround them using a lump hammer and a sharp plugging chisel. This process is made much quicker and easier, particularly on cavity walls, by drilling holes through the mortar joints at frequent intervals with an 8mm masonry drill bit. Damage to surrounding bricks should be avoided as far as possible, which is why it is not a good idea to use anything bigger than an 8mm bit or a heavy-handed tool such as a cold chisel.
Replacement bricks should, obviously, match the existing brickwork and the new mortar should be of the same mix as the original. Any significant difference in the mix proportions between new and old mortar could result in differential movement and cracking. From an aesthetic point of view it should be noted that an invisible repair will be impossible due to introducing new materials into a wall that has weathered and, therefore, changed colour somewhat over time.
Having removed the old bricks and mortar, ensure that all the dust and debris has been removed and dampen the area with water to ensure good adhesion for the new mortar.
When replacing an individual brick, or indeed the last brick in a group, it can be difficult to achieve a tight fit and a compact and full mortar joint on top of that brick. This is most significant when repairing a cavity wall since there is nothing but air behind the brick against which to ‘pin’ the new mortar for the top joint, and pointing mortar simply falls into the cavity. To overcome the problem, it is necessary, having positioned the last brick, to ‘caulk up’ the top joint with pieces of slate. This ensures a strong, tight fit and allows the joint to be pointed up easily. At all times, care must be taken to ensure that no debris falls into the cavity.
The situation is much easier when replacing bricks at the front of a solid wall that is one brick thick or more. In order to achieve a tight joint at the top of the new or last brick, it is necessary to excessively butter the back of the brick with mortar and to force the brick into the opening. This is best achieved with a wooden or rubber mallet that will not damage the face of the brick. As the brick is forced in, the mortar at the back gets squeezed forward over the top of the brick – the mortar needs to have a high degree of workability for this to be effective. A tight joint should then be achievable with some final pointing up, without the need for any additional caulking up with slate.
For free-standing walls, there is a second option for replacing damaged brickwork. It may be economically viable partially to take down the wall to the point where the defective brick/s occur, utilizing the principle of racking back, to renew the defective brick/s and then to rebuild the wall back up. A value judgement would need to be made on a case-by-case basis as to whether such a course of action is practical, since the lower down the wall the defective bricks are located, the more significant the amount of brickwork that needs to be taken down. The time and cost of rebuilding must also be taken into consideration. If any original bricks are to be reused, they must be cleaned of old mortar (‘dressed’), which will demand more time, and will further impact on any decision relating to viability.