EXTENDING THE LENGTH OF EXISTING WALLS
When constructing extensions or making internal alterations, there may be a need to extend the length of an existing wall. There are two recognized methods for this.
Toothing involves cutting out every alternate brick at the stopped-end of a wall in order to build on new brickwork and continue the bonding arrangement through from the existing wall into the new section. For this reason, toothing is favoured as a method for extending walls on facing work, where the bonding arrangement will be visible and must be seen to be a continuation of the original wall.
The depth of the toothed indents cut into the existing wall will depend on the existing bonding arrangement. For quarter-bond walls the depth of the indent will be 56mm (a quarter-brick + 10mm cross-joint); for stretcher bond the depth will be 112mm (a half-brick + 10mm cross-joint).
Fig. 272 Using toothing to extend the length of an existing wall.
Toothed indents should be cut into the wall working from the top down, to avoid any risk of breaking off the projecting tails of the existing bricks. The method of cutting out starts with drilling the mortar joints around the bricks to be removed and is essentially the same as for replacing perished bricks. When all the bricks and mortar have been removed the indents should be brushed out to remove any dust.
Before any new construction starts, it is vital to acquire bricks that are the same as those in the existing wall. In addition, the mortar being used for the new brickwork should match that used in the existing wall, in terms of both strength and colour. The projecting tails of the toothing on the existing wall should be checked for plumb, to establish whether any cutting is needed when joining up the new brickwork to the old – hopefully there will be no deviation. If any is present, it will be small and can be accommodated with slight adjustment to the cross-joints of the new brickwork without the need for cutting. Finally, dampen the toothing with water, to ensure good adhesion for the new mortar.
When building away from toothings, the far end of the new wall should be set up and then the brickwork should be run-in between it and the toothings, using a string-line to ensure horizontal alignment. The string-line will most likely need to be attached to the face of the wall with a pin knocked into the mortar joints. (This will need to be made good with mortar when the work is completed.) Every tie brick (the brick inserted into the toothed indent) on alternate courses must be the first brick laid in that course and never the last. This is vital to provide the opportunity to get a full cross-joint inside the indent and to enable the bed joint above the tie brick to be caulked up solidly with mortar under the projecting tail of the existing brick above. Caulking up solidly at this point is best done using a semi-stiff mortar that can be compacted into the joint without squeezing out. Failure to achieve solid joints where
new brickwork joins to old will result in a weak point within the wall.