. EXPANSION JOINTS IN LONG BOUNDARY WALLS

Masonry structures expand, contract, lengthen and shorten due to changes in temperature, moisture content of the ground and relative humidity of the surrounding air. As a rule of thumb, brick walling expands and contracts by approximately 1mm in every 1m length of walling so the
longer a wall is, the greater the extent of the movement overall. This is less of a problem for most domestic construction but is more significant on very long free-standing walls, which can be many metres in length.

Provision needs to be made in such walls to allow for this structural movement, usually in the form of an expansion joint (see Fig 244), which effectively acts as a ‘separating point’ between two adjacent lengths of walling. A continuous, straight, vertical 12mm joint is introduced, and filled with a strip of semi-rigid fibre-board, instead of continuing the bonding arrangement through. It is good practice to introduce an expansion joint for every 6m length of walling. The width of the expansion joint, in this instance 12mm, is derived from two adjacent walls of 6m both having the capacity to

expand/contract 6mm each (in other words, 1mm for every linear metre). Expansion joints are usually only installed above DPC level, and not below, but must continue through any coping at the top of the wall.

. EXPANSION JOINTS IN LONG BOUNDARY WALLS

Fig. 244 Expansion joint with slip tie in situ.

Ancon Building Products

Structural stability across expansion joints is provided by special metal de-bonded wall ties, placed every three courses. These are known as ‘slip ties’, since they tie the two sections of walling together across the expansion joint, but one end of the tie is fitted with a moveable plastic sleeve that acts as a ‘de-bonding element’ between the mortar joint and the metal tie. This allows the walling to expand and contract – the tie is rigidly fixed at one end but at the other only the plastic sleeve and not the metal part of the tie is gripped by the mortar bed joint. The plastic part of the tie remains static while the metal section ‘slips’ within it during expansion and contraction.

. EXPANSION JOINTS IN LONG BOUNDARY WALLS

Fig. 245 Expansion joint slip tie. Ancon Building Products

Despite the presence of expansion joints effectively dividing the wall into separate sections, the wall is still constructed as if it were one continuous length, with the two extreme ends set-up and the middle run-in to a string-line. As the wall is run-in, provision must be made to allow for the inclusion of the vertical expansion joint as the work proceeds and great care must be taken to ensure that the expansion joint is truly vertical, or plumb. The most accurate method is to ‘build in’ a straight 1200mm long timber lath at the front of the expansion joint (see Fig 246), which acts as a

profile to form an open expansion joint. The lath must be equal in thickness to the width of the expansion joint but not the full depth of the brick skin, as this would prevent the installation of the slip-ties across the expansion joint. The timber lath should be positioned vertically and, initially, braced in position. (This temporary bracing should be set behind the brickwork so that it does not interfere with running-in the wall to a string-line.) As the wall increases in height to half-way up the lath, the temporary bracing can be removed, as the brickwork either side will hold the lath in place. The lath should be periodically checked for plumb to maintain the accuracy of the expansion joint. When the brickwork reaches the top of the lath, the lath is raised vertically by about half its length so that it remains plumb and gripped in position. To make it easier to raise the lath and, eventually, to remove it completely, it is a good idea to tap it on completion of every four or so courses to loosen the mortar bond inside the expansion joint.

. EXPANSION JOINTS IN LONG BOUNDARY WALLS

Fig. 246 Method of forming an open expansion joint.

Once the wall has been completed, the timber lath is removed and the open expansion joint is thoroughly cleared of any mortar droppings and debris. The fibre-board strip is then inserted into the open joint. Slits will need to be made where slip-ties occur or separate short lengths of board could be used that fit vertically in between the ties. The fibre-board that is used to fill the expansion joint during construction is not at all weather-resistant and will rot away over time. To protect it and also to provide a neat finish to the expansion joint, the exposed edge is completely sealed with flexible mastic (in a colour to tone or match with the brickwork). This means that the fibre-board needs to be set back 5-10mm from the face of the wall as it is installed. Strips of polyethylene offer a weather-resistant alternative to fibre-board, although the joint will still need to be sealed with mastic, as an aesthetic consideration.

. EXPANSION JOINTS IN LONG BOUNDARY WALLS

Fig. 247 Finished expansion joint sealed with mastic.