Blockwork partitioning is usually used only on a solid base such as a solid concrete ground floor. If the blockwork is to be built on a timber floor, it is advisable to check and see if it runs in the same direction as the floor joists. In this case, the joists would need strengthening in view of the excessive “point load” put upon them. If the joists run at right angles to the partition, it is normally safe to erect, say 3 in (7.5 cm) thick lightweight non-loadbearing blocks.
Blocks come in many different materials, shapes and sizes. For normal non-loadbearing lightweight partitions 3 in. thick by 1 ft 6 in. long by 9 in. high (7.5 by 46 by 23 cm), block is the most common. Lightweight cellular concrete blocks, which are load-bearing, can also be used, in place of aggregate blocks.
Slots. Before starting to build the partition, mark out a line on the floor vertically on the abutting wall. If the existing wall is of brick or blockwork, knock out 4 in. by 9 in. (say 10 by 23 cm) vertical slots in the wall about 3 in. (7.5 cm) deep. For a room of about 8 ft (or 2.44 m) high, these slots are sufficient to allow for the keying in of the new block wall and also to reduce drying shrinkage and subsequent cracking in the plaster. It is possible to knock 4 in. (or 10 cm) cut nails into the existing wall and build them into the horizontal joints of the new wall so that they form a tie. Butterfly brick ties may also, and preferably, be used in place of nails.
1 Bed the blocks in a 1:1 (lime): 6 mix prepared in small quantities, since this should not be re-mixed after it has begun to set.
2 Apply the mortar to the blocks using a bricklayer’s steel trowel; the horizontal and vertical joints should be as near 10 mm thick as possible. It is a good idea to use a gauging rod — a piece of timber marked off in block course heights — so that a constant thickness of joints can be achieved.
3 Make sure the vertical joints are staggered, or adequate bonding will not be achieved.
4 Use a builder’s spirit level and straight edge to check that the blockwork is straight and level at each course.
5 Once in position, the members can be nailed to avoid any slipping. If, because of the spacing of the existing joists, a larger hole than needed exists, you can insert another member between the trimming members to bring the hole down to the required size.
6 Put back and trim the floorboards around the hole.
7 Fix, around the ceiling edge, an expanded metal lathing with a corner stop to the underside of the joists and plaster level with the ceiling.
8 The supporting members can now be taken carefully away, avoiding sudden movements.