This article explains how to build a block wall (brick, or other) at right angles to an existing wall. Start by planning the wall – after you have checked that the floor you are laying the wall on is solid enough to support the weight of your new wall, check that there are no pipes or cables you can damage in the existing wall, at the point where the new wall will join it. Mark out the position of the new wall, and attach wall profiles. These metal brackets need to be exactly vertical (check with a spirit level) and the correct height (use a hacksaw to shorten).

Now you should attach a taut string line to each wall profile at floor level. The new wall will be built along this line. Measure and mark the position of the doorway if you plan to have one in the new wall. Remove the string line and tie it to two blocks, placed next to the wall profiles. We will use this line to lay the first row evenly. Use a trowel to butter one end of the first block with mortar, and then lay a bed of mortar the same length as a block on the floor where the line was. Press the buttered end of the block tight to the wall profile, and tap it in to compress the mortar. It should sit evenly against the string, and the mortar should be about 1cm thick. Remove any excess mortar using the sharp edge of the trowel, and check that the block is level using a spirit level.

Repeat this process until you reach the doorway, or if you are not installing a doorway until you reach about half way along. Start the process again from the other end of the wall, as we want to lay all of the whole blocks before measuring for any cut blocks needed to finish off. To cut a block you should lay it on some sand, then use a bolster chisel and club hammer to etch out the line where you want it to break, on all four surfaces of the block. Then a stronger blow will cause it to break along this line. Butter the cut end of the block to finish off (or both ends if there is no doorway).

Raise up the string line by sitting it on top of the first course of blocks, and start at the doorway end this time (or the middle if no doorway). You want to stagger the blocks, so that the join sits in the middle of the block on the course above. Repeat this process, remembering that when you are five courses high you need to leave the mortar to dry before continuing. The top of the doorway is finished using a lintel, which may require you to notch the top of the last whole row of blocks. Cut down blocks may be needed to complete the very top row, as the gap may be too small for another whole row.

If you are looking for flats to rent in London Bridge check out my site. It also has information on local attractions like London Bridge in the area.