Inserting the support
The method you use when making a hole in a wall to bend the support into position depends on whether you are using a lintel or a rolled steel joist.
Lintels: These must be lifted and twisted, slid or fed carefully into position. Because of the weight of the lintel and the need to position it accurately, it is essential to have several people to help you do this.
Lay a bed of fairly dry 1:3 mortar on all parts of the lintel that are likely to come into contact with the wall above the opening before you slide it into its final resting place. Before you go any further, make sure that the lintel lies exactly as specified in the plans.
The next step is to cut away more holes underneath the lintel and insert two or more needles – with props – so that you can jack it up against the wall above. Pack any gaps above the lintel with mortar before you do this, and make sure that the lintel remains horizontal at all times.
Then, with the lintel forced upwards by the needles, pack the gaps underneath both supporting ends with pieces of slate tapped firmly into place with a hammer. Finally, point all around the cracks between the lintel and brickwork with mortar ready for re-plastering.
Rolled steel joists: These are not bedded in cement but instead set on pads’ of natural stone or pre-cast concrete. Pads are available at most large builder’s merchants and are usually sold along with the joist. Depending on the size of the joist, they should be at least 50-70mm thick and long and wide enough to guarantee good support at both ends. The pads must rest on a firm, level base and this may involve renewing or adding to the brickwork piers which support them.
Use a trowel to spread an even bed of 1:3 mortar below the pads and leave this to set for at least 48 hours so that there is no risk of movement. Then lift the joist into place with the help of your assistants and position it accurately. Gaps around the outside of the joist should be packed with slate and pointed in a similar way to the pre-stressed lintel.
Removing the props
Do not be in a hurry to remove the temporary supports they are best left where they are for at least another two days so that the bedding around the lintel or joist can harden fully. When you do remove them, bring down the weight on the support gradually by loosening off the props a little at time. And while you are taking the props away, look carefully at the area around the top of the support to make sure that it is not suffering from an undue stress.
Finishing a rolled steel joist
Because of their shape, rolled steel joists do not look particularly attractive when left uncovered. In most cases the easiest way to improve their appearance is by covering them with a shell of plasterboard.
In order to do this, you need to make up wooden noggins to act as supports for the board. Cut these fractionally longer than the distance between the upper and lower lips of the joist then tap them in place at 300mm intervals.
Most joints protrude slightly from the wall surface, which makes it difficult to cover them neatly. But with care, a piece of plasterboard car be made to bend around the three protruding sides.