The inherent stability of an opening depends upon the means of spanning over the opening. Lintels, short beams of timber, reinforced concrete or steel are stable because the loading at the supports is vertical. Arches and wedge arches are inherently unstable and may only be used safely if they are adequately buttressed by the walling on either side of the opening and are to some extent loaded, which has the effect of compressing the sectors of the arch together.
This means that an arched opening will be unsafe if it is constructed near a corner of a building. It must be abutted by at least 4 ft. (or 1.2 m) of walling. This rule applies only to the opening nearest to the corner. Arches may be built into arcades supported by quite slender columns or piers, as the thrust from each arch is opposed by that of its neighbour, provided that the arches at either end are properly buttressed.
There is a tendency for walls to arch over openings and usually an arch or lintel effectively supports only a triangle of walling directly over the opening. If a window is located beside a corner, the lintel has to support the entire wall of the building located above the opening.
Cutting openings near a corner is one of the surest ways to encourage structural collapse. No attempt should be made to form such an opening without establishing whether this can be done safely.
Door opening: internal wall
Before starting to make a door opening in an internal wall it is essential to find whether the wall is load-bearing or nonload-bearing. This can be done by checking if there are any floor joists above the proposed door opening which bear on to the wall or if any of the roof timbers rely on the wall for their support.
1 Mark out on the wall the opening size required. The size of the lintel needed to span the opening depends on the load which the wall is carrying and the width of the opening. For a single door opening and openings up to 6 ft 6 in. (or 2 m) a prestressed concrete lintel is suitable. Lintels are obtainable from stockists who can normally recommend a suitable lintel for the situation. Larger openings may need to be calculated by a professional.
2 Knock openings through the wall above the door position to take a piece of wood, minimum 4 in. by 4 in. (or 10 by 10 cm), long enough to project some 18 in. (say 46 cm) on each side of the wall. This is to prop up the wall before the new lintel is installed. The props can either be of an adjustable type which can be hired or consist of lengths of 4 in. by 3 in. (10 by 7.5 cm) or 4 in. by 4 in. (10 by 10 cm) timber. Timber wedges can be used to make the props fit tightly beneath the member that passes through the hole in the wall.
3 After positioning the props, cut the slot and locate the lintel. The area of wall beneath can then be removed. The lintel needed for a 12 in. (or 30 cm) opening is 4 ft (or 1.22 m), allowing a 6 in. (or 15 cm) bearing at each end.
4 Next, 1 in. (2.5 mm) door linings can be fitted, allowing 4 mm tolerance for the width of the door and 10 mm for height. The linings can be fixed to the wall with 3 in. (or 5.5 cm) cut nails, driven through the linings into the mortar joints of the brickwork.
5 Make good around the linings with a 1:3 mix, which can be finished off later with plaster.
6 Fix 11 in. by bin. (or 4 by 1.2 cm) softwood door stops in position into the linings.
7 Fix the architraving.
8 Knot, prime and stop before painting and then hang the door.
Window opening: external wall
Before forming a window opening in an external wall you must submit drawings to the Local Authority, showing a plan section and elevation through the wall with the new window in position. There are various regulations governing the minimum distance a window can be from a boundary, its size and shape and the structural stability of the proposed works.
Standard window joinery can be bought. There is a large range of lintels but the best type to go over a window is a prestressed concrete or galvanized-steel “boot” lintel.
1 Knock holes through the wall above the lintel position, using props in the same way as when making an opening in a load-bearing wall. When cutting through the bricks of the outer skin, make sure that a neat, clean-cut straight line is achieved. Use a 3 in. (or 7.5 cm) bolster and a club hammer.
2 Position the lintel over the new opening If a concrete lintel is used, install a lead cavity tray and make sure that it has equal end bearing at the opening jambs.
3 Before putting the window in position, tack a strip of DPC felt to the side of the frame. This is built into the cavity.
4 Place the window in position and fix with the help of building-in cramps. These are screwed to the side of the frame and built into the brickwork at the jambs. You may need to knock out a few more bricks. The cramps are positioned at 2 ft (or 60 cm) centres.
5 Block off the cavity by sandwiching the DPC felt between the inside skin and outside wall skins. This stops damp from penetrating through the wall.
6 The next job is to glaze the windows. Place the mastic in position on the outside of the window where the jambs meet the brickwork, using a hired mastic gun.
7 Most window stockists hold a range of standard window joinery boards. Cut these to the appropriate length and fix, using masonry nails. Be sure to knock the heads of the nails below the surface of the wood and fill the hole with putty or a cellulose filler before knotting, priming and painting.
8 Plaster around the window opening on the inside.
9 Paint the window frame before fixing the window furniture.