The most important factor to consider when cutting a new door is its location. If the door is at the side of your house to improve access to your garden, you will need to cut through an external wall, which requires a specialist steel cavity wall lintel. When cutting an internal doorway from a new extension into the existing house you can use concrete lintels to support the brick and blockwork around the doorway.
Accurately measure the position of your door; then mark out the required opening -- this will be 20 mm wider and 10 mm higher than the door frame. Use the spirit level to ensure all vertical and horizontal lines are perfect.
Mark out the opening above the door frame for the lintel; your lintel should overlap each side of the door frame by at least 150 mm. Stick gaffer tape along all the pencil lines to make cutting easier when the site get dusty.
Drill through both walls using a long masonry drill bit. Drill the holes to plot the door outline so that you can mark and tape the doorway accurately on the other side of the opening. Ensure the drill is straight and level when carrying out this procedure.
Protect the rest of the house from dust before cutting. Shut any internal doors leading from the room in which you are working and seal them with polythene sheeting and gaffer tape because dust will blow through any gaps. Also cover furniture with dust sheets. Get a friend or family member to follow your cutting with an industrial hoover to reduce mess.
Cut a horizontal slot in the centre of the door just above your top line above the lintel with the disk cutter. The slot must be large enough to accommodate the strong boy support attachment. Fit the support and wind up the shoring prop until tight to take the weight of the wall above the cutting zone. Wear safety goggles, a dust mask and gloves for this procedure.
Continue to cut around the doorway, accurately following the lines you marked out earlier. Push the disk cutter blade as deep into the wall as possible.
Remove any plaster from the wall with the lump hammer and bolster, and then remove just enough bricks to accommodate the lintel.
Install the lintel using the 1 to 3 cement mix; make sure the lintel is perfectly level. Leave it to dry overnight before removing any more bricks from the area below.
Remove the remainder of the bricks using the hammer and bolster; try not to break off lumps that are too big to handle.
Repeat all these steps on the other side of the opening. The inside skin of blockwork may be harder to knock out with the bolster. When you've cut through both walls successfully you can then install the door frame.
Fitting the lintel is a two-man job so recruit a friend, neighbour or family member to help you.
Safety is the most important factor when carrying out structural work on your house; if in doubt, always seek professional advice.