Install an angle iron

At a lintel (the top of an opening), install an angle iron for support. If possible, plan the installation so you can set the angle iron on top of a block at each side of the opening.

If that is not possible, use a circular saw equipped with a masonry blade to cut a channel that you can slide the lintel into. Mark and cut carefully so the angle iron will be level. The masonry saw blade makes a cut that is just thick enough for the angle iron.