There are two methods you can use for blocking off a redundant doorway: you can fit a wood framework around the inside and panel it with wallboard on both sides, or you can use bricks or lightweight building blocks if the floor is solid. In each case, plaster is used to finish it off.
Alternatively, consider whether you can put it to some other use such as paneling in the back and filling the recess with shelves.
Removing the old frame
To remove the old door and frame, unscrew the door hinges, lift the door away and lever off the molding. The lining frame may be fixed by masonry nails, screws or metal ties cemented into the brickwork. You should be able to lever it free, but if not, cut through its fixings by working a saw blade between the back of the frame and the wall.
Paneling with wallboard
If you are filling an opening in a stud partition, the supporting framework should be made of wood to match the framework of the partition — usually 2 x 3 or 4in lumber. On the other hand, if paneling a masonry wall, you might need two separate frames of something like 2in sq wood to panel each side flush.
The frame should comprise a head plate, sole plate, two upright studs and a central brace. Toe-nail these together and to the insides of the opening, making sure the frame is set back from the face of the wall to allow for the thickness of the skimmed wallboard.
Nail a panel of wallboard to each side of the frame, and to prevent the skim coat shrinking back from the edges, nail lengths of metal lath around the join. Apply a skim coat of finish and when the plaster has hardened, fit a length of new baseboard across the opening.
To fill the opening with bricks or concrete blocks, you must tie the new masonry to the old. The easiest method is to hammer 6in masonry nails half-way into the side of the opening to correspond with the mortar joints of alternate courses of the new bricks which will eventually be buried in the mortar.
Lay the bricks or blocks in the normal overlapping fashion and point all the mortar joints flush with the face of the masonry when finished.
Finally, apply floating and finish coats of plaster, using the surrounding original plaster as a thickness guide.
Tags: alternate courses, baseboard, bricks, brickwork, building blocks, concrete blocks, fixings, masonry wall, metal lath, molding, mortar joints, old frame, plaster, recess, saw blade, sole plate, toe nail, wall paneling, wallboard, wood framework