Masonry contruction uses blocks that are typically 8″ in height. When installing a door in a masonry or block wall opening, you need to consider the height of the blocks in relation to the height of the door.
6/8 Door Example
A 6/8 door is 80″ tall. If each block is 8″ then the door itself is equivalent to 10 blocks. On top of any door is head jamb, which in the case of a Commercial door frame, is 2″ in height. There will be 6″ between the head jamb and the next block, which will be filled with a lintel. A lintel is a horizontal support of timber, stone, concrete, or steel across the top of a door or window.
A 7/0 door is 84″ tall, which is equivalent to 10.5 blocks. If you use a 2″ head jamb, there will be 2″ remaining. The lintel would need to be installed in line with the next set of blocks which will leave a 2″ gap between the top of the door frame and the lintel.
In comparison, by using a 4″ head, the entire unit would be equivalent to 11 blocks in height. The lintel would be installed above the door unit and would fall in line with the next set of blocks. The 4″ head jamb is important for ensuring the best fit between the door unit and the lintel.