There are very often issues with cracking at the heads of openings, which are caused by several factors.
1. The wall panel above the head of a lower opening, and sill of a higher one, or at the top of a wall at roof level, represent a point of least cross sectional area in a wall, where any movement stresses tend to concentrate as they try to flow around an opening. This increases the chance of a tension crack forming at these points in a wall.
2. If a metal lintel has been used, this will have a significantly different response to ambient temperature changes than the masonry surrounding it. not normally a problem with shorter lintels / openings, but can cause
differential movement cracking at the jambs/reveals on wider door / window openings.
Where perforated base plate lintels are used a slip plane, under one end only of the lintel is now often used to prevent differential thermal movement of the lintel from `dragging' at one or other of the reveals.
3. If a box lintel was used, the back is covered with a metal mesh to pick up renders / plasters, (sometimes with a timber batten trapped behind it, to provide a fixing point for curtain rods etc) which may result in a hollow area occurring behind the finish. this is sometime broken into when fixing curtain rails etc.
Given the cracking pattern. it may be best to cut back the finish to enable the lintel to be revealed / checked, and if all is well with it, use a render mesh over the lintel / masonry interface (which may require doing this to the entire lintel areas) so that the finish is reinforced over the points where
the two materials (metal and masonry) meet.
