Well it is kind of irrelevant to you but houses built with stone tend to have very uneven walls and you tend to have very deep areas of plaster in order to straighten them out a bit, this is where you normally see wooden lintels also, I had initially assumed by the depth of plaster missing and the colour of it that you had a stone house, sorry for any confusion I may have caused.
The batten idea above seems the best route to take
The batten idea above seems the best route to take