Wall Base Plinths – Are they a Good Idea?
Are Wall Base Plinths A Good Idea?
Wall base plinths are generally poorly understood and I attribute this to the confusion caused by a damp proofing industry fad for installing cementitious wall base plinths in the 50’s, 60’s and 70’s. It was a perceived treatment for wall base damp long before retrofit chemical injection became heavily marketed. The basic idea for installing brick wall base plinths, as seen in many thousands of old properties, was a good one, but a whole damp remediation industry ran with a sound principle and because they didn’t understand it, they ruined it. Original brick plinths generally preceded the widespread installation of a physical damp proof course, though you will occasionally find both an original brick wall base plinth and a physical damp proof course, usually of slate. Generally speaking, our experience has been that if you see an original brick wall base plinth then there won’t be a physical DPC installed.
The thicker wall base gave extra protection against rainsplash and penetrating damp and generally acted as a larger buffer for damp. If you read my last blog then you may remember that I talked about buildings being built on the ‘overcoat’ principle. Well this is precisely how original wall base plinths work, they are generally constructed with lime mortar and so long as breathability is maintained then this thicker ‘buffer’ zone at wall base does a great job of protecting against wall base damp. Unfortunately a remedial damp proofing industry latched onto this idea and started to install retrofit cementitious plinths in the mistaken belief that these too would help protect against wall base damp, which couldn’t be further from the truth. Anecdotally, retrofit cementitious plinths are said to help ‘shield’ the wall base from rainsplash but that function is outweighed by virtue of the fact that they shield against moisture evaporation from wall base.
When it comes to protecting against wall base damp, wall base ventilation and subsequent moisture evaporation at wall base are absolutely critical. If a retrofit cementitious wall base plinth is installed then this critical ability is lost, moreover we often see retrofit plinths that have actually bridged an existing physical damp proof course so now you have double trouble, a bridged DPC and zero wall base ventilation.
Occasionally we also see heavily spalled brickwork to original brick plinths, which results in a render coat being applied to form a new cementitious plinth over the top of the original brick plinth, an action that will most likely lead to severe wall base damp, as in the images below.
To round up, if you see a retrofit cementitious plinth installed then you should have it removed. If an original brick plinth is installed then you should respect the importance of this technical detail and maintain its breathability by keeping the brickwork bare and only repoint using breathable lime mortar.