We have had a couple of projects recently, where clients have requested parapet walls instead of conventional eaves. Parapets are less popular these days, this could possibly be because they can be a maintenance problem if they are not detailed correctly. If you are unsure as to what a parapet looks like, then try to bring to mind the traditional Georgian façade, and you may recall the stone capping at the top of the wall (which is called a coping) instead of the usual gutter and painted eaves detail found on almost all modern houses.
It became a very popular detail for use with 18th century grand classical buildings because it emulated the Greek temple, but it also helped to emphasise the height of the building, both of these aspects contribute to giving the architecture a sense of grandeur.Often the façade would sport stone columns and a large portico too.
The parapet on these old buildings can give rise to a big headache for the owners, since the lead gutters hidden from view behind the parapet will often have deteriorated, therefore allowing water into the building fabric. The parapet itself may also be responsible for allowing water in as well, even more recently built parapets can cause major problems, and in many ways this is unforgivable, given that we have the benefit of better materials and a better perspective, which should make visible the past failures of this type.
The main causes of parapet failures are usually water ingress, and insufficient structural stability. The structural stability aspect is pretty obvious in that if a parapet wall extends up too far beyond any other structure giving sufficient lateral stability, it will probably blow over in a high wind (which there seems to be a lot of recently here in the UK). Garden walls, even though they are not a parapet wall, are a perfect example of this, and are often too high for their thickness.
So if a parapet wall is thick enough, or designed to combat overturning in the wind, then you are heading in the right direction. What you need to do is to pay attention to the detailing to ensure water cannot get in. The watertightness will depend on an adequate coping, and a front and rear surfae which will not allow water to penetrate in a way that will damage the structure. Modern bricks are perfectly able to withstand the effects of water and freezing temperatures, unlike many bricks made in past centuries, but driving rain must not be allowed to enter a cavity beyond the facing brick unless there is a cavity tray and a weep hole to allow the rain to drain back out. Before cavity walls, rain would sometimes attack the parapet wall from both sides and collect in the middle of this exposed piece of masonry, which would then freeze when conditions were bad enough, causing the water to expand and gradually push the structure apart.
As new cracks and joints form, the structure is severely weakened and over a period of time (sometimes years) this can lead to a dangerous situation with masonry falling from a great height onto public and populated spaces.
Parapets should be inspected from time to time to check for such failures. At the start of the design, it is a good idea to implement means of being able to get to the roof to check and view such a part of the building for any faults.
Written by Tony Keller – Building Tectonics Ltd.