How to build a safe and stable boundary wall
When I was a small boy growing up on the outskirts of Johannesburg a free-standing wall fell on my late sister and me resulting in broken bones and for my sister a lengthy stay in hospital. Poor foundations, bad design, lack of expansion joints and piers (support pillars) can result in masonry boundary walls developing unsightly and sometimes dangerous cracks.
HouseCheck inspectors therefore carefully check the condition of free-standing boundary walls.
If an existing wall is leaning, unstable, or has significant cracks, then urgent attention is indicated. It is sometimes not cost-effective to repair badly designed and poorly built free=standing boundary walls and in some cases it is cheaper in the long run to tear down the dodgy wall and to start again.
Many boundary walls are poorly built by people trying to cut costs and using bakkie builders with an inadequate budget and skills base.
Often cracks in plaster, walls splitting apart or leaning over only becomes noticeable long after the builder has left the site. When the home owner thinks they have received a wall at a bargain price, the old saying of “you get what you pay for” often applies.
The most important elements to consider when installing a new boundary wall are:
- The design of the wall. Apart from local by-laws as regards height restrictions and set-backs on street corners, sound engineering principles also need to be applied to the design of any free-standing wall. Both the thickness of the wall and the spacing of piers governs the maximum height of the wall. above ground level
- Foundations: Design of the foundations must take into account soil conditions and slopes. Steel reinforcement of the foundations and of the wall itself may be necessary. A general rule of thumb is that the wider and deeper the foundation is, the more stable the support of the wall will be. Again sound engineering design principles should be employed.
- If the boundary wall also acts as a retaining wall and is subject to water and soil thrust, then the foundation footing should extend further under the upper side of the wall. This ensures that the weight of the retained soil further stabilizes the foundation structure.
- Adequate expansion joints (every 3 meters on a straight run) and adequate piers (support pillars). A failure to install proper expansion joints and sufficient piers is possibly to most common error seen in South African brick and block boundary walls – and inevitably results in cracked walls. Any cavities in the piers of free-standing walls (including hollow units) should be filled with concrete.
- Drainage: If the boundary wall is on a slope then adequate weep holes need to be installed to cope with storm water build up. If the wall also serves as a retaining wall sub-soil drainage should also be installed. NHBRC (Part 3. 3:24) “Weep holes to be provided in all retaining walls at a height not exceeding 300 mm above the lower ground level at centres not exceeding 1.5 metres.” Weep holes to be formed using 50 mm plastic pipe covered on the non-exposed end with a geofabric.
- Damp: Boundary walls are normally “free-standing”. This means that these walls do not generally form part of a structure (like a house) where the connected walls are self-bracing. Because free-standing walls are not supported by a structure of other walls, boundary walls are usually built without a damp proof course (DPC) – plastic membrane – in order to improve the bond between the free-standing wall and its foundation. A DPC would break the bond between wall and foundation, resulting in instability. As a result of the lack of a DPC rising damp is often seen low down on plastered and painted boundary walls. One way to avoid unsightly damp on boundary walls is to use harder (less porous) bricks for the lower courses (first 150mm) and to ensure that the plaster does not extend to the level of the soil (otherwise the plaster will act as a wick and allow water from the ground to wick upwards).