4 ways to stop walls cracking under pressure
Movement in buildings is inevitable. As such it is essential to allow for these subtle changes during the construction stage to prevent unnecessary problems later.
Particular attention should be paid when designing masonry walls to include provision for movement. If movement in masonry walls is not taken into account it could lead to unwanted bowing and/or cracking in the building because it is not free to expand or contract.
What measures need to be taken to prevent these problems from happening?
1. Building in of movement joints
These should be included in the masonry to allow for expansion and/or contraction caused by changes in temperature and the moisture characteristics of the masonry units. These joints should be built-in as construction proceeds.
2. Designing and positioning slip planes
Slip planes enable elements of the construction to slide in relation to each other to help reduce shear stresses in the adjacent materials. The design and positioning of movement joints and slip planes should be carefully considered to ensure they do not impair the stability of the wall or any of its functions.
3. Prevention of water penetration on external walls
When using movement joints and slip planes on external walls it is important that they are sealed and protected properly to prevent water penetration.
4. Effective fixings and services
Make sure that fixings and services do not interfere with the performance of the movement joints or slip planes. Finishes should be discontinuous at the joints and slip planes and any fixings and fittings should not tie across the joints.
How to decide on the size of movement joints?
Unrestrained or lightly restrained unreinforced walls generally expand 1mm/m during the life of a building, due to thermal and moisture movement changes.
When it comes to the spacing and width of movement joints there is no hard and fast rule because there are a number of influential factors. These include the compressibility of fillers and the performance of the sealant, not to mention different compositions of masonry.
However as a general guide, the width of the joint in millimetres should be about 30% more than the numerical value of the distance between joints in metres. For example, movement joints at 12m centres should be a minimum of 15mm wide.
It is important to note that spacing between movement joints should never exceed 15m in unreinforced walls or cracking will occur due to thermal contraction.
In external walls containing openings, movement joints may need to be provided at more frequent intervals or the masonry above and below the opening may need to be reinforced to restrain movement. Particular attention should be paid to long low horizontal panels of masonry, e.g. those under windows.
- Issue 7 of our Technical Newsletter update contains an article on using deflection heads in reinforced concrete frame buildings
For more information on the requirements for different types of masonry please refer to the LABC Warranty technical manual.
By Anna Symington