What Is An Expansion Joint
Expansion joints in render are generally cut into the render to a depth of 5mm – 10mm into the below brick or block work. The joints are created to ensure that if any movement or cracking occours it will likley happen in said joint. Many expansion joints that we look at today are either done poorly or not done at all. When assessing a rendered area its important to look at the block works exisiting cracking issues and remedy with the use of proper expansion joint practice.
Once the need for an expansion joint has been identified we need to then look at the solution we are going to put in place.
Some areas require heavy cutting and back blocking with flexable foam and sealant as the need for the expansion joint is not for the allowance of surface cracking but rather movement of the actual block work, this is what we call block work expansion joints. Block work expnasion joints are litterly the space given between to seperate block walls which join together as some point. The issue with these kinds of joints is that when they are not done correctly and simply rendered over the top of the movement in the block work will crack the render and result in the photo below.
By cutting out a weak point in the render along a known “fault line” we can then take measures to ensure the cracking wont continue to aid in the damage of the exisiting masonary.
You see the real issue is not the cracking but the moisture that enters the crack once it appears. By allowing moisture to consitantly enter cracking joints it will lead to further cracking of the surrounding render and possibly concrete cancer which is caused by moisture which develops rust to the steel reinfocingments within the concrete. Hard to believe all this could be caused by a simple cracked joint right? The issue of expansion joints and proper protective coatings to masonary become even more important when near oceans which have the thing that causes rust, salt!
What Is Mastic & How Is It Important To Expansion Joints In Render Houses
Mastic is a super sticky rubber like sealant which has extremely impressive +\- expansion tolerance, which is ideal for the issue we have with expansion joints. It is also used to create a water tight seal around an expansion joint or known “faul line”. The thought process is that once the masonary issues have been corrected and a nice neat joint line has been cut, the mastic sealant will be applied to the joint. This is done so if the masonary joint does decide to crack in the future ( which it will ) then it will crack BEHIND the mastic flexiable sealant! The mastic will continue to expand and contract with the movement of the block work but due to its water tight seal it prevents water from entering the joint and exasperating the issue.