Soft (control) joint in an ashlar masonry wall

In a masonry assembly the masonry units are held together by mortar joints. That is pretty obvious, but what a lot of people don’t know is that in a masonry wall it’s also normal and correct to have some joints wall not be built with mortar but instead be built with a resilient or elastomeric sealant. For example in most residential walls, every 20 or so linear feet around the facade there will be a vertical control joint. That control joint will be cut directly through every other brick so that the control joint runs in a straight line.

In the photo below you can see a control joint that was intentionally installed in the knee wall.

In the case above, the soft joint has failed, but that works out perfect for us to see the inside of the joint, just for us to examine and understand. Also, it’s ok that it has failed, it should be expected that control joints fail many times within the life of a masonry wall. They require a comparatively advanced maintenance schefule. Typically a masonry wall should last over 50 years without much maintenance. However a control joint for only last approximately 3 to 10 years before need to be reworked and replaced.

Also, it should be noted that in this open during, since the sealant has failed, you can see there is a backer Rod or expansion board inside of the space of the joint between the stones. That is intentional and done correctly in this photo. That expansion board does a couple things to benefit the joint. It allows for movement of the stern without having a hard pinch point and simultaneously provides a backer so that the ceiling does not have a deeper opening which needs to be filled. Sealants have special limits to which they can be applied. For a sealant to work properly it can’t be applied beyond a certain Dimension, in this case having a opening that is roughly 3/4 of an inch x 3/4 of an inch, that is inappropriate space.

In the photo below you can see a regular hard mortar joint in the same wall. We show this here just for comparison.

And also, here for comparison in the photo below you can see a control joint which has not failed. The sealant used has not been applied very smoothly, but all the same functionally this control joint is still working the way it should. It is an impermeable membrane which keeps water or moisture out of the joint. And also, because this is soft and has much higher flexibility then a regular mortar joint, it allows the wall to move a little within the 20 foot long section without breaking the mortar or stone the way would otherwise if the control joint was not there.

In this context when we say hard mortar joint we mean a regular mortar joint and when we say ‘soft’ joint we mean a joint that is filled with a sealant and no mortar at all.

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