Caps look like trim and behave like roofing. They shed water off the top of the wall, they stop frost getting into the joints of the top course, and they take every knock from mowers, boots and whatever gets set down on them. A missing cap is not a cosmetic gap, it is an open joint on top of a wall.
Water entering the top course of a wall is the beginning of most wall failures we see. It gets into the block cores and the joints, freezes, and lifts the course, which opens the joint further and lets more water in the following year.
Caps prevent that by covering the joints and throwing water clear of the face, which is why a proper cap has an overhang rather than sitting flush. When caps come loose it is usually because the adhesive was skipped, the wrong adhesive was used, or the course beneath them has already moved.
That last one matters: recapping a wall whose top course is shifting is money spent on a surface that is about to move again, so the course gets checked before anything is glued down.
A shifting top course will move whatever is glued to it, so the wall under the caps is assessed before any of them go back down.
Clean both faces
The cap underside and the course top are cleaned back to sound block, because adhesive on dirt is adhesive that lets go by spring.
Set the overhang
Caps are set to project past the face rather than flush, which is what throws water clear instead of letting it run down the wall.
Adhere and weight
An outdoor rated adhesive goes down in a continuous bead and the caps are weighted while it cures rather than left to hope.
Missing Caps
One gap lets water into the whole top course
Block cores and joints run continuously along the top of a wall, so a single missing cap is not a local problem. Water entering at that point travels along the course, and by the second winter the joints either side have opened too.
Replacing one cap is a small job; waiting until three are loose turns it into a top course rebuild. See how wall rebuilding handles that, what a leaning wall tells you about the course below, and how wall repair costs are measured.
Either the adhesive was skipped or the wrong type was used, or the top course underneath is moving and taking the caps with it. The second one needs fixing first.
Both. They cover the joints of the top course and throw water clear of the face, which is what keeps frost out of the block cores below them.
If the course beneath is sound, yes, with an adhesive rated for outdoor block and both faces cleaned first. If the course has moved, gluing it is temporary.
Replace it sooner rather than later. Water entering at that point travels along the top course, so one gap becomes three loose caps within a couple of winters.
Roughly $25 to $45 per running foot including the caps, assuming the course underneath is sound and does not need work first.
It does, and the cap course takes the worst of it because it is the part that gets walked and shovelled. Where a wall doubles as a step edge we suggest sand for grip instead, because a spalled cap is a replacement rather than a clean.
