Cap Block

Without a cap block, the top course of a wall would have open cores exposed straight to the weather, letting rain and melting snow pour directly down into the wall where it can pool, freeze, and crack the block apart from the inside. Cap block comes solid rather than hollow and is set in a full mortar bed across the top course, so the wall's finished edge sheds water off the sides instead of funneling it down into the cells below. It also gives the wall a clean, finished line to look at instead of an exposed row of open cores. Cap block is available flat or in styles with a slight overhang, so it can shed water further out from the wall face if that's what the design calls for. On a retaining wall, the cap is also usually the widest, most visible course in the whole run, so it's worth getting right both structurally and visually.

On retaining walls and any block wall left exposed to Blue Ridge winters, the cap is doing real structural work, not just cosmetic work, since it's the single detail most responsible for keeping freeze-thaw cycling out of the wall's core. We set caps sloped slightly and mortared solid so water sheds away from the wall face instead of standing on top of it through a hard freeze. A cap that's cracked or coming loose is usually the first sign a wall needs attention before the damage spreads any further down, which is why we check it first on any wall inspection.