diff --git a/src/viewport.cpp b/src/viewport.cpp
index e9be650a25..b944a2aa4d 100644
--- a/src/viewport.cpp
+++ b/src/viewport.cpp
@@ -1785,7 +1785,8 @@ static void SetSelectionTilesDirty()
 
 		static const int OVERLAY_WIDTH = 4; // part of selection sprites is drawn outside the selected area
 
-		MarkAllViewportsDirty(l - OVERLAY_WIDTH, t - OVERLAY_WIDTH, r + OVERLAY_WIDTH, b + OVERLAY_WIDTH);
+		/* For halftile foundations on SLOPE_STEEP_W the sprite extents some more towards the top */
+		MarkAllViewportsDirty(l - OVERLAY_WIDTH, t - OVERLAY_WIDTH - TILE_HEIGHT, r + OVERLAY_WIDTH, b + OVERLAY_WIDTH);
 
 		/* haven't we reached the topmost tile yet? */
 		if (top_x != x_start) {