DO THIS, NOT THAT

Patching stone steps.

Stone steps (or stoops) are common, particularly on urban row houses. While chopping at ice or moving heavy furniture can chip edges, a more common cause of fractured stone comes from the effect of iron oxide, sometimes called “rust jacking.” Lacking good design and regular maintenance, water enters the space between the embedded metal portion of the iron railing and the stone. Resulting corrosion exerts tremendous pressure on the surrounding material, causing it to fracture and spall. Start any repair by coating the remaining portion of the embedded iron with a rust preventive, or replace the part if it is badly corroded.

Then address a repair to the stone.

WRONG WAY USING JUST MORTAR

Avoid patching the stone with an off-the-shelf mortar, or a concrete patching or anchoring compound. These are rarely compatible with the stone in hardness, color, or texture. (They’re best for concrete repairs.) Without a mediating filler of epoxy or lead, the bond between the iron and the patching material will break down, allowing water to enter the gap and restarting the rusting process.

RIGHT WAY CUT OUT AND PATCH STONE

A large defect can be cut and the space fitted with a similar stone patch—a process best undertaken by an experienced stonemason. For a DIY repair, try using a masonry fill formulated to match the stone in color and texture. (For an accurate mix, send a sample to such suppliers as LimeWorks.us or Jahn system, cathedralstone.com.) You’ll need to replace the iron element first. Use a removable “form” around the base of the iron so that the space can be filled with epoxy or lead after mortar has cured, forming a flexible and renewable seal.