Stone Chimney Repair: How It Differs From Brick

Why stone chimneys behave differently

A brick chimney is built from identical units with thin, even joints laid to a line. A stone chimney is built from pieces of different sizes and shapes, fitted together with joints that can be narrow in one spot and two fingers wide in the next. More of the chimney's face is mortar, and the mortar does more of the work of holding it together and spreading the load from stone to stone.

Stone also handles water differently. Many building stones, such as granite and dense fieldstone, absorb far less water than brick, so rain runs off the face of the stone and into the joints. Softer stones like some sandstones absorb more and can weather on their own. Either way, when the joints wear out, water gets deep into the chimney and loosens stones from the inside.

Where you find stone chimneys in the region

Stone chimneys turn up on older farmhouses and colonials in north and central New Jersey, Bucks and Chester counties, the Hudson Valley and parts of Westchester, often built from local fieldstone or quarried stone. Some newer houses have stone veneer over a block or brick chimney, which looks similar but behaves very differently.

That difference matters before any repair. Full stone masonry carries its own weight; veneer is a thin face attached to a backing, and its problems usually start with water getting behind it rather than with the joints between stones.

Common problems, most common first

Most stone chimney repairs start with one of a few signs you can see from the ground or from inside.

• Mortar recessed, sandy or missing between stones
• A cracked or missing crown, or flue tiles with no cap
• Cracks running through the mortar from top to bottom
• A stone that has shifted, tilted or fallen out
• Water stains inside on the wall around the fireplace
• Gaps or loose flashing at the roof line

Matching the mortar

The mortar has to suit the stone and the age of the chimney. Older stone chimneys were usually laid in soft, lime-rich mortar that flexes slightly and lets moisture out. Repointing them with a hard cement mix can trap water and force cracks into the stone or along its edges, because the hard joint will not give as the masonry moves through hot summers and freezing winters. The new mortar should be no harder than the stone and close to the original in color and texture, so the repair does not stand out.

Joint shape matters too. Stone joints may be raked back, struck flush, or finished with a raised ribbon or a tooled line, and copying the original finish is what makes a repair blend in. A sample panel tried on a hidden face is a good way to settle color and texture before the whole chimney is done.

Weather also shapes the schedule. Lime-rich mortars cure slowly and need protection from frost and from drying too fast in hot sun, so stone repointing is usually planned for spring and fall rather than midwinter or the hottest weeks of summer.

• Lime-rich, soft mortar suits older stone laid in lime, and needs mild weather to cure well
• A blended lime and cement mortar suits mid-age work and harder stones, as long as it stays softer than the stone
• A hard cement mix is rarely right on old stone, because it traps water and cracks stone edges

Resetting and rebuilding

A loose stone is taken out, the old mortar is cleaned off it and from the opening, and the stone goes back in the same spot on fresh mortar, wedged and packed so it sits as it did. When a whole section has let go, usually at the top where weather hits hardest, it is taken down and rebuilt stone by stone. Photos and marks are used so the original stones go back in their original places wherever they are sound.

That work takes a mason who is used to fitting irregular pieces. It is slower than brickwork, because each stone has to be chosen and placed by eye rather than laid to a line, and joints have to be packed deep rather than buttered on the face.

A stone chimney that bulges, leans or shows a long crack running through stones as well as joints is a different situation. That usually points to movement in the foundation, a failed footing, or water that has washed out the core of the chimney over many years. Those chimneys need a structural assessment before any repointing, and the area below should be kept clear until a mason has looked at it.

The top: crown, cap and flue

Many stone chimneys have a thin mortar wash on top instead of a proper crown, and that wash cracks first. A cracked top lets water straight down into the core of the chimney, behind the face stones. Rebuilding the top as a sloped crown with an overhang and drip edge, and capping each flue, protects everything below it.

The flue inside a stone chimney is often clay tile, sometimes unlined in very old houses. It is checked for cracks and sizing like any other flue, especially if it serves a boiler or a wood stove.

What shapes the cost

The main factors are how much mortar has to be cut out and replaced, how many stones need resetting, the height of the chimney and how hard it is to reach, whether scaffolding is needed, and whether the crown, flashing or flue also need work. Matching an unusual mortar color or replacing a stone that is missing can add time. Historic properties may also need review before exterior work.

A proper estimate looks at the whole chimney and lists each part of the job separately, so you can see what is urgent and what can wait.

Checking a stone chimney without a ladder

Use binoculars on a dry day. Look for joints that look deep and shadowed rather than flush, sand or mortar crumbs on the roof or ground below, any stone that sits proud of its neighbors, and plants growing from joints. After a rain, look for faces that stay dark long after the rest has dried. Inside, check the walls around the fireplace and in the attic beside the chimney for stains. Leave the roof itself to a crew with the right equipment.