Natural Stone: Exterior Care and Cleaning

Stone has clad buildings for thousands of years and will happily outlast everything you fasten around it. That reputation for indestructibility is deserved, and it's also the source of the one mistake homeowners reliably make: treating all stone as if it were the same tough, inert material. It isn't. The word covers granite and marble, sandstone and slate, limestone and rough fieldstone — materials formed differently, with wildly different porosity and chemistry. The care that suits one can permanently mar another.

So the useful starting point isn't a cleaning recipe. It's a question: what kind of stone is this, and if I can't tell, how do I clean it without gambling? Answer that, and everything else falls into place.

Two families that behave differently

For care purposes, the sprawling variety of stone collapses into a simple distinction based on what the stone is chemically made of. Calcium-based stones react with acid; silica-based stones largely don't. That single fact governs which cleaners are safe, and it cuts across the more familiar categories of hardness and color.

You'll meet stone in a lot of roles around a house — veneer and full walls, foundations and water tables near grade, chimneys weathering from every side, steps and patios and pavers, retaining walls and sills and caps. Each exposure is different: paving sees traffic and standing water, walls near grade stay damp, chimney stone takes the full brunt of weather. But whatever the location, the family the stone belongs to decides how you're allowed to clean it.

The acid trap

The most avoidable damage to stone comes from acid meeting a calcium-based surface. Acidic cleaners — and household standbys like vinegar — etch, dull and pit limestone, marble and travertine, leaving a permanent frosted patch where the acid dissolved the surface. The frustrating part is that you often can't tell a calcium-based stone from a silica-based one just by looking, and acidic algae removers marketed for masonry make no exception for the stones they'll harm.

The safe rule writes itself: unless you positively know the stone is acid-tolerant, keep acids off it, and test any product in a hidden spot before committing. Silica-based granite and most sandstone tolerate a wider range of cleaners, but "wider" is not "anything," and the test patch costs nothing.

Porosity, staining and spalling

Where acid is the risk for calcium stones, water absorption is the risk for porous ones. Dense granite and slate are nearly impervious; many sandstones and some limestones drink water readily, and with it, stains — oil near a grill, tannins from leaves and mulch, rust from iron fixings or iron within the stone itself. Because porous stone absorbs deeply, prompt cleaning matters far more than it does on a dense surface, and a poultice is often needed to draw out a stain that has soaked in.

Absorbed water carries a second hazard in cold climates. When water trapped inside porous stone freezes and expands, it pushes the face apart — spalling, the same flaking that troubles brick. Softer sedimentary stones are the most prone to it. And as with all masonry, a non-breathable coating that seals moisture inside rather than letting it escape makes the problem worse, not better.

Washing stone without wrecking the joints

Hard, dense, sound stone tolerates moderate pressure much as brick does — but the caution is identical: the mortar joints are softer than the stone, and a concentrated stream erodes them and drives water into the wall behind. Softer, porous or historic stone shouldn't be pressure washed at all. The sensible default is moderate pressure at most, a wide fan tip kept moving and held well back, and a shift to soft washing on any stone you're unsure about.

The gentle method suits nearly every situation: rinse loosely with plain water first, apply a pH-neutral, stone-safe cleaner (or a diluted bleach solution for organic growth, kept off plants), agitate with a soft-bristle brush rather than anything wire that scratches and leaves rust-prone flecks, then rinse thoroughly so no residue lingers. For green growth specifically, the approach in removing algae from exterior walls works well, adjusted for acid sensitivity. The broader trade-off is laid out in pressure washing versus soft washing, and the mortar-joint warning mirrors the guidance for brick.

Sealing: help or trap

Sealing divides along the porosity line. A porous stone that stains or absorbs water benefits from a breathable, penetrating sealer, which reduces both while still letting the stone dry outward; it wears over time and needs reapplying. A dense stone may not need sealing at all. The one thing to avoid everywhere is a film-forming, non-breathable coating: it traps moisture inside the stone, and in a freeze-thaw climate that trapped water is exactly what causes spalling. As with any product, test a sealer on a small area first and confirm it's suited to the specific stone.

What deserves action, and what doesn't

Not every mark on a stone wall is a problem. Surface soiling, atmospheric grime and a white chalky haze of efflorescence are cosmetic — efflorescence in particular is just mineral salts left behind as water evaporates, and it often brushes off dry (our efflorescence guide covers it, and persistent bloom points to a moisture source worth reducing).

The signs that genuinely call for action are different in kind: dull, etched patches on calcium-based stone; face flaking or crumbling from spalling; crumbling or gapped mortar that needs repointing before water gets behind it; and deep-set oil or organic stains in porous stone. Beyond cleaning, keep the water managed — confirm gutters, drainage and flashing steer runoff away from stone walls and paving — repoint failing joints with mortar matched to the stone's hardness, keep de-icing salts off stone paving, and clear organic growth before moss and lichen mats hold moisture against the surface. Anything structural, such as stepped cracking or a bulge in a stone wall, warrants a professional look.

Do that, and the material lives up to its reputation. Dense stones like granite and slate sit at the very top of the durability spectrum, outlasting brick, concrete and every siding material. The practical limits aren't the stone at all — they're the mortar, which weathers faster and needs periodic repointing, and the vulnerabilities of the softer, porous stones. Keep the joints sound and the water moving away, and stone will comfortably outlast the rest of the house.

Keep reading

• Brick: exterior care and cleaning
• Concrete: exterior care and cleaning
• Pressure washing vs. soft washing
• How to remove algae from exterior walls
• Efflorescence: white residue on brick