Ammonia kills mold on the surface of wood but struggles to reach mold growing deeper inside the grain. Because wood is porous, mold sends root-like threads (called mycelium) well below the visible layer. Ammonia sits on top and can’t follow them down, which means the mold often grows back within weeks.
Why Ammonia Falls Short on Wood
Ammonia is effective on hard, non-porous surfaces like glass, tile, and countertops. On those materials, all the mold sits on the surface where ammonia can make direct contact. Wood is a different story. Its grain is full of tiny channels and pores that mold exploits to anchor itself deep into the material. Ammonia doesn’t penetrate far enough into those channels to kill the embedded spores and roots hiding below.
Research from the USDA Forest Products Laboratory illustrates how difficult it is to get alkaline solutions (ammonia included) deep into wood. In lab tests, wood blocks soaked in ammonia solutions for just five minutes showed some resistance to fungal attack, but only under mild conditions. When exposed to aggressive fungal growth, ammonia-treated wood decayed readily. Achieving consistent protection required either prolonged soaking or vacuum impregnation, neither of which is practical for a homeowner wiping down a moldy shelf or floor joist.
So if you spray ammonia on moldy wood, the visible staining may fade and surface spores will die. But the underlying colony often survives and regrows.
How to Use Ammonia on Wood Surfaces
If the mold is only on the surface of a sealed or semi-porous piece of wood (like a painted shelf or polyurethane-coated furniture), ammonia can work as a cleaning agent. Mix a 50/50 solution of clear ammonia and water, spray it onto the moldy area, let it sit for two to three hours, then rinse. Repeat if discoloration remains.
For raw, unfinished, or heavily porous wood, this approach is unlikely to solve the problem long-term. You’ll clean what you can see, but regrowth is common because the roots remain intact inside the wood fibers.
Ammonia Can Damage Wood
Ammonia is alkaline enough to chemically alter wood. It softens lignin, the compound that gives wood its rigidity. Woodworkers actually use concentrated ammonia on purpose to bend and reshape lumber. On furniture or structural wood, repeated ammonia applications can soften the surface, change the color (ammonia fuming is a known technique for darkening oak), and strip or cloud existing finishes. If you’re cleaning a piece of wood you care about, test a hidden spot first.
Never Mix Ammonia With Bleach
This is the single most important safety rule when using ammonia for any cleaning purpose. Mixing ammonia with bleach produces chloramine gases that cause tearing, nausea, and respiratory tract irritation. In serious exposures, these gases can lead to chemical pneumonia and pulmonary edema. The reaction happens quickly, and even residual bleach left on a surface from a previous cleaning session can combine with ammonia. If you’ve recently used bleach in the same area, do not switch to ammonia without thoroughly rinsing first.
When working with ammonia indoors, open windows and use a fan to create cross-ventilation. Wear a respirator or gas mask rated for ammonia vapor, chemical-resistant gloves, and eye protection. The fumes are irritating even at low concentrations, and in enclosed spaces like crawlspaces or basements (where mold problems are most common), they build up fast.
Alternatives That Penetrate Wood Better
White vinegar is one of the most commonly recommended alternatives for mold on wood. It kills most mold species and, unlike ammonia, works on both porous and non-porous surfaces. Its acetic acid can penetrate into wood grain more effectively. Apply undiluted white vinegar with a spray bottle, let it sit for an hour, then wipe and allow the wood to dry completely.
Hydrogen peroxide (3% household concentration) is another option that penetrates porous materials. Spray it on, let it fizz for about 10 minutes, then scrub and wipe clean. It can lighten dark wood, so test it in an inconspicuous spot.
For heavy mold growth on structural wood, like framing, joists, or subfloor sheathing, surface cleaning with any household product has limits. When mold has visibly penetrated deep into the wood or the wood feels soft and spongy, the affected section typically needs to be sanded down aggressively or replaced. Widespread mold in wall cavities or crawlspaces often calls for professional remediation, because simply killing surface mold doesn’t address the moisture source that caused it in the first place. Mold needs moisture to grow, and until that underlying issue (a leak, condensation, poor ventilation) is fixed, it will return regardless of what you clean with.