Wet Rot vs Dry Rot: How to Tell the Difference and Why It Matters

They Both Damage Timber — But That's Where the Similarity Ends

If you've spotted crumbling, discoloured wood in your property, your first instinct might be to assume the worst and call it rot. That's fair. But which kind? Wet rot and dry rot are two distinct problems, and treating one as if it were the other is one of the more expensive mistakes a homeowner can make.

Here's how to tell them apart, what causes each, and what actually needs to happen to fix them properly.

What Is Wet Rot?

Wet rot is the more common of the two. It happens when timber absorbs excess moisture — usually from a leak, poor drainage, or a damp external wall — and begins to decay. The rot is generally confined to the wet area itself, which is actually good news. Fix the moisture source, treat the affected timber, and you've largely solved the problem.

Signs you're looking at wet rot:

• Timber feels soft or spongy when pressed
• Wood looks darker than surrounding areas
• Paint is bubbling or lifting from the surface
• There may be a musty smell near the affected zone
• Cracking tends to follow the grain of the wood

In many cases, wet rot is caught during routine maintenance — around window frames, skirting boards, or floor joists in older properties. The damage is real, but it's usually manageable if you act before structural timber becomes compromised.

What Is Dry Rot?

Dry rot is a different beast entirely. It's caused by a specific fungus — Serpula lacrymans — and once it takes hold, it can spread through timber, masonry, and even behind plasterwork without needing much moisture to keep going. That's what makes it so destructive in older London properties, where it can travel undetected for months.

Signs you might be dealing with dry rot:

• Timber looks cubed or cracked into small brick-like pieces
• A grey or white cotton-like growth on the surface of the wood
• Orange or rust-coloured spore dust nearby
• Mushroom-like fruiting bodies (this means the outbreak is advanced)
• Wood feels dry and brittle despite the fungus being active

The name is slightly misleading — dry rot still needs an initial moisture source to start. But once
established, it can keep spreading into drier areas, which is why a small patch of dry rot visible to the eye is rarely the full extent of the problem.

Why You Shouldn't Self-Diagnose

It's tempting to poke at the timber yourself, look up a few photos online, and make a call. The problem is that misidentification leads to incomplete treatment. Wet rot treated with a surface fungicide won't stop dry rot that's already behind your wall. And dry rot eradication — which typically involves removing affected timber, treating masonry, and addressing the original moisture source — is considerably more involved than patching soft wood around a leaky window.

A qualified surveyor will assess not just the visible damage but where the moisture is coming from, how far the rot has spread, and whether structural elements are at risk. Without that context, any repair is essentially guesswork.

What Treatment Actually Involves

For wet rot, treatment usually means removing and replacing decayed timber, applying preservative treatments, and resolving the damp source. For dry rot, the approach is more involved — affected timber is cut back beyond the visible growth, masonry may need to be treated with fungicide, and the entire area must be thoroughly dried out before any reinstatement takes place.

Both require a proper damp and moisture survey first. There's no point treating rot if the underlying cause — whether that's rising damp, a roof leak, or poor ventilation — is still present. The rot will simply return.