Timber Decay: A Guide to Signs, Causes and Fixes

Timber has long been the backbone of residential construction in the UK and Ireland, trusted for its strength, reliability, and natural warmth. But, like anything organic, timber isn’t invincible. When timber decay sets in, it creeps quietly at first – softening joists, hollowing out lintels, and turning what once felt solid into something alarmingly fragile.

For surveyors, spotting timber decay early is not just helpful, but essential. You’re often the first line of defence between a homeowner and a hefty repair bill. Whether you’re inspecting a Victorian terrace in Belfast or a post-war semi in the Midlands, understanding timber decay in detail helps you catch problems before they escalate.

In this guide, we’re unpacking everything you need to know: how to identify timber decay, what causes it, what fixes actually work, and where tech tools can make your life easier on site.

What We’ll Cover

• What Is Timber Decay?
• Common Signs of Timber Decay 
• Primary Causes of Timber Decay in Residential Properties
• Dry Rot vs Wet Rot: What’s the Difference?
• The Hidden Role of Moisture: How Damp Fuels Timber Decay
• Fixes: From Spot Treatments to Full Structural Repairs
• Preventing Timber Decay in Residential Surveys
• How Digital Surveying Tools Help You Identify and Report Timber Decay
• FAQs: Quick Answers to Common Questions 

Timber decay is the gradual breakdown of wood caused by fungal organisms. When timber is exposed to persistent moisture, fungi begin feeding on the cellulose and lignin, weakening the wood’s structure from within. Left unchecked, it compromises not just aesthetics, but load-bearing integrity.

Decay typically falls into two categories: wet rot and dry rot. Both can look deceptively mild on the surface – until you prod a skirting board and your finger sinks in further than it should.

And here’s the kicker: decay doesn’t need a flood to get started. A dripping gutter or a poorly ventilated loft can be all it takes to set the process in motion.

Most homeowners won’t notice timber decay until it’s visible or audible. For surveyors, though, subtle clues are part of the job. If you’re conducting a Level 3 building survey, it pays to keep your senses sharp, especially in older or unventilated buildings.

Keep an eye (and ear) out for:

• Softened or spongy timber – often detectable underfoot or with a probing tool.
• Crumbling wood – especially around skirting, window frames, or joists.
• Fungal growth – ranging from white mycelium to mushroom-like fruiting bodies.
• Musty odours – particularly in enclosed spaces like cellars or beneath suspended floors.
• Warping or distortion – timber may twist or crack as it loses structural integrity.
• Paint that bubbles or flakes away easily – a common sign the wood beneath is deteriorating.

Even seemingly minor signs like squeaky floorboards or slight discolouration can indicate deeper issues. Basically, if the wood doesn’t feel or behave like timber should, there’s probably more going on beneath the surface.

You’ll rarely find timber decay in isolation. It’s usually a symptom of a wider issue, and one that’s tied to how the property breathes, retains heat, or deals with water.

Here are the most common culprits:

• Water ingress – from rising damp, penetrating damp, or leaks
• Poor ventilation – particularly in lofts, basements, or crawl spaces
• Blocked or damaged gutters – leading to saturated walls and adjacent timber
• Inadequate maintenance – like untreated timber exposed to weather
• Condensation – especially in high-humidity areas such as bathrooms or kitchens
• Defective flashing or pointing – water seeps in where it shouldn’t
• Historic repairs using cement mortar – which traps moisture against internal timbers

Moisture is almost always the instigator. Whether it’s a roof leak or long-standing rising damp, it creates the perfect habitat for decay to thrive.

It’s easy for clients to confuse the two, but the distinction matters—especially when it comes to treatment and costs. If you want a more in-depth comparison, we’ve broken it down fully in our dry rot vs wet rot guide, but here’s a quick overview to keep in mind during surveys.

Dry Rot (Serpula lacrymans)

Dry rot is the most serious form of timber decay. It spreads through timber even in relatively dry conditions and can travel across brick and plaster in search of moisture.

Tell-tale signs:

• A mushroom-like smell
• White mycelium on timber
• Rust-red fruiting bodies
• Deep cracking along the grain

Dry rot behaves more like an organism than a simple fungal bloom; it moves, spreads, and eats away at the structure almost silently.

Wet Rot (Coniophora puteana and others)

Wet rot is more common but generally less aggressive. It’s confined to the damp area and doesn’t leap from timber to brickwork.

Look for:

• Wood darkening in patches
• A wet, spongy feel
• Fungal strands, usually black
• Paintwork cracking or flaking

The key difference between the two: Wet rot stays put; dry rot spreads.

Treatment depends on three things: the type of decay, how far it’s spread, and what the timber is doing structurally.

Here’s how you might approach it:

• Isolate the moisture source first – Without this, treatment is pointless 
• Replace irreparably damaged timber – Especially if it’s load-bearing 
• Apply fungicidal treatments – Always choose those appropriate for dry or wet rot 
• Improve ventilation and air movement – Particularly in subfloors and roof spaces 
• Use timber preservatives – To prevent recurrence, especially in vulnerable areas 
• Install physical barriers – Such as damp-proof membranes or air bricks 

Treatments need to be tailored not just to the damage, but to the property’s age, construction method, and materials. A Georgian townhouse demands a different approach from a 1990s detached.

Here’s where surveyors really earn their keep. Prevention advice is part of your duty of care to clients, and getting it right is essential.

Offer clear, practical advice that doesn’t just fix the issue, but stops it from coming back. That might include:

• Checking gutters and downpipes regularly 
• Keeping ventilation pathways clear 
• Using breathable materials in heritage properties 
• Installing extractor fans in bathrooms and kitchens 
• Repairing external defects (like pointing or flashing) promptly 

The trick is explaining all this in plain English. Your clients don’t want jargon, they want to know what to do and why it matters.

Spotting decay is one thing. Capturing it accurately without missing details is something else entirely.

That’s where surveyors are increasingly turning to digital surveying tools and apps. They help streamline condition reports, flag issues with photographic evidence, and maintain a consistent trail of what was seen, where, and when.

With GoReport, for example, you can:

• Record decay-related issues on-site via mobile 
• Auto-generate consistent reporting structures 
• Annotate photographs directly within the app 
• Tailor your findings to Level 2 or Level 3 reports 
• Store historical data for future comparison 

If you’re not already using tech in your timber inspections, you’re working harder than you need to. Book a free GoReport demo and see what it looks like in practice.

What’s the main difference between timber decay and woodworm?
Timber decay is fungal; woodworm is insect-based. The former softens timber; the latter riddles it with holes.

Is timber decay covered by home insurance?
Often not. Most insurers classify it as a maintenance issue unless sudden damage results from an insured event like flooding.

Can dry rot come back after treatment?
Yes, if the moisture issue isn’t fully resolved. Dry rot spores are hardy and persistent.

How long does it take for timber decay to cause structural damage?
It varies. Wet rot can take years. Dry rot can compromise structures within months if conditions are right.

What’s the best way to explain timber decay to a client?
Keep it simple. Compare the wood to a sponge or loaf of bread; it looks fine, but once it’s damp inside, it breaks down fast.