# How does dry rot occur? Dry rot needs damp, unventilated timber to start, then spreads through masonry to reach fresh wood. What causes it, how to spot it, and what a proper repair involves. Dry rot is the most damaging thing that can happen to the timber in a building, and its name is thoroughly misleading. It does not occur in dry conditions. It needs damp wood to start — and once established it can carry moisture with it, which is what makes it uniquely destructive. ## What has to happen for dry rot to start Four conditions, all required: - **Timber above roughly 20% moisture content.** This is the trigger. Below that, dry rot cannot germinate. - **A source of that moisture.** A leak, a failed damp proof course, penetrating damp through masonry, condensation, or ground water in a cellar. - **Poor ventilation.** Moving air dries timber. Still air keeps it wet. - **Moderate warmth.** Which is why it thrives in the heated parts of a building rather than a cold outbuilding. Remove any one of those and it cannot establish. In practice the one that gets removed accidentally is ventilation — blocked airbricks under a suspended floor, a sealed-up chimney, new windows fitted to a room that previously breathed, insulation packed into a roof space over the eaves ventilation. ## Where it starts, and why you do not see it Dry rot begins in the places nobody looks: **under floors, beneath stairs, in roof voids, behind skirtings, inside boxed-in pipework**. Warm, damp, unventilated and out of sight. By the time it shows on a surface, it has usually been developing for a long time. That is the central problem with dry rot — the visible symptom is late. ## What makes it worse than wet rot Wet rot stays where the water is. Cut off the moisture and it stops. Dry rot does something else: it produces **hyphae (rhizomorphs) that grow through masonry, plaster and behind render** to reach timber elsewhere. Those hyphae can carry moisture with them, so it can attack timber that was dry to begin with, several metres from the original outbreak. That is why a dry rot repair is rarely just "replace the affected board". The fungus has to be traced. ## How to recognise it - **Cuboidal cracking** — the timber splits into rough cubes across and along the grain, rather than just along it. This is the most distinctive sign. - Timber that is **light, dry-feeling and crumbles** between the fingers despite having been wet - **Cotton-wool white mycelium** — sheets or fluffy growth on timber or masonry - **Grey hyphae** running across brickwork or plaster, sometimes with a slight sheen - A **fruiting body**: a flat, rubbery, rust-orange pancake with a white edge, often shedding brick-red dust (spores) - A strong **damp, mushroomy smell** — frequently the first thing anyone notices If you find a rust-coloured fruiting body, the outbreak is well established and shedding spores. That is the point to get it looked at quickly. Dry rot on or through **masonry** is common in solid-walled property, and it is worth knowing it does not mean the wall itself is rotting — the fungus is using the wall as a route to timber. Treatment addresses both. ## How it should be treated There are three parts, in this order. Any repair that skips the first one will fail. ### 1. Find and fix the moisture Not optional, and not secondary. Whatever was keeping that timber above 20% moisture content has to be dealt with — the leak repaired, the ventilation restored, the [damp proof course](/services/rising-damp-control) renewed, the [external masonry sealed against driving rain](/services/waterproofing-masonry), or the cellar [tanked](/services/wall-tanking). Deprive the fungus of water and you have already beaten it. ### 2. Establish the full extent Trace the outbreak, including where hyphae have travelled through masonry — the outbreak often goes further than you think. This is why stripping back involves destructive works to calculate the full outbreak — lifting boards, removing panelling, checking adjacent voids. Guessing the boundary is how outbreaks recur. ### 3. Remove, treat, and protect Structurally compromised timber comes out, with a sensible margin. Remaining timber and adjacent masonry are treated with fungicide. Any necessary replacement timbers are pressure treated. ## In older and historic buildings We have treated dry rot in churches, halls and listed property across the province, including [First Ballymena Presbyterian Church](/blog/first-ballymena-presbyterian-church-est-1627) and [Bushmills Distillery](/blog/bushmills-distillery-bushmills). In buildings like these the specification matters as much as the treatment, which is why we work to [Property Care Association](/about-us) standards. ## If you think you have it Dry rot is the one where waiting genuinely costs money, because the spread is ongoing and it is reaching timber you cannot see. A surveyor will establish whether it is dry rot, wet rot or something else entirely, find what is keeping the timber wet, and set out the work in writing before you commit. See [wet and dry rot treatment](/services/wet-dry-rot-treatment), or ring **028 9061 8343**. Not sure whether you are looking at rot or beetle damage? Our guide on [telling active woodworm from old damage](/blog/is-my-woodworm-still-active) covers that. If the main symptom is damp rather than damaged timber, start with [rising damp or condensation](/blog/rising-damp-or-condensation).