Dry rot: early signs, how serious it is and what treatment costs

Dry rot is a fungus, Serpula lacrymans, that decays the timber in a house once the wood stays damp at around 20% moisture. Unlike wet rot, it can send strands across brick and masonry to reach timber further away, which is why its treatment goes further than the damage you can see. We cover the early signs, how to tell it from wet rot, how serious it is, how it is treated and what treatment costs.

What dry rot is

Dry rot is a wood-decaying fungus. Its spores are in the air in most buildings and do no harm until they land on timber in the right conditions: a moisture content of around 20%, and for fastest growth a temperature of 20°C or more.

Despite the name, dry rot needs damp timber to start. What it leaves behind is dry: wood that is warped, shrunk back and crumbles to the touch.

It grows in four stages. Spores land and germinate, fine threads called hyphae spread through the wood, the hyphae mass into mycelium, and the fungus then produces a fruiting body that releases new spores.

What sets dry rot apart is that its strands can cross brickwork, masonry and other surfaces with no food in them to reach timber further away. An outbreak that starts at one wet timber can spread through the building from one timber to the next.

Early signs of dry rot

Dry rot is hard to spot early, because it grows in places you do not see into. The first sign is often a smell or a patch of dust rather than visible damage.

Where dry rot starts in a house

Dry rot starts in dark, unventilated spaces: the void under a suspended timber floor, behind wall panelling or plaster, and in ceiling voids. A leak can keep timber wet in these places for months without anyone seeing it.

Where you smell it but see nothing, lift a floorboard or ease off a length of skirting where the smell is strongest, and look with a torch. Photograph whatever you find.

Dry rot or wet rot: how to tell them apart

Both are fungi that decay damp timber, and both need a moisture problem to start. The difference is how far they go. Wet rot stays in the area that is wet, while dry rot can spread to timber that was never wet at all.

Early on, the two are hard to tell apart even for a trained eye. Dry rot in its early stages can need a building mycologist and a laboratory test to confirm, and the clearest sign, the fruiting body, only appears once an outbreak is established.

Wet rot

Wet rot is the name for several fungi, grouped into white rots and brown rots, including Coniophora puteana, the cellar fungus. It turns up in cellars, in roof timbers and in the timber around windows, wherever a leak or rain keeps wood wet, and it needs that moisture to keep going. It is the less destructive of the two.

Rot and woodworm beetles often turn up together. The wood boring weevil is linked to wet rot, and the death watch beetle to hardwood already softened by fungal decay, so a timber survey looks for both.

What causes dry rot

Dry rot starts with a building fault that lets water into timber and keeps it there. A survey looks for every cause, not just the first one found, because an outbreak fed by two leaks and fixed at one comes back.

• Rain getting in. A leaking roof, a blocked gutter, a cracked downpipe or a defect in a wall, often after years of missed maintenance.
• Plumbing leaks. A slow leak from a waste or heating pipe, or a bath or sink that overflows, keeping a floor void wet.
• Flooding. Timber soaked in a flood or a burst pipe and covered back up before it dried.
• Voids with no air. Dark, unventilated spaces under floors, behind walls and in ceilings, where damp timber never dries out.

How serious is dry rot?

Dry rot is the most serious form of fungal decay in a house. It attacks the timber that holds the building up, and it does not stay where it started. Left untreated, it weakens joists, floors and roof timbers and spreads from one timber to the next for as long as the moisture lasts.

Dry rot spores are not harmful to health in themselves. The risk is structural: a floor, a ceiling or a roof that loses strength.

Caught early, while it is one wet timber and a patch of growth, dry rot is a contained repair. Found late, after the strands have crossed a wall, it means opening up more plaster and flooring and replacing more timber, and the bill follows the amount of timber that comes out.

How dry rot is treated

Dry rot treatment runs in a set order, and the moisture comes first, because without it the fungus cannot grow. A standard treatment goes like this:

1. Every source of moisture is found and fixed, whether a leak, a gutter or a blocked airbrick.
2. The affected area is opened up, taking off plaster, render or flooring to find how far the growth has spread.
3. Decayed timber is cut out to at least 500mm beyond the last sign of fungal growth or strands.
4. Fruiting bodies and spore dust are removed, and the wall surfaces are brushed clean.
5. The removed timber is replaced with pre-treated timber.
6. A fungicide is applied to the sound timber left around the outbreak.
7. The masonry the strands crossed is sterilised with a masonry dry rot treatment.
8. The structure is left to dry for as long as possible before replastering and redecorating.

Wood preservatives and rot fungicides are biocidal products, and in Great Britain a biocidal product must be authorised before it can be sold, under the GB Biocidal Products Regulation. Northern Ireland follows the EU Biocidal Products Regulation instead.

Where the rot is in a roof space in England, the loft is checked for bats before any chemical goes on. Chemical timber treatments cannot be used in or near a known bat roost while bats are present.

Environmental control or chemical treatment

Not every outbreak needs every timber sprayed and every wall drilled. An environmental approach investigates without tearing out more than it must, removes the cause, dries the structure and keeps it dry, with chemicals kept to the timber and masonry that need them.

Stripping large areas of plaster, removing timber that is not infected, spraying sound timber and drilling and irrigating walls with chemicals can all be overtreatment, with the cost and damage that go with it. Where a quote includes them, ask what the survey found that makes each one necessary.

Can you treat dry rot yourself?

Dry rot is not a DIY job. The strands reach further than you can see, the timber has to be cut well past the damage, and the masonry has to be treated as well. Dry rot is treated by damp and timber specialists, who survey the outbreak, find the leak behind it and decide which timbers come out.

Wet rot is different. Someone with basic skills can repair a small area: fix the moisture, cut out the soft timber, treat the sound wood around it, and use a wood hardener on a door or window frame where the damage is shallow. Safeguard lists a 5-litre woodworm and rot treatment at £29.99 and a 5-litre wood preserver at £31.99.

Dry rot treatment cost

A dry rot survey costs £150 to £400, and treating an outbreak costs £300 to £3,000. The spread comes from how much timber has to come out, whether the ventilation has to change, and whether other fungi are present as well.

Nobody can price a dry rot job from the doorway. As with most damp and timber treatment, the quote comes after a survey, because the price depends on what is behind the plaster and under the floor.

Buying a house with dry rot

In England, dry rot usually comes to light in the buyer's survey, as a smell, a patch of spore dust or a floor that feels soft. The surveyor flags it, and a timber specialist confirms which rot it is and prices the work.

Get the treatment quoted in writing before exchange, including fixing the leak that caused it, and use the figure to renegotiate or ask the seller to have the work done first. If the house was treated before, ask for the specialist's report and the guarantee. Some firms give 30 years, backed by guarantee protection insurance so the cover outlasts the firm.

Scotland works differently. The seller or selling agent must give a buyer a Home Report within 9 days of being asked. Its single survey covers the condition of the home and the repairs it needs, and its property questionnaire covers past damage and guarantees, which is where an earlier dry rot treatment would show.

How to prevent dry rot

Dry rot needs timber at around 20% moisture to start, so prevention is about keeping water out of the structure and letting it dry when it gets in.

• Fix leaks quickly. A dripping waste pipe, a leaking shower tray or a blocked gutter can keep nearby timber wet enough for dry rot to start.
• Keep airbricks clear. The void under a suspended timber floor needs air moving through it, so keep soil, paving and planters away from the airbricks.
• Dry out after a flood or a burst pipe. Let wet floors and walls dry properly before they are covered back up.
• Deal with condensation on window timber. Persistent condensation runs down window frames and soaks the timber below them, winter after winter.
• Check again after any leak. Lift a board where the water went, and over the following months check for the smell and for dust.