What is dry rot and how to treat it

Dry Rot Fruiting Body

Dry Rot Specialists

HarperBD has been investigating and helping control dry rot outbreaks in all types of buildings and structures for over twenty years.
We utilise an advanced range of non-destructive instrumentation to assess the severity and extent of damage, while minimising exposure works and disruption.

Our recommendations follow a conservation-minded approach that focuses on rectifying the causes of moisture and recreating a ‘dry’ internal environment, which is the only true way of controlling dry rot.

Chemical-free solutions
As we are passionate about helping to create healthier buildings,  our recommendations will ensure the use of chemical preservatives is absolutely minimised if not negated altogether.

What is dry rot

Dry rot (Serpula lacrymans) is the most aggressive fungus found in UK buildings, gaining much notoriety for its ability to travel across all sorts of surfaces and spread through masonry and infect adjoining properties. Its resilience and reputation for destruction have lead to overzealous remedial  ‘treatments’ that have often been more destructive than the dry rot outbreak itself.

The dry rot fungus predominately attacks softwood building timbers, and although it will readily move across hardwood timber components, damage is far less significant.

A few key features

  • Dry rot belongs to the ‘brown rot’ group of fungi
  • Attacks softwood
  • Can spread over most surfaces
  • Can spread through masonry
  • The fungus is very resilient
  • Fungal material can remain viable for several years
  • Spores are in most buildings

What causes dry rot?

Moisture, moisture and … more moisture!

Although it is the nutrient content of timber that wood-decaying organisms seek to feed off; they are unable to exploit the timber unless it has become sufficiently damp.

‘Dry’ timber does not rot, so while dry rot spores are omnipresent, therefore likely to be in most buildings, the environmental conditions need to be right for them to germinate.

Wood is a hygroscopic material, it absorbs and releases moisture in reaction to environmental conditions, wand therefore the moisture content of construction timbers fluctuate throughout the year in reaction to the humidity within a building (this is most commonly observed with timbers within roof spaces).

Timber components in buildings typically have a wood moisture content (%WMC) around 16%, with timbers deemed to be safe from fungal decay when below 20%WMC.

Dry rot initiation requires timber components to have become sufficiently damp and have sustained a wood moisture content (at approx. 28%) for an extended period; these levels of moisture are usually found to be the consequence of one or more of the following;

  • Leaking plumbing
  • Defective rainwater goods
  • Building fabric defects
  • Roof defects
  • Inappropriate alterations
  • Poor design detailing
  • Sustained period of high humidity in confined and poorly ventilated areas

How to identify dry rot

The dry rot fungus has four distinctive stages to its lifecycle, the sporophore, spore, mycelium, and strand stages, which helps us differentiate dry rot from other types of fungi.
The appearance of the fungus will also present differently depending on environmental conditions and its maturity.

Dry Rot Identification

Life CycleAppearance of fungal stageImages
Fruiting body (reproductive stage)Rusty red/brown with a white edge.
The spore-bearing surface has a folded appearance, having a flat or angled (bracket) shape.
SporeAppear as a red/brown dust covering on surfaces.
Mycelium White or grey, sheets or cotton wool-like balls depending on humidity.
Strands White or grey (grey when mature), thick strands, which are brittle when dry.
Appearance of damaged woodThe timber shrinks and ‘cuboidal’ cracks form. The timber becomes darker.

What does dry rot smell like

Depending on the severity and extent of the problem, you may notice a musty, earthy smell before noticing any physical evidence of a dry rot outbreak.

How far can dry rot spread

Damage can be extensive, how far it spreads will be dependent on the source of water ingress, the resulting environmental conditions (humidity, temperature, light, yes light!), the supply of softwood… and how long it goes undetected.

How do you investigate dry rot

Hopefully you will have discovered the fungus early and it can controlled before it spreads, however, the most severe dry rot outbreaks can spread a significant distance from the location of the original outbreak.

The investigation of dry rot must be thorough, with its success reliant on a good understanding of the fungus, its characteristics, and a sound knowledge of the construction design.

The damage caused by investigating and treating dry rot can be minimised, which actually is a legal requirement for works involving listed buildings; even in modern buildings, damage can be minimised by following a conservation-minded approach.

The search for dry rot will start with a thorough visual inspection, extending the search from infected areas in all directions.

Depending on the construction of the property, it may be necessary to inspect within voids and behind surfaces.

Common areas that conceal fungal growth;

  • Ceiling voids
  • Suspended floors
  • Behind panelling
  • Basements and cellars
  • Behind cupboards
  • Built-in furniture
  • Under vinyl and other impervious floor coverings.
  • Behind skirting boards
  • etc

Exposure works

In order to minimise damage, obviously infected materials would be lifted rather than sound material e.g. removing a small section of rotten floorboards rather than a sound section.

Borescopes and other visual aids can be utilised to inspect voids and other difficult to access areas suspected of concealing dry rot.

How to control and treat dry rot

As with all issues relating to timber decay and deterioration in buildings; it is essential that the source of moisture is identified and rectified.

Controlling moisture is the only guaranteed way to control dry rot – chemical preservatives will not control dry rot if the underlaying moisture issues have not been addressed, no matter what you are told!

A robust strategy to control is needed in order to minimise the risk of reinfection, and will include;

  • Solving the moisture issues
  • Controlled drying/dehumidification
  • Considering the extent of infection and how much infected timber (if any),  needs to be removed
  • Clean infected areas
  • Oversee the application of any targeted chemical treatments that are thought justified
  • Monitor remedial works
  • Reinstate with care…
  • Monitor


As mentioned, the only guarantee against dry rot affecting building timbers is to ensure the wood moisture content is maintained below 20%.

Remedial treatment contractors will usually offer their ‘company guarantee’, and for an additional charge they may offer an ‘insurance-backed guarantee’.

Although an in-depth discussion about the merits of remedial guarantees is beyond this article, the following abstract from one such document typifies the extent of their worth...

Unless the following conditions are satisfied, the guarantee would become nil and void.

(d) where the property has not been kept in a good and proper state of maintenance including, by way of example only, maintenance of rain water goods and disposal systems, soil and waste disposal, hot and cold water systems, internal and external ground levels relative to damp-proofing courses and internal floor levels, adequate sub-floor through ventilation and general structure of the property;

Condition (d) immediately made the guarantee void for this particular Listed property, as external ground levels were the same or slightly higher than internal floor levels, which could not be practically modified.

(e) where the moisture content in any timber treated by the Company has been allowed to exceed 20% at any time subsequent to the treatment by the Company;

The remedial company still wanted to charge the client for providing this ‘Guarantee’ despite that timber floor joists within several locations were likely to continue to have moisture contents in excess of 20%, and therefore be unable to satisfy ‘Condition (e)’.

Call to learn more about our scientific approach to investigating dry rot and environmental controls