5 Causes of Dry Rot & How to Avoid It

5 Causes of Dry Rot & How to Avoid It

In this article we will be looking at dry rot and some of the more common causes of it in the UK, as well as what actions can be taken to prevent it starting in the first place.

What is dry rot?

Dry rot is a fungal growth that feeds off of the cellulose that exists in timber. It can spread through most building materials, including brick walls, and when left untreated causes serious structural problems.

Luckily the treatment of dry rot is effective, and the earlier you can spot the rot the easier, and cheaper, it will be to resolve the problem.

The most common fungus that is associated with dry rot is called Serpula Lacrymans. As it feeds off timber it grows and spreads leaving the timber dry and brittle with square-edged, cuboidal cracking that is easily recognisable. This ultimately leaves the timber significantly weakened and incapable of performing its structural role.

Dry rot is considered a far more significant problem than wet rot because it is capable of spreading a long distance from the original source of the fungal growth, whereas wet rot is contained to the location where the timber has become saturated.

How to spot dry rot

Dry rot forms reddish-brown / white fungal growths, often in corners of roofs, or behind plasterboard or under floorboards. In addition to fungal growths, dry rot lets off a musty smell which can be an indicator of concealed dry rot.

As the dry rot spreads it produces strands that look similar to a spider's web as it seeks out fresh timber after depleting an existing source.

What are the main causes of dry rot in UK houses?

1. Lack of adequate ventilation

It goes without saying that in order for a fungus that requires humidity levels of between 18% - 30% in order to survive, that poor ventilation will contribute to its formation. Ventilation is designed to remove stale, humid air from buildings and continuously replace it with fresh air. This serves many purposes, but most important here is to reduce humidity levels.

Dry rot is often found in lofts of older buildings where ventilation has not been installed. Another place that it often forms is to floor joists below floors, particularly where the air bricks or vents are either blocked or not working effectively.

The first step that you should take to prevent dry rot from developing is to install ventilation. For roofs that will include mushroom or other types of roof vents - note that ventilation is not required when the roof is a warm roof design. For floor areas you should ensure that the ventilation systems are working effectively, including making sure that air bricks and vent openings are not blocked or removed during adaptations to the building.

2. Leaking roofs or other water ingress

Regardless of its name, dry rot cannot survive without a certain amount of water. Leaky roofs which lack ventilation create the perfect breading ground as the moisture that gets in through the leak will add to the humidity in the air. Given enough time, ever present dry rot spores will eventually start to develop into the fungus and the problems will start.

In order to avoid this, ensure that any leaks to your roofs, under floorboards, into ceilings, or any leaks generally, are dealt with as quickly as possible and that they are allowed to fully dry out. A dehumidifier can be used to speed up the drying process.

3. Condensation

The main source of moisture in the UK that forms dry rot is not from leaks, but instead from condensation. High levels of condensation form in our homes when we shower, wash dishes, wash clothes, and when we lie in bed all night breathing into a room with the doors and windows closed.

To minimise the risk of dry rot starting due to condensation you should ensure that all bathrooms and rooms with sinks are fitted with effective mechanical extract fans, ideally that meet the flow rates set out in the Building Regulations Approved Document F.

You should also ensure that your home is properly heated in the winter to prevent condensation forming on cold surfaces.

4. Poor insulation

Poorly insulated homes will have colder internal surface which will lead to condensation forming during the winter. Even in modern homes, if the building was not constructed properly, a gap in the insulation or a cold bridge in the structure can allow condensation to form. This will increase the risks of dry rot developing.

It is no surprise that dry rot is most prevalent in older buildings and this is in part due to poor, or lack or, insulation. You can improve your property's insulation by finding out the feasibility of having cavity insulation injected into the wall cavity (post 1950s houses only), or external or internal wall insulation installed. You can also insulate your loft to reduce the risk of dry rot.

5. Missing / broken damp proof course

A damp proof course is a waterproof layer of material in the external walls of a building that is designed to prevent moisture from moving any further up the wall than where the damp proof course is fitted.

Older buildings built during the early Victorian period or earlier may not have a damp proof course at all, or they may be made from materials such as slate which can easily crack with the movement of the building.

Missing or failed damp proof courses can lead to rising damp, where water moves up the wall. Whilst rising damp does not always lead to dry rot, it can increase the risk of it forming, particularly to the floor boards and joists, or to timber studs lining an external wall, or in the plasterboard itself.

To avoid this, you should ensure that if you have rising damp you get it resolves as quickly as possible by having a chemically injected damp proof course installed, or by contacting a damp proof specialist to provide further recommendations.


Dry rot can be disastrous for UK homes if left untreated. And whilst treatment is effective, it can be both invasive and costly due to the spread of the rot that can occur.

In order to prevent dry rot from developing in your home, you should ensure the following:

  • Ensure that underfloor areas, lofts, or other concealed spaces are well ventilated to reduce humidity.
  • Rectify any water ingress issues as soon as possible.
  • Ensure that your property is fitted with mechanical extract fans in shower rooms, bathrooms, utility rooms, and kitchens.
  • Keep your house well heated during the winter.
  • Consider using a dehumidifier if poor ventilation cannot be easily rectified, particularly during winter.
  • Try to ensure that even during the winter, bedrooms and other living spaces are ventilated when possible by opening windows or through trickle vents to avoid excess condensation building up.
  • Install insulation to external walls, lofts, and floors where possible.
  • If you have rising damp, or a defective DPC, ensure that it is repaired professionally to reduce the risk of dry rot forming under floor boards or in other adjacent areas.

We hope you have enjoyed reading this article. If you have any other recommendations on causes and methods of avoiding dry rot, please let us know in the comments below.

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