Rule 5: Damp/Rot OK?

Damp’s not the problem, is the rot!

Damp can have an aesthetic impact on decoration, but it isn’t a health problem per se, after all up to 60{f898b7ccba74aedaf6f9f165b1651d39d829939f1b62e7036dcda8fdc0245a9d} of the human adult body is water.

Is mould a problem?

There is no peer reviewed scientific evidence that mould causes a problem in residential properties, after we eat cheese, yogurt, bread and drink beer and other beverages made from mould.

Some extreme levels of mould, such as found in hay, can cause health complications (known as farmer’s lung), but not the level found on walls in properties.

Yes the problem can be psychosomatic, that is to say a mouldy environment can make people feels, queasy with a very real negative feeling, but there’s no scientific peer group reviewed evidence of residential mould causing a health risk.

No, mould is your friend!

Why is mould a friend;

  1. because it’s telling you that there’s a damp issue that needs to be addressed,
  2. you know mould is caused by excess humidity (between 85{f898b7ccba74aedaf6f9f165b1651d39d829939f1b62e7036dcda8fdc0245a9d}RH – 100{f898b7ccba74aedaf6f9f165b1651d39d829939f1b62e7036dcda8fdc0245a9d}RH) , and
  3. not by Rising Damp, that inhibits mould.

The greatest damp risk to a property is rot

The most virulent and damaging rot to a property is dry rot (serpula lacrymans). Dry rot can spread rapidly, but may only become noticeable when a fruiting body forms under biological threat, such as during a summer’s heat wave, loss of food or competition from another rot or outbreak of dry rot.

Brown rots are the most dangerous to structural timber, as they cause cuboidal cracking (cracking across as well as along the grain), such as cellar rot (coniophora puteana).

White rots, such as window rot (phellinus contiguus) are common, particularly around external timber frames. Generally there are no a structural problem.

Simple rules for rot

  1. Rot needs wet timber to exceed 28{f898b7ccba74aedaf6f9f165b1651d39d829939f1b62e7036dcda8fdc0245a9d} moisture to break open cells.
  2. Rot spores are omni-present (everywhere), so expect rot if you have wet timber.
  3. Lack of ventilation is the main issue with brown rots (e.g. dry rot).
  4. Rots (like humans) need oxygen and produce water as a by-product of respiration.
  5. So once a rot gets started only evaporation through ventilation will stop it.
  6. Stop the source damp and ventilate – the rot will die – even dry rot (but it may take a year).
  7. If there’s insignificant structural decay, then leave the timber to dry throughly (few weeks) then check strength again once dry.
  8. If there is structural damage then replace the timber that it damaged and look to support from sub-floor walls or brackets.
  9. Overall – no source of damp = no rot
  10. Overall – ventilate = no brown rot

If you look at a wooden post, the rot is only found a few centimetres from the ground, the area where there is sufficient water (minimal ventilation), but it gets sufficient oxygen to respire.

Dust mites

Dust mites (dermatophagoides pteronyssinus) are a significant health risk causing allergies.


Woodworm is caused by the larvae of beetles burrowing into timber. Period properties tend not to have sufficient nutrients remaining in timber to sustain woodworm. Modern timbers are treated for woodworm (and rot), however the vac-vac (change in pressure to force fungicide and insecticide into timber) is not always good at protecting timber – especially low cheap timber.

The common furniture beetle (anobium punctatum) is very common in London residential properties but is almost certainly inactive, because period timber is too old to be edible to woodworm. The way to find out is look for fresh frass, that is larvae excretion – often trapped in spider’s webs.

Otherwise most infestation takes place in the forest or timber yard, such as is common with ptilinus, or is associated with rot, such as cellar rot for the woodboring weevil or the white rot, donkioporia for the death watch beetle (Xestobium).

Termites are virtually unheard of in UK.

The house longhorn beetle that destroyed roof structures from Camberley to South London in the 1970’s and 80’s has largely died out.

There’s more to know, but these are the basics.

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