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104S: Dry Rot – Rising Damp Treatment

Victorian cottage with a squeaky floorboard, what lies below? Dry rot.

Surveyor Tips:

  • In most instances damp proofed properties have been misdiagnosed, which can mean that the dampness is ongoing causing destruction elsewhere.
  • To check for significant and or ongoing rot:
    • Always check the sub-floor relative humidity – high sub-floor humidity accompanies rot growth.
    • Perform a floorboard walk test – significant structural damage will be accompanied by unexplained movements in floorboards.

Root cause

Historic Dr rot, probably caused by a leak, later fixed, but was almost certainly misdiagnosed and mistreated as rising damp.


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The house has been damp proofed against rising damp by injecting chemicals. Typically damp proofers replace ground floor internal plaster with an impermeable plaster, known as slurry. This increases condensation risk.

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The primary concern was rot in the rear reception.

A piece of timber, used to support a floorboard was poorly fitted causing the timber to move. I understand that you wish to overlay floorboards to the rear, therefore wish to make sure that there is no ongoing rot or structural concern.

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The rot is probably old Dry rot, originally resulting from a leak. There are no sources of ongoing dampness.

I found no evidence of current rot, see surveyor.tips/rot. I guess the predecessors had a leaking main which they fixed. I measured the sub-floor humidity, it was normal, see above.

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Here is a selection of images from the sub-floor in all directions.

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There is no sign of the rot spreading to other timbers.

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The light patches are not a sign of rot, but typically remains of building works.

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I would leave access to the sub-floor and place a hygrometer there to monitor sub-floor humidity.

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The Dry rot has corroded the copper pipe.

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This image shows the extent of the rot damage. This joist is 1 joist in front of where we removed the floorboard.

I would support either side of this joist up to the next cross joist, with a new joist, all three drilled and bolting all the way through, two bolts for each end.

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Rather than replace timber I would support the lower joist either side.

Use approximately 1.5M length of a similar joist to the current one, probably 4 X 2 Timber (47 X 100MM). Use tanalised timber (pre-treated and treat the cut ends with a fungicide).

Similarly support the floorboard each with its own supporting joist centred on the lower joist for support and balance.

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There was slight dampness on the kitchen external wall up down from about 1M to the ground.

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Through a thermal imaging lens where blue is ~ 5˚C colder than yellow, we see that the base of the wall is cold.

In my opinion the primary cause of dampness here is condensation from excess humidity.

I understand that there was a smell of mould in the cupboards. Mould grows where relative humidity exceeds 85%RH for 6+ hours. Excessive humidity results from insufficient ventilation, poor air circulation and a cold surface. See surveyor.tips/mould. The secondary cause of high relative humidity is low temperature relative to the source of humidity, see surveyor.tips/humidity.

Consider installing an extraction fan into the kitchen and improving insulation.

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There are many cracks in external brickwork that should be raked out and filled.

There was no sign that water was penetrating through to the inside, as there would have been a stain.

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Horizontal render cracks draw in the greatest amount of water.

A wet external skin of wall increases heat loss in turn increasing the risk of condensation.

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I am not a fan of French drains (drains invented by Mr French, not from France).

The worse French drains are those that are not connected to the drain itself, increasing the time and amount of water coming into contact with a wall. However, I don’t think the root cause of your damp is the French drains.

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Your front drain does not drain away from the house.

I would attach a horizontal downpipe to drain rainwater away from the property about 2 M.

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