Flooding advice for Listed Building owners

The following advice is for listed building owners dealing with the after-effects of flooding.

Dealing with flooding

Buildings that are insured will often be appointed a professional to assess the damage on behalf of the insurance company. It is important that the person appointed has experience in addressing such issues in historic buildings.

Extent and degree of flood damage depends on –

  • depth water reaches
  • course it takes
  • length of time it remains
  • type of materials
  • amount of contaminants carried
  • length of time for drying out

10 basic steps to take

Wait for water to recede  

  • in basement or semibasements, if surrounding water level is high, pumping could increase external pressure on foundation walls
  • pumping before floodwater has receded is futile as so long as the water table remains higher, water will continue to seep through the walls until the ground water finds equilibrium

H&S risk assessment

  • wear protective, waterproof clothing and face mask
  • consider if it is safe to enter (there may be hidden dangers below floodwater level)
  • ensure electricity turned off at mains before standing in flood water
  • unplug and remove electrical items
  • take photographs / video to record damage

Decontamination and cleaning

  • floodwater can contain contaminants from drainage and sewage systems, or from surface runoff;avoid high pressure spray systems which can damage historic materials; and release micro-organisms, which are harmful to operatives
  • selective removal of items or fittings considered beyond economic repair (e.g. carpets, soft furnishings)
  • remove mud, dirt and flood debris with fresh water
  • clean services ducts

Post-flood survey

  • photograph again to record damage previously covered by water
  • signs of structural damage include bulging or dislodged masonry, leaning, tilting or cracks >6mm above openings, not evident before flood –consult Building Control and seek professional advice
  • don’t use electricity or heating unless inspected and certified by suitably competent professional

Drains and sewers

  • monitor / manage own private drains as best as possible until mud and silt blocking mains systems cleared by local / statutory authorities
  • if water rising through trap (ubend) of lowest appliance (e.g. toilet), report immediately
  • once cleared, flush sinks and toilets regularly to rinse system

Salvage detached and damaged items

  • steady drying and careful repair can save badly damaged items or provide template for replicating


  • opening doors and windows for ventilation  may require temporary perforated plywood sheeting or metal grilles screw fixed to existing frames
  • consider secure salvage store and/or intruder alarm

Drying out

  • depends on; time of year, building fabric (porosity and permeabilty), flood duration, water table and relative humidity
  • ‘forced’ drying can produce temporary and superficial results, where surfaces seem dry but core remains wet
  • should be undertaken slowly - too quick may warp, twist or split timber, encourage salts to migrate through old stone and plaster and painted surfaces to peel and flake
  • >18-20oC may encourage mould growth; <4 oC allows ice crystals to form in moisture laden materials and can lead to spalling / delamination
  • monitor using damp meters or temperature and humidity probes – note prong type damp meters only measure surface moisture and readings can be affected by salts


In order of priority, adopt natural ventiation; with fan; with moderate background heating; and with dehumidifier


  • clear mud and silt from base of external walls (ideally >150mm below internal floor)
  • remove flood detritus from air bricks
  • open all doors, windows, rooflights to increase airflow / reduce humidity
  • strip carpets and underlay
  • move furniture and pictures away from walls
  • lift every 6th floorboard to ventilate underfloor spaces may require a carpenter for tongued and grooved boards
  • saturated boards expand and buckle upwards, these need to be lifted, left on edge and wedged apart, or turned frequently if laid flat (excessive ‘cupped’ boards need replaced)
  • investigate hidden voids, behind panelling, door lining and window reveals and remove saturated insulation
  • drill small holes at base of skirting to provide access to cavities and promote air flow behind


  • where natural drying is too slow it risks mould growth and timber decay, so controlled air movement is recommended
  • 4 x air speed across surface = 2 x rate of drying
  • can focus air movement in strategic position on wetted area, to concentrate drying


  • during winter months, when external humidity is high and temperatures low so air cannot hold moisture, natural ventilation not sufficient
  • introduce modest background heat (around 18-20 oC) assisted by fan, in final stage of drying
  • too much heat will make the situation worse, i.e. if vapour production exceeds vapour removal


  • requires doors, windows and rooflights to be kept closed
  • aggressive dehumidifying dries lime plasters too quickly, results in cracks / deterioration and can deform timber as surface dries more rapidly than core.
  • use dehumidifier with humidistat control to monitor air moisture content (relative humidity) – ideally maintained at 40-50%
  • aim is to balance rate of evaporation with rate of vapour removal by ventilation / dehumidifiers

Remedial work

  • general principle is to deal with drainage, through ventilate cavities and use compatible finishes (usually means breathable or vapour permeable)
  • water will often flood materials, which then act as ‘moisture reservoirs’ providing conditions for long term damp and decay
  • finishes vulnerable to damp and fluctuating moisture contents such as MDF, should be avoided

Materials commonly found in listed buildings:


  • often untreated in historic buildings 
  • deforms when wet but typically recovers to original form when dry
  • moisture content >20% = susceptible to fungal attack
  • prolonged wetting increases risk of rot
  • rapid drying causes warping / splitting


  • porous and absorbent – can take months to dry out
  • drying can leave staining
  • risk of damage through salt crystallisation (efflorescence) and frost action
  • can often be brushed off

Plaster and render

  • soft lime plasters tend to be more resilient than gypsum
  • gpysum plasterboard tends to crumble and disintegrate when wet
  • insulation behind plasterboard will need to be removed
  • internal surfaces should be thoroughly washed down with water and suitable disinfectant to inhibit mould and bacterial growth
  • external cement based renders hinder drying out of wall core as impermeable to moisture

Internal decoration

  • modern paints containing plastic can inhibit drying and may blister and peel following saturation
  • same applies for vinyl wallpaper

New services and heating

  • install wiring and fitings above floor level and as high as possible
  • consider installing a sump and additional wall vents and manholes

Other sources of advice

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