Drying
A unique approach is required to dry a historic or traditional building because the materials respond to removing moisture and high levels of heat differently to modern materials. If this difference is not taken into account, structural damage or complete loss of a historic element may occur.
Prior to any drying works commencing a thorough survey, likely using moisture meters, thermal cameras and drones, will be conducted to establish the level of moisture present. In some situations, salt testing kits, calcium carbide testers and air and surface sampling for mould might also be undertaken.
Security
You may need to secure the building to keep out people who are not supposed to be there, as leaving doors and windows open to help a building dry out creates many opportunities for thieves to enter, particularly when many different people are coming and going.
Perforated plywood sheets or wire grilles can be temporarily screwed to existing frames to secure openings at night without inhibiting through ventilation in the building. An alternative way to secure sash windows without inhibiting ventilation is to open the sashes a little way and screw them to the frame, leaving a gap of no more than 100mm at the top and bottom.
The provision of secure salvage stores and temporary alarm systems may also be worth considering.
Drying out
Once excess water has been removed from the building and initial cleaning has taken place to remove solid matter, rubbish, and materials that cannot be recovered (for example insulation or non-historic carpets) then a planned programme of drying out can start.
It is essential not to attempt to dry buildings of traditional construction too quickly (via forced drying) or at extremely high temperatures, as this could cause a great deal of damage particularly where timber is evident. This is because timber when subjected to extreme high temperatures may warp, twist or split. Painted surfaces are likely to peel and flake and salts may migrate through old stone and plasterwork, causing blistering, powdering and exfoliation. Forced drying can also produce a temporary and superficial result as the surface is dried quickly while the bulk of the wall remains wet. This can lead to secondary damage, often manifesting itself just as repair works appear to be finished.
Research has shown that drying is a two-stage process: the first stage is drying by liquid transfer to the surface, and the second drying by vapour transfer. The first stage is fast and effective and governed by ambient conditions of evaporation, the second stage is much slower and almost independent of the ambient conditions.
The best general advice is to dry the building out gently and slowly, first through natural ventilation (possibly aided by mechanical fans). Natural ventilation means opening windows and doors, and where possible elements of the building such as floor voids and behind timber panelling to allow airflow. However, when the humidity levels are higher outside this method will provide little benefit.
Mechanical ventilation, background heating and dehumidifiers (known as Combination Drying) can be used to support in drying out a traditional building. It is important that the background heating is preferably around 19-20°C and that aggressive dehumidifiers are not used, in favour of refrigerant or desiccant dehumidifiers controlled by humidistats.
If heaters are used without adequate ventilation the absolute humidity may actually increase, potentially creating high-humidity conditions in other previously unaffected rooms.
In some situations, injection drying might also be required. This method requires holes to be drilled into the construction, and warm, dry air forced into construction layers through the holes. This warm air dries the hidden materials and the damp air is brought back into the room where it should sucked into a dehumidifier and dried. A detailed method statement as to why this method is being used and how the holes will be repaired after should be requested.
Monitor the drying process
It is essential that the relative humidity and temperature are monitored and recorded both in and outside the building in order to know and assess the rate of evaporation and therefore the speed of water removal.
Key-hole injection-drying techniques may assist in these situations.
Organic materials such as timber can swell and distort when wet and may suffer from fungal and insect decay if left damp for too long. If dried too quickly and at temperatures that are too high, organic materials can shrink, split, or twist.
The following criteria will determine whether sufficient drying has been achieved. Ideally a building or its contents should be returned to the same condition that existed prior to the flood.
- Internal conditions are normal for that particular property. Old buildings are damper than new buildings so the criteria has to be 'dry for the purpose' - i.e. equilibrium in moisture content has been achieved
- The remaining moisture will not support the active growth of fungal spores, mould, mildew or insect infestation
- The building materials and contents will finish returning to equilibrium under normal room conditions by themselves without further damage. Acceptable levels of moisture content will differ depending on the materials involved
The 'drying-out certificate' is what loss adjusters and contractors strive for; however, many old buildings might not have been particularly dry in the first place and it may not always be clear what the previous pre-flood condition of the building really was. Often a subjective judgment needs to be made, backed up with drying-monitoring data to establish when it is safe to commence with reinstatement and repair works.
Timber specifically
When saturated (i.e. when the moisture content goes above 25 per cent), timber is vulnerable to rot as a result of fungal and insect attack. Decay mechanisms must exist for a long period of time in order to become destructive. A one-off event such as a flood should not cause serious damage as long as the wood is allowed to dry afterwards.
Problems occur when moisture is trapped in the wood and cannot escape. This can happen in wall cavities, in sill plates under floors, behind panelling and under impermeable finishes such as oil-based gloss paint. If the moisture content exceeds 28 per cent then dry rot spores could germinate.
Flaking paint does not necessarily signify that the underlying timberwork is rotten, merely that it is temporarily wet. In most cases timber can be dried and returned to a fair state ready for repainting.
Independent experts can be hired for advice on drying and treating woodwork to prevent decay. They can carry out surveys to assess the potential risks of fungal and other infestations and have the tools to monitor the welfare of concealed or remote historic timbers over time.
Panelling
Where there is timber panelling in a property, some can be easily removed, whilst others might be hard to dismantle without the risk of damage to the panelling. Working with a specialist carpenter to assess whether it is possible to dismantle and reinstate without damage is the first step. Secondly, photographing and numbering the panels as they are removed to support in reinstatement. The panelling should be dried in a well-ventilated space. The pieces should be stacked with spacers, turned over periodically and loaded with uniformly distributed weights to counteract warping.
Where it is not possible to dismantle the panelling, injection drying might be the only option to prevent timber decay.
Floorboards and skirting
Where floorboards are buckled they will need to be taken up and carefully stacked and allowed to dry slowly. If the buckling has been considerable they may never regain their previous profile and may need to be replaced.
It may be necessary to remove skirting boards to allow them to dry and prevent warping.
Doors and windows
Doors and windows that are submerged may need to be removed to prevent warping, buckling, or splintering but more often then not they should be fine being allowed to dry in situ, as long as they are left open to help in air circulation. In some situation them may need stablising, or their joints resealed. Repairs should not be attempted until the door or window is completely dry.
Staircases
Timber staircases can suffer swelling and shrinkage in their many components during and after flooding, and this can lead to the loosening and weakening of structural joints. If necessary, stabilise loose treads with additional underside blocks and fixings once the staircase is dry. Ensure that natural ventilation reaches both the upper and lower side of the stair so that the timber dries evenly; this will reduce distortion.
Monitor the long term effects
As the building and surrounding land dry out, monitor the stability of the walls and floors and the integrity of the mortar joints. Any cracks that appear in foundation walls or around openings should be investigated. If they are the result of temporary hydration and expansion of the underlying clay soil around the foundations, the cracks should shrink or at least cease to expand when the water content of the soil returns to normal.
Cracks from foundation erosion, however, can be expected to worsen over time as the building settles. Cracks that widen or move are signs of structural instability that warrants careful examination by a structural engineer experienced in building conservation.
Under-floor timbers should be inspected six months after flooding and then annually, for evidence of fungal infestation and rot. Fibre-optic boroscopes of diameters as small as 6mm are useful for investigation within the fabric via small boreholes or where skirting boards have been temporarily removed. Experience and skill are required to interpret such observations accurately.