Understanding Property with PCA Flood Recovery

This technical article first appeared in the March 2010 issue of Construction Manager magazine. www.construction-manager.co.uk

Thousands of UK residents have suffered the effects of flooding in recent years.  From Hull and Gloucestershire in 2007 to Cumbria in late 2009 many people have seen their homes and businesses devastated, explains Steve Hodgson, technical director of the trade body, the Property Care Association.

These catastrophic events have highlighted the need for professional advice from contractors with specific knowledge and experience of flood remediation when recovering flood damaged properties.

Flood recovery work is a specialised skill, get it wrong and six months down the line the property may have developed a serious damp problem, adding to the distress and trauma of the homeowners who may need to vacate their home for a second time.

Understanding the Flood

Before any investigation of the building takes place it is essential to first establish the nature of the flood. Existence of sewage effluent, chemical or biological contaminants or silts, sands and soils in the floodwater will dictate the way recovery is managed.

The height reached by the water and its rate of flow both inside and around the building must also be considered as floodwater and the debris carried with it can exert extreme pressure on the structure of a building.

Understanding the Building

It is also necessary to fully understand the construction of the building.

Water affects different materials in a variety of ways.  For instance, concrete and brick are very resistant to water damage.   Timber can be resistant to short term wetting but decays when it remains wet for a long period.  Dense materials retain little water but dry slowly, whereas porous materials can retain large amounts of water but can often be dried quickly.

Historic repairs and alterations can affect the remediation of a building.  For instance, the presence of a chemical damp proof course can indicate a previous problem with rising damp and, even though this may not have been affected by the flood, the presence of ground salts within the structure must be considered.

Previous problems with dry rot in a building may substantially increase the risk of future outbreaks if the building structure becomes wet again.

Building extensions can result in voids being created between the original structure and the newer addition and can often conceal original drains and service ducts or restrict ventilation to sub-floor areas.

Diagnosis and Investigation

Site investigation should begin as soon as standing water has been removed to allow the surveyor to see the building prior to the stripping out of damaged materials. 

A thorough investigation at this time is important in order that the surveyor can establish a base level from which the drying process can be monitored. 

Monitoring should be undertaken using methods and techniques that can be repeated throughout the flood recovery process. The results of these tests must then be recorded in a way that allows easy retrieval and interpretation during the drying stage.  This will then allow the surveyor to decide when a building has reached a point when drying activities can cease and will influence the materials and techniques that will be used in the restoration process.

Moisture monitoring can be undertaken using a number of devices and techniques including:

Gravimetric method – drilled samples from the test area are placed in securely sealed bottles. The moisture content is then determined by oven drying in a laboratory.

All materials contain some moisture when in equilibrium with the environment. This water is held within the structure or exists within soluble salts that may be present.  Gravimetric testing allows the surveyor to determine accurately the total volume of water in the sample, the amount that would be present when the materiel is in its usually dry state (equilibrium) and therefore the amount of free water that is present.  It is the free water that is often introduced by the flood and that should be removed to allow the recovery of the building.

Chemical method – a sample of standard weight is mixed with calcium carbide powder in a pressure vessel fitted with a gauge. Moisture within the sample reacts with the calcium carbide and produces a volume of acetylene directly proportional to the level of moisture.

Conductivity meters – moisture levels can be found by placing two probes into a wall and measuring the electrical resistance.

Hand held conductivity and capacitance moisture meters can be very useful when screening a property for damp and flood water and can be used to quickly identify areas of wood and plaster that deserve further investigations. They give reliable readings in timber but cannot used to derive the moisture content in plaster or masonry.

Recovery Strategies – Drying buildings

Natural drying can be accelerated using dehumidification, ventilation and heat.  In most circumstances a combination of all three will be used. 

The speed of drying is dictated by the amount of energy input and how this is then managed in order to expel the water vapour.

Speed Drying – The use of heaters and directed hot air. These take several forms, from large diesel powered boilers that pump heated glycol to heat exchanges, propane fired space heaters to systems that have fans that run air over electric heating elements.

Dehumidification – Desiccant and heat exchange types are commonly used by flood drying companies.  Desiccant dehumidifiers operate by absorbing moisture from the air that is then removed from the desiccant materiel as water. These systems are best suited to use in unheated spaces.  Heat exchange dehumidifiers draw air over a cold surface were water is condensed out of the atmosphere. These are most effective where the air is warm and relative humidity can be held at a high level.

Fresh air – When conditions are right the passage of large volumes of relatively dry warm air over a damp surface will achieve very effective drying. Sometimes at certain times of the year the best way to achieve drying is to open all the doors and windows and let the air blow through!

How Do You Know When the Building is Dry?

Different materials hold different levels of moisture in their dry state.  It is not possible therefore to define a level of ‘dryness’ for a building as each will be different.

In most cases a surveyor does not need to decide that a building is dry, but rather that the building is in a suitable state for repair and reoccupation - without risk to the health of the occupants or further deterioration of the building and its contents.

A structure is essentially “dry” when it reaches a state of equilibrium with its wider environment. This cannot be expressed as a % moisture content but is derived by having an understanding of the building and its constituent parts. In some cases this “dry” state will be well above what is normally consider dry in new construction but if all the free water has been removed the extraction of further water from the building fabric is folly.

By monitoring the drying out of the building the surveyor will be able to determine the full extent of any repairs that may be required and go on to draw up a schedule of works to return the building to occupation.

The repair is dictated by the amount of free water that remains in the building, levels of salt contamination and the materials and techniques that are adopted in the repair.

Salts are often Hygroscopic (able to absorb atmospheric water). If they exist within the building at significant levels they can not only migrate with the hydration water to effect the new finishes but will also absorb water from the air, go into solution and migrate to the surface of the wall as the water evaporated off. This process readily affects some modern building products particularly those that contain gypsum.

Repair or Refit

When it is considered that the drying phase is complete, the flood damaged property should be subject to further investigation to determine what needs to be removed and renewed.

Special attention must be paid to the condition of timbers, plaster or other wall finishes and any other element of the building that may have been affected by the floodwater or the drying process.

Further defects may be found as the building is prepared for reinstatement.  These should be reported to the client and acted upon.

Longer Term Effects of Flooding

Common problems reported by occupants of buildings that have undergone flood recovery include:

Condensation

It is essential to the recovery of a building following flooding that environmental controls such as heating, ventilation, air conditioning, dehumidification are put in place to prevent the transfer of atmospheric water to areas where it can condense and cause further damage. If condensation persists following flood recovery, detailed investigations will be required to determine the cause, which may be completely unrelated to the flood water.

Mould Growth

Mould or mildew growth may be the result of an underlying problem which may include trapped flood water, condensation, plumbing leaks or water ingress.  A thorough investigation by a qualified surveyor will be required to assess the cause.

Fungal Decay in Timbers

Where the drying, ventilation or isolation of timbers has been inadequate, the  timber will be prone to attack from wood destroying fungi.

If wet or dry rot become apparent following a flood, a detailed investigation will be necessary to determine the extent of the outbreak and to identify and eliminate any source of water, while drawing up specifications for treatment.

Damp Staining

This occurs when the water within the building contains salts or other forms of contamination.  The water can transport minerals in solution as it moves through the building and is often deposited in the wall and floor finishes from where the water evaporates.

When staining is the result of hygroscopic salts, this should be fully investigated and may result in the removal and renewal of contaminated materials.

Salt Deposition

In some circumstances crystals may be deposited on the surface of new plaster. Usually these can be removed without damage and will only be a short term problem.  If they are persistent then further investigation and rectification will be necessary.

Fluctuating Moisture Levels

Some old brickwork can contain relatively large volumes of water and this can be artificially reduced using drying apparatus. However, once the dryer has been removed the brickwork will absorb water from the atmosphere and become ‘wet’ again. Allowances must be made for this when considering how best to carry-out reinstatement.

It is also possible that misleading moisture readings are being obtained from materials that are naturally conductive where the primary diagnostic equipment is an electronic conductivity meter.

Lamination Swelling and Shrinkage

Materials will naturally absorb and release water in response to fluctuating atmospheric conditions.
It is essential that items such as laminate, timber strip flooring and timber wall panelling are only reinstated when the adjacent structures are thoroughly dry or can be permanently isolated from damp materials.

Flood Resilient Repair

It may not be possible to make an existing building totally flood proof, but very high levels of water resistance and flood resilience can be achieved.  It is possible to create a perfectly dry, habitable underground room in a building that is completely saturated, so recovering wet buildings is entirely possible.

Good design techniques are essential to making a building resilient to future flooding.

Effective ways of preventing water from entering the building include:

Installation of temporary or permanent flood barriers around the building
Closing openings and creating a waterproof outer shell 
Appliance of a waterproof membrane to the inner surfaces of the walls and floors
Closing openings and then managing and expelling water that has entered the building using drainage channels and pumps.

Code of Practice

The Property Care Association (PCA) has developed a new code of practice for the recovery of flood damaged buildings.

Available as a free download from the PCA’s website, www.property-care.org" www.property-care.org the document is designed to assist companies and agencies involved in all aspects of flood restoration and the recovery of flood affected buildings.

The guide covers a broad subject area, including information on the assessment of flooded buildings, drying, repair considerations and examples of failures where reinstatements are not carefully considered – and details on making a building more flood resilient.

The new code highlights the importance of understanding the building. It considers the need for thorough investigation at the earliest possible opportunity, and the importance of identifying pre-existing defects.

It also looks in general terms at the drying process, concentrating on the importance of monitoring and understanding what is going on within the structure at this critical time.

Information is provided too on the techniques and materials that can be utilised in the reconstruction phase of the recovery project – and consideration given to the processes that reduce the likelihood of future problems.

Finally, some of the failures that are encountered when reinstatements are undertaken without proper consideration are featured – and the means by which the building can be made resilient to future flooding.

Summary

The recovery of flooded buildings is a complex process that will involve a number of specialists working together.

The principals set out in this CPD will allow buildings to be reoccupied sooner and will significantly reduce potential future problems.

Steve Hodgson is the technical director of the Property Care Association, which represents the UK’s structural waterproofing, wood preservation, damp-proofing, flood remediation and structural repair sectors.
 
Guidance about the PCA’s wide-ranging work, including training courses for construction professionals, are available from the PCA website at  www.property-care.org.