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Technical briefing 1 – timber frame construction

This first briefing is about timber frame constructions and the steps that should be taken following a flood or escape of water.

Timber frame construction

The term ‘timber frame’ typically describes a system of panelised structural walls and floors constructed from small section timber studs that are clad with board products, in which the timber frame transmits vertical and horizontal loads to the foundations. In simple terms a timber framed building consists of a cavity wall built off a concrete base or lower wall.

The outer leaf of the wall is commonly masonry and the inner timber frame is built off a foundation or concrete slab. It is commonly finished with plasterboard and a vapour barrier, insulation, air voids and a breathable membrane between. This method of construction is widely used to speed up construction, particularly in new build homes. The effects of water ingress either through flooding or an escape of water differ to those on traditional construction methods. As a result the methods required for drying are also different.

Points to note when dealing with water ingress to rimber framed buildings

When dealing with flood damage (i.e. where water has entered the building at ground level) it is important to establish the nature of the floor/timber junction. In theory, it should be a base plate sitting on a damp-proof course (DPC) on the concrete base, but in some cases the base plate may be set into the concrete or masonry. This can result in a ‘moisture reservoir’ that can hold water, increasing the risk of decay affecting the timbers. The visible finished floor of the property may not be the base, and there may be a void which will also hold moisture if not treated.

The key to the restoration of timber framed premises is speed of drying

At moisture levels of greater that 16% there is a risk of rot in untreated timbers which can include cut-ends of treated timbers. During the drying process following a severe incident there is a period during which the timber moisture levels can be between 18 – 25% which means there is a risk of decay. Ensuring that the drying period is minimised will reduce that risk of decay. It is important to start drying at the earliest opportunity to reduce the likelihood of expansion of the timbers.

To produce an effective and efficient drying process, it’s important to establish the extent of damage and spread of moisture.

In a timber framed construction this cannot necessarily be done with surface moisture meters alone. The internal surface maybe dry, but the unseen areas may have been affected. Also, it cannot be assumed that the construction methods will be the same in similar looking timber framed properties. A thorough inspection of the property is required.To minimise disruption this can be carried out in several ways by using tools such as endoscopes to inspect voids and thermal imaging to provide information on anomalies in the construction, together with traditional methods of moisture detection.

Stripping out

Normally only those materials irrevocably damaged by the water should require removal. This may include plasterboard, insulation and floor coverings. However, it may sometimes be advantageous to expose areas to assist in the drying process. Pressurised drying systems can be used to reduce the need to expose voids and a correctly carried out inspection will identify the areas of damage and enable a ‘focused drying system’. The extent of stripping out works should always be discussed with the drying specialist and the re-instatement contractors before commencement to ensure that all parties agree with the method and are clear on their roles and responsibilties.

Drying systems

Due to the construction of the property, simply placing dehumidifiers in the affected rooms is likely to be inefficient. This is because the moisture can be trapped in the wall cavity, insulation and in the timber itself by the vapour barrier. Therefore, the drying must be targeted or focused on the areas which require drying. Using focused drying methods will also minimise the amount of strip-out work required. In most cases, the nature of the construction provides a closed system which will assist in establishing an efficient, focused drying system, enabling the restoration operative to control drying speed and temperature.

Reinstatement works

Depending on the severity of the incident, reinstatement is likely to be minimal. It is important to ensure that reinstatement contractors are familiar with the method of construction and that they re-fit and bond new vapour barriers to maintain the integrity of the building envelope. Also, it may be prudent to have wood preservation treatment carried out to further reduce the risk of future problems.


Drying methods for timber framed buildings are very different to those for traditional buildings. A full inspection must be carried out to identify where the moisture is trapped so that the nost efficient drying regime can be installed. It will also help establish the requirements for strip-out works and the areas for targeted drying. The drying process should target the specific areas and account for the ‘sealed’ nature of the construction. Drying should start as soon as possible and achieve rapid results to prevent mould growth. Reinstatement works should ensure that vapour barriers and moisture control membranes are re-instated and should include wood treatment processes if the requirement is identified by the initial assessment.