Wattle & Daub: Craft, Conservation & Wiltshire Case Study
3.6 Daub
4.1 Soils
4.1.3 Strength
4.2 Dung
4.2.2 Lignin
4.2.3 Urine
4.3 Fibre
5.4.2 Renewal

Title Page Previous Next | Conservation >Defects and Decay >Decay of Daub and Plaster

5.2.1 Decay of Daub and Plaster

The most significant cause of daub decay is caused by water absorption and erosion.

An unprotected daub is essentially exposed soil. Direct contact with rain may therefore cause the clay matrix to be dissolved. The loss of binder may then lead to the remaining aggregate rapidly falling away. A daub that is predominantly sand/silt has characteristics making it especially prone to such erosion.[78] Wind may also contribute to loosening unbound aggregate. A daub is protected by the shedding effects of a limewash, but the lime additionally provides a binder to the surface of the daub in the form of calcium carbonate. A lime render also protects daub from erosion.
Erosion by water may be significantly accelerated by run-off from impervious materials situated above exposed daub such as impermeable modern paints and cement renders.

Water Penetration
The action of absorbed water is probably the single most damaging cause of daub failure. The linear and volumetric expansion and contraction of the clay causes cyclical fatigue of the daub, leading to increased cracking and eventual failure. Cracks may run either perpendicular to the plane of the panel surface or parallel to it. The former are visible on the surface and will themselves allow increasing levels of water penetration into the daub. Cracking parallel to the surface is manifested as delamination. This may occur within the bulk of the daub (especially at residual interfaces remaining from where cats were melded together) or where the daub protrudes between the withies (or as ‘nibs’ between laths).

At the base of a panel, the groove to accept the staves can act as a water trap if maintenance is neglected. If cracks are left unattended or impervious materials used, water may accumulate in the groove and cause accelerated rot of the timber. The problem is avoided by appropriate maintenance of the wattle and daub so that rain cannot penetrate into cracks or into gaps adjacent to the surrounding timber frame. By filling the gaps with daub or lime mortar, any accumulating moisture is quickly reabsorbed into the bulk of the panel and is removed by evaporation through its large surface area.

It is imperative to the conservation of wattle and daub that a survey or routine inspection correctly identifies the source of the damp. The universal moisture tester based on electrical resistance is not a reliable guide since it only measures surface conditions and is affected by changes in electrical conductivity caused by any natural or added salts in the daub. The presence of moderate moisture levels is also not automatically indicative of a problem unless decay is evident.[79] For example, moisture content will be higher after rainfall or during the winter months. Damp is often caused by the trapping of moisture, such as by the use of impermeable paints, cement renders (including repair patches), proprietary wood treatments to the surfaces of timbers, or by excessive moisture due to poor detailing or maintenance of rainwater goods, flashings, etc.

Frost Action
At temperatures of 0°C and below, water within the pores of a daub will freeze and expand, forcing the daub apart. This may appear as cracking or spalling (‘blowing’ of daub by an outward force). Once this type of decay has started, water may accumulate within the cracks and cause the process to accelerate.

Organic Growth
The effect of plant growths on wattle and daub may vary from those that add interest and patina, through plants that have little effect, to those that cause complete failure of a panel.
Growth of algae and lichen may occur where local conditions are favourable. There is potential for harmless growth on a limewash surface due to casein or tallow proteins added to some limewashes, but is often counteracted by the causticity of limewash. Regular limewashing is likely to minimise growth.
Lichens are unlikely to cause harm. Small quantities of moss are also likely to be harmless but, where excessive, they may trap moisture and should be removed. Fungal growth may be an indication that decomposition is occurring due to excessive dung or organic matter within the daub and excessive levels of moisture.
The sprouting of small plants from the daub is also an indication of excessive moisture and there is a risk that their root systems may cause fragmentation of the daub. The underlying problem should be rectified and the plants removed. Larger plants, especially ivies and creepers, should not be planted in the proximity of wattle and daub. Such plants easily find their way into crevices between daub and frame or into cracked daub, the root system quickly establishing within the daub and around the withies. The roots are likely to breakdown the daub and cause delamination from the wattle. Existing growth should not be forcibly removed since this will cause further damage. Instead, the plant should be killed by severing the main stem at its base and removing a 2-3cm section to prevent it regrafting. Dead growth may be carefully removed where loose, but where it remains bonded to the daub it should be left and limewashed over.[80]

Mechanical Damage
The strength of wattle and daub tends to decrease with time due to localised decay, but usually retains sufficient strength to support its own weight. Imposed lateral loads may damage a panel by cracking or deformation. A typical cause is that of leaning of ladders onto a panel, such as by contractors or window cleaners. Weak, but otherwise sound panels may be damaged by a person leaning their hand on them. Damage can be prevented by informing contractors of the risk and ensuring ladders are rested only on the timber frame.[81]

The effects of ground and air-borne vibration have been assessed for masonry and plasters, but not for daubs. Vibration is of legitimate concern due to the large number of historic buildings that were built close to the highway and due to the increasing levels of passing traffic. Other sources include nearby railways, mine-blasting and nearby building works, especially pile driving and on-site use of jackhammers.[82] Nailing, such as during the repair of laths, also presents a significant concern. The risk is of cracking or failure of the panel by debonding with the wattle and since the effects are cumulative, exposure should be minimised.

Rats and mice may find their way into panels, but are likely to do little harm to wattle and daub compared to solid earth walls.[83] However, an infestation should be eradicated since rodents may damage electrical wiring and thatch. Masonry bees are unlikely to significantly harm a panel since the relative thinness of daub makes an unattractive home. The occasional hole can be repaired with lime plaster.

[78] Robson (1999), pp.32-33.
[79] A thorough discussion of the monitoring and assessment of damp is presented by Ashurst and Ashurst (1988a), pp.1-21 and by Oxley (2003), pp.137-157.
[80] Ashurst and Ashurst (1988b), pp.20-24.
[81] Ladders may also damage the surface of a timber frame, especially where frassy or where shakes lie close to the timber edges. A stout lateral board should be attached to the top of the ladder side rails to prevent excessive pressure on the timbers.
[82] Hulme (1985) summarises recent research on vibration and concluded that traffic is unlikely to cause significant damage to masonry and plasters. Snider (2003) similarly concludes that the effects of blasting are several orders less than the thermal effects of the weather.  However, daubs are weaker than lime renders and plasters and so published research cannot be directly applied to the performance of daubs.
[83] Rat runs in earth walls can cause structural damage. Ashurst and Ashurst (1988a), p.98.